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1 | <?xml version="1.0" encoding="UTF-8"?> |
1 | <?xml version="1.0" encoding="UTF-8"?> |
2 | <chapter id="mm"> |
2 | <chapter id="mm"> |
3 | <?dbhtml filename="mm.html"?> |
3 | <?dbhtml filename="mm.html"?> |
4 | 4 | ||
5 | <title>Memory management</title> |
5 | <title>Memory management</title> |
6 | 6 | ||
7 | <para>In previous chapters, this book described the scheduling subsystem as |
7 | <para>In previous chapters, this book described the scheduling subsystem as |
8 | the creator of the impression that threads execute in parallel. The memory |
8 | the creator of the impression that threads execute in parallel. The memory |
9 | management subsystem, on the other hand, creates the impression that there |
9 | management subsystem, on the other hand, creates the impression that there |
10 | is enough physical memory for the kernel and that userspace tasks have the |
10 | is enough physical memory for the kernel and that userspace tasks have the |
11 | entire address space only for themselves.</para> |
11 | entire address space only for themselves.</para> |
12 | 12 | ||
13 | <section> |
13 | <section> |
14 | <title>Physical memory management</title> |
14 | <title>Physical memory management</title> |
15 | 15 | ||
16 | <section id="zones_and_frames"> |
16 | <section id="zones_and_frames"> |
17 | <title>Zones and frames</title> |
17 | <title>Zones and frames</title> |
18 | 18 | ||
19 | <para>HelenOS represents continuous areas of physical memory in |
19 | <para>HelenOS represents continuous areas of physical memory in |
20 | structures called frame zones (abbreviated as zones). Each zone contains |
20 | structures called frame zones (abbreviated as zones). Each zone contains |
21 | information about the number of allocated and unallocated physical |
21 | information about the number of allocated and unallocated physical |
22 | memory frames as well as the physical base address of the zone and |
22 | memory frames as well as the physical base address of the zone and |
23 | number of frames contained in it. A zone also contains an array of frame |
23 | number of frames contained in it. A zone also contains an array of frame |
24 | structures describing each frame of the zone and, in the last, but not |
24 | structures describing each frame of the zone and, in the last, but not |
25 | the least important, front, each zone is equipped with a buddy system |
25 | the least important, front, each zone is equipped with a buddy system |
26 | that faciliates effective allocation of power-of-two sized block of |
26 | that faciliates effective allocation of power-of-two sized block of |
27 | frames.</para> |
27 | frames.</para> |
28 | 28 | ||
29 | <para>This organization of physical memory provides good preconditions |
29 | <para>This organization of physical memory provides good preconditions |
30 | for hot-plugging of more zones. There is also one currently unused zone |
30 | for hot-plugging of more zones. There is also one currently unused zone |
31 | attribute: <code>flags</code>. The attribute could be used to give a |
31 | attribute: <code>flags</code>. The attribute could be used to give a |
32 | special meaning to some zones in the future.</para> |
32 | special meaning to some zones in the future.</para> |
33 | 33 | ||
34 | <para>The zones are linked in a doubly-linked list. This might seem a |
34 | <para>The zones are linked in a doubly-linked list. This might seem a |
35 | bit ineffective because the zone list is walked everytime a frame is |
35 | bit ineffective because the zone list is walked everytime a frame is |
36 | allocated or deallocated. However, this does not represent a significant |
36 | allocated or deallocated. However, this does not represent a significant |
37 | performance problem as it is expected that the number of zones will be |
37 | performance problem as it is expected that the number of zones will be |
38 | rather low. Moreover, most architectures merge all zones into |
38 | rather low. Moreover, most architectures merge all zones into |
39 | one.</para> |
39 | one.</para> |
40 | 40 | ||
41 | <para>For each physical memory frame found in a zone, there is a frame |
41 | <para>For each physical memory frame found in a zone, there is a frame |
42 | structure that contains number of references and data used by buddy |
42 | structure that contains number of references and data used by buddy |
43 | system.</para> |
43 | system.</para> |
44 | </section> |
44 | </section> |
45 | 45 | ||
46 | <section id="frame_allocator"> |
46 | <section id="frame_allocator"> |
- | 47 | <indexterm> |
|
- | 48 | <primary>frame allocator</primary> |
|
- | 49 | </indexterm> |
|
- | 50 | ||
47 | <title>Frame allocator</title> |
51 | <title>Frame allocator</title> |
48 | 52 | ||
49 | <para>The frame allocator satisfies kernel requests to allocate |
53 | <para>The frame allocator satisfies kernel requests to allocate |
50 | power-of-two sized blocks of physical memory. Because of zonal |
54 | power-of-two sized blocks of physical memory. Because of zonal |
51 | organization of physical memory, the frame allocator is always working |
55 | organization of physical memory, the frame allocator is always working |
52 | within a context of some frame zone. In order to carry out the |
56 | within a context of some frame zone. In order to carry out the |
53 | allocation requests, the frame allocator is tightly integrated with the |
57 | allocation requests, the frame allocator is tightly integrated with the |
54 | buddy system belonging to the zone. The frame allocator is also |
58 | buddy system belonging to the zone. The frame allocator is also |
55 | responsible for updating information about the number of free and busy |
59 | responsible for updating information about the number of free and busy |
56 | frames in the zone. <figure> |
60 | frames in the zone. <figure> |
57 | <mediaobject id="frame_alloc"> |
61 | <mediaobject id="frame_alloc"> |
58 | <imageobject role="html"> |
62 | <imageobject role="html"> |
59 | <imagedata fileref="images/frame_alloc.png" format="PNG" /> |
63 | <imagedata fileref="images/frame_alloc.png" format="PNG" /> |
60 | </imageobject> |
64 | </imageobject> |
61 | 65 | ||
62 | <imageobject role="fop"> |
66 | <imageobject role="fop"> |
63 | <imagedata fileref="images.vector/frame_alloc.svg" format="SVG" /> |
67 | <imagedata fileref="images.vector/frame_alloc.svg" format="SVG" /> |
64 | </imageobject> |
68 | </imageobject> |
65 | </mediaobject> |
69 | </mediaobject> |
66 | 70 | ||
67 | <title>Frame allocator scheme.</title> |
71 | <title>Frame allocator scheme.</title> |
68 | </figure></para> |
72 | </figure></para> |
69 | 73 | ||
70 | <formalpara> |
74 | <formalpara> |
71 | <title>Allocation / deallocation</title> |
75 | <title>Allocation / deallocation</title> |
72 | 76 | ||
73 | <para>Upon allocation request via function <code>frame_alloc</code>, |
77 | <para>Upon allocation request via function <code>frame_alloc</code>, |
74 | the frame allocator first tries to find a zone that can satisfy the |
78 | the frame allocator first tries to find a zone that can satisfy the |
75 | request (i.e. has the required amount of free frames). Once a suitable |
79 | request (i.e. has the required amount of free frames). Once a suitable |
76 | zone is found, the frame allocator uses the buddy allocator on the |
80 | zone is found, the frame allocator uses the buddy allocator on the |
77 | zone's buddy system to perform the allocation. During deallocation, |
81 | zone's buddy system to perform the allocation. During deallocation, |
78 | which is triggered by a call to <code>frame_free</code>, the frame |
82 | which is triggered by a call to <code>frame_free</code>, the frame |
79 | allocator looks up the respective zone that contains the frame being |
83 | allocator looks up the respective zone that contains the frame being |
80 | deallocated. Afterwards, it calls the buddy allocator again, this time |
84 | deallocated. Afterwards, it calls the buddy allocator again, this time |
81 | to take care of deallocation within the zone's buddy system.</para> |
85 | to take care of deallocation within the zone's buddy system.</para> |
82 | </formalpara> |
86 | </formalpara> |
83 | </section> |
87 | </section> |
84 | 88 | ||
85 | <section id="buddy_allocator"> |
89 | <section id="buddy_allocator"> |
- | 90 | <indexterm> |
|
- | 91 | <primary>buddy system</primary> |
|
- | 92 | </indexterm> |
|
- | 93 | ||
86 | <title>Buddy allocator</title> |
94 | <title>Buddy allocator</title> |
87 | 95 | ||
88 | <para>In the buddy system, the memory is broken down into power-of-two |
96 | <para>In the buddy system, the memory is broken down into power-of-two |
89 | sized naturally aligned blocks. These blocks are organized in an array |
97 | sized naturally aligned blocks. These blocks are organized in an array |
90 | of lists, in which the list with index i contains all unallocated blocks |
98 | of lists, in which the list with index i contains all unallocated blocks |
91 | of size <mathphrase>2<superscript>i</superscript></mathphrase>. The |
99 | of size <mathphrase>2<superscript>i</superscript></mathphrase>. The |
92 | index i is called the order of block. Should there be two adjacent |
100 | index i is called the order of block. Should there be two adjacent |
93 | equally sized blocks in the list i<mathphrase />(i.e. buddies), the |
101 | equally sized blocks in the list i<mathphrase />(i.e. buddies), the |
94 | buddy allocator would coalesce them and put the resulting block in list |
102 | buddy allocator would coalesce them and put the resulting block in list |
95 | <mathphrase>i + 1</mathphrase>, provided that the resulting block would |
103 | <mathphrase>i + 1</mathphrase>, provided that the resulting block would |
96 | be naturally aligned. Similarily, when the allocator is asked to |
104 | be naturally aligned. Similarily, when the allocator is asked to |
97 | allocate a block of size |
105 | allocate a block of size |
98 | <mathphrase>2<superscript>i</superscript></mathphrase>, it first tries |
106 | <mathphrase>2<superscript>i</superscript></mathphrase>, it first tries |
99 | to satisfy the request from the list with index i. If the request cannot |
107 | to satisfy the request from the list with index i. If the request cannot |
100 | be satisfied (i.e. the list i is empty), the buddy allocator will try to |
108 | be satisfied (i.e. the list i is empty), the buddy allocator will try to |
101 | allocate and split a larger block from the list with index i + 1. Both |
109 | allocate and split a larger block from the list with index i + 1. Both |
102 | of these algorithms are recursive. The recursion ends either when there |
110 | of these algorithms are recursive. The recursion ends either when there |
103 | are no blocks to coalesce in the former case or when there are no blocks |
111 | are no blocks to coalesce in the former case or when there are no blocks |
104 | that can be split in the latter case.</para> |
112 | that can be split in the latter case.</para> |
105 | 113 | ||
106 | <para>This approach greatly reduces external fragmentation of memory and |
114 | <para>This approach greatly reduces external fragmentation of memory and |
107 | helps in allocating bigger continuous blocks of memory aligned to their |
115 | helps in allocating bigger continuous blocks of memory aligned to their |
108 | size. On the other hand, the buddy allocator suffers increased internal |
116 | size. On the other hand, the buddy allocator suffers increased internal |
109 | fragmentation of memory and is not suitable for general kernel |
117 | fragmentation of memory and is not suitable for general kernel |
110 | allocations. This purpose is better addressed by the <link |
118 | allocations. This purpose is better addressed by the <link |
111 | linkend="slab">slab allocator</link>.<figure> |
119 | linkend="slab">slab allocator</link>.<figure> |
112 | <mediaobject id="buddy_alloc"> |
120 | <mediaobject id="buddy_alloc"> |
113 | <imageobject role="html"> |
121 | <imageobject role="html"> |
114 | <imagedata fileref="images/buddy_alloc.png" format="PNG" /> |
122 | <imagedata fileref="images/buddy_alloc.png" format="PNG" /> |
115 | </imageobject> |
123 | </imageobject> |
116 | 124 | ||
117 | <imageobject role="fop"> |
125 | <imageobject role="fop"> |
118 | <imagedata fileref="images.vector/buddy_alloc.svg" format="SVG" /> |
126 | <imagedata fileref="images.vector/buddy_alloc.svg" format="SVG" /> |
119 | </imageobject> |
127 | </imageobject> |
120 | </mediaobject> |
128 | </mediaobject> |
121 | 129 | ||
122 | <title>Buddy system scheme.</title> |
130 | <title>Buddy system scheme.</title> |
123 | </figure></para> |
131 | </figure></para> |
124 | 132 | ||
125 | <section> |
133 | <section> |
126 | <title>Implementation</title> |
134 | <title>Implementation</title> |
127 | 135 | ||
128 | <para>The buddy allocator is, in fact, an abstract framework wich can |
136 | <para>The buddy allocator is, in fact, an abstract framework wich can |
129 | be easily specialized to serve one particular task. It knows nothing |
137 | be easily specialized to serve one particular task. It knows nothing |
130 | about the nature of memory it helps to allocate. In order to beat the |
138 | about the nature of memory it helps to allocate. In order to beat the |
131 | lack of this knowledge, the buddy allocator exports an interface that |
139 | lack of this knowledge, the buddy allocator exports an interface that |
132 | each of its clients is required to implement. When supplied with an |
140 | each of its clients is required to implement. When supplied with an |
133 | implementation of this interface, the buddy allocator can use |
141 | implementation of this interface, the buddy allocator can use |
134 | specialized external functions to find a buddy for a block, split and |
142 | specialized external functions to find a buddy for a block, split and |
135 | coalesce blocks, manipulate block order and mark blocks busy or |
143 | coalesce blocks, manipulate block order and mark blocks busy or |
136 | available.</para> |
144 | available.</para> |
137 | 145 | ||
138 | <formalpara> |
146 | <formalpara> |
139 | <title>Data organization</title> |
147 | <title>Data organization</title> |
140 | 148 | ||
141 | <para>Each entity allocable by the buddy allocator is required to |
149 | <para>Each entity allocable by the buddy allocator is required to |
142 | contain space for storing block order number and a link variable |
150 | contain space for storing block order number and a link variable |
143 | used to interconnect blocks within the same order.</para> |
151 | used to interconnect blocks within the same order.</para> |
144 | 152 | ||
145 | <para>Whatever entities are allocated by the buddy allocator, the |
153 | <para>Whatever entities are allocated by the buddy allocator, the |
146 | first entity within a block is used to represent the entire block. |
154 | first entity within a block is used to represent the entire block. |
147 | The first entity keeps the order of the whole block. Other entities |
155 | The first entity keeps the order of the whole block. Other entities |
148 | within the block are assigned the magic value |
156 | within the block are assigned the magic value |
149 | <constant>BUDDY_INNER_BLOCK</constant>. This is especially important |
157 | <constant>BUDDY_INNER_BLOCK</constant>. This is especially important |
150 | for effective identification of buddies in a one-dimensional array |
158 | for effective identification of buddies in a one-dimensional array |
151 | because the entity that represents a potential buddy cannot be |
159 | because the entity that represents a potential buddy cannot be |
152 | associated with <constant>BUDDY_INNER_BLOCK</constant> (i.e. if it |
160 | associated with <constant>BUDDY_INNER_BLOCK</constant> (i.e. if it |
153 | is associated with <constant>BUDDY_INNER_BLOCK</constant> then it is |
161 | is associated with <constant>BUDDY_INNER_BLOCK</constant> then it is |
154 | not a buddy).</para> |
162 | not a buddy).</para> |
155 | </formalpara> |
163 | </formalpara> |
156 | </section> |
164 | </section> |
157 | </section> |
165 | </section> |
158 | 166 | ||
159 | <section id="slab"> |
167 | <section id="slab"> |
- | 168 | <indexterm> |
|
- | 169 | <primary>slab allocator</primary> |
|
- | 170 | </indexterm> |
|
- | 171 | ||
160 | <title>Slab allocator</title> |
172 | <title>Slab allocator</title> |
161 | 173 | ||
162 | <para>The majority of memory allocation requests in the kernel is for |
174 | <para>The majority of memory allocation requests in the kernel is for |
163 | small, frequently used data structures. The basic idea behind the slab |
175 | small, frequently used data structures. The basic idea behind the slab |
164 | allocator is that commonly used objects are preallocated in continuous |
176 | allocator is that commonly used objects are preallocated in continuous |
165 | areas of physical memory called slabs<footnote> |
177 | areas of physical memory called slabs<footnote> |
166 | <para>Slabs are in fact blocks of physical memory frames allocated |
178 | <para>Slabs are in fact blocks of physical memory frames allocated |
167 | from the frame allocator.</para> |
179 | from the frame allocator.</para> |
168 | </footnote>. Whenever an object is to be allocated, the slab allocator |
180 | </footnote>. Whenever an object is to be allocated, the slab allocator |
169 | returns the first available item from a suitable slab corresponding to |
181 | returns the first available item from a suitable slab corresponding to |
170 | the object type<footnote> |
182 | the object type<footnote> |
171 | <para>The mechanism is rather more complicated, see the next |
183 | <para>The mechanism is rather more complicated, see the next |
172 | paragraph.</para> |
184 | paragraph.</para> |
173 | </footnote>. Due to the fact that the sizes of the requested and |
185 | </footnote>. Due to the fact that the sizes of the requested and |
174 | allocated object match, the slab allocator significantly reduces |
186 | allocated object match, the slab allocator significantly reduces |
175 | internal fragmentation.</para> |
187 | internal fragmentation.</para> |
176 | 188 | ||
- | 189 | <indexterm> |
|
- | 190 | <primary>slab allocator</primary> |
|
- | 191 | ||
- | 192 | <secondary>slab cache</secondary> |
|
- | 193 | </indexterm> |
|
- | 194 | ||
177 | <para>Slabs of one object type are organized in a structure called slab |
195 | <para>Slabs of one object type are organized in a structure called slab |
178 | cache. There are ususally more slabs in the slab cache, depending on |
196 | cache. There are ususally more slabs in the slab cache, depending on |
179 | previous allocations. If the the slab cache runs out of available slabs, |
197 | previous allocations. If the the slab cache runs out of available slabs, |
180 | new slabs are allocated. In order to exploit parallelism and to avoid |
198 | new slabs are allocated. In order to exploit parallelism and to avoid |
181 | locking of shared spinlocks, slab caches can have variants of |
199 | locking of shared spinlocks, slab caches can have variants of |
182 | processor-private slabs called magazines. On each processor, there is a |
200 | processor-private slabs called magazines. On each processor, there is a |
183 | two-magazine cache. Full magazines that are not part of any |
201 | two-magazine cache. Full magazines that are not part of any |
184 | per-processor magazine cache are stored in a global list of full |
202 | per-processor magazine cache are stored in a global list of full |
185 | magazines.</para> |
203 | magazines.</para> |
186 | 204 | ||
- | 205 | <indexterm> |
|
- | 206 | <primary>slab allocator</primary> |
|
- | 207 | ||
- | 208 | <secondary>magazine</secondary> |
|
- | 209 | </indexterm> |
|
- | 210 | ||
187 | <para>Each object begins its life in a slab. When it is allocated from |
211 | <para>Each object begins its life in a slab. When it is allocated from |
188 | there, the slab allocator calls a constructor that is registered in the |
212 | there, the slab allocator calls a constructor that is registered in the |
189 | respective slab cache. The constructor initializes and brings the object |
213 | respective slab cache. The constructor initializes and brings the object |
190 | into a known state. The object is then used by the user. When the user |
214 | into a known state. The object is then used by the user. When the user |
191 | later frees the object, the slab allocator puts it into a processor |
215 | later frees the object, the slab allocator puts it into a processor |
- | 216 | private <indexterm> |
|
- | 217 | <primary>slab allocator</primary> |
|
- | 218 | ||
- | 219 | <secondary>magazine</secondary> |
|
192 | private magazine cache, from where it can be precedently allocated |
220 | </indexterm>magazine cache, from where it can be precedently allocated |
193 | again. Note that allocations satisfied from a magazine are already |
221 | again. Note that allocations satisfied from a magazine are already |
194 | initialized by the constructor. When both of the processor cached |
222 | initialized by the constructor. When both of the processor cached |
195 | magazines get full, the allocator will move one of the magazines to the |
223 | magazines get full, the allocator will move one of the magazines to the |
196 | list of full magazines. Similarily, when allocating from an empty |
224 | list of full magazines. Similarily, when allocating from an empty |
197 | processor magazine cache, the kernel will reload only one magazine from |
225 | processor magazine cache, the kernel will reload only one magazine from |
198 | the list of full magazines. In other words, the slab allocator tries to |
226 | the list of full magazines. In other words, the slab allocator tries to |
199 | keep the processor magazine cache only half-full in order to prevent |
227 | keep the processor magazine cache only half-full in order to prevent |
200 | thrashing when allocations and deallocations interleave on magazine |
228 | thrashing when allocations and deallocations interleave on magazine |
201 | boundaries. The advantage of this setup is that during most of the |
229 | boundaries. The advantage of this setup is that during most of the |
202 | allocations, no global spinlock needs to be held.</para> |
230 | allocations, no global spinlock needs to be held.</para> |
203 | 231 | ||
204 | <para>Should HelenOS run short of memory, it would start deallocating |
232 | <para>Should HelenOS run short of memory, it would start deallocating |
205 | objects from magazines, calling slab cache destructor on them and |
233 | objects from magazines, calling slab cache destructor on them and |
206 | putting them back into slabs. When a slab contanins no allocated object, |
234 | putting them back into slabs. When a slab contanins no allocated object, |
207 | it is immediately freed.</para> |
235 | it is immediately freed.</para> |
208 | 236 | ||
- | 237 | <para> |
|
209 | <para><figure> |
238 | <figure> |
210 | <mediaobject id="slab_alloc"> |
239 | <mediaobject id="slab_alloc"> |
211 | <imageobject role="html"> |
240 | <imageobject role="html"> |
212 | <imagedata fileref="images/slab_alloc.png" format="PNG" /> |
241 | <imagedata fileref="images/slab_alloc.png" format="PNG" /> |
213 | </imageobject> |
242 | </imageobject> |
214 | </mediaobject> |
243 | </mediaobject> |
215 | 244 | ||
216 | <title>Slab allocator scheme.</title> |
245 | <title>Slab allocator scheme.</title> |
217 | </figure></para> |
246 | </figure> |
- | 247 | </para> |
|
218 | 248 | ||
219 | <section> |
249 | <section> |
220 | <title>Implementation</title> |
250 | <title>Implementation</title> |
221 | 251 | ||
222 | <para>The slab allocator is closely modelled after OpenSolaris slab |
252 | <para>The slab allocator is closely modelled after OpenSolaris slab |
223 | allocator by Jeff Bonwick and Jonathan Adams with the following |
253 | allocator by Jeff Bonwick and Jonathan Adams <xref |
224 | exceptions:<itemizedlist> |
254 | linkend="Bonwick01" /> with the following exceptions:<itemizedlist> |
225 | <listitem> |
- | |
226 | empty slabs are immediately deallocated and |
255 | <listitem>empty slabs are immediately deallocated and</listitem> |
227 | </listitem> |
- | |
228 | 256 | ||
229 | <listitem> |
257 | <listitem> |
230 | <para>empty magazines are deallocated when not needed.</para> |
258 | <para>empty magazines are deallocated when not needed.</para> |
231 | </listitem> |
259 | </listitem> |
232 | </itemizedlist>The following features are not currently supported |
260 | </itemizedlist>The following features are not currently supported |
233 | but would be easy to do: <itemizedlist> |
261 | but would be easy to do: <itemizedlist> |
234 | <listitem> |
- | |
235 | cache coloring and |
262 | <listitem>cache coloring and</listitem> |
236 | </listitem> |
- | |
237 | 263 | ||
238 | <listitem> |
264 | <listitem>dynamic magazine grow (different magazine sizes are |
239 | dynamic magazine grow (different magazine sizes are already supported, but the allocation strategy would need to be adjusted). |
265 | already supported, but the allocation strategy would need to be |
240 | </listitem> |
266 | adjusted).</listitem> |
241 | </itemizedlist></para> |
267 | </itemizedlist></para> |
242 | 268 | ||
243 | <section> |
269 | <section> |
244 | <title>Allocation/deallocation</title> |
270 | <title>Allocation/deallocation</title> |
245 | 271 | ||
246 | <para>The following two paragraphs summarize and complete the |
272 | <para>The following two paragraphs summarize and complete the |
247 | description of the slab allocator operation (i.e. |
273 | description of the slab allocator operation (i.e. |
248 | <code>slab_alloc</code> and <code>slab_free</code> |
274 | <code>slab_alloc</code> and <code>slab_free</code> |
249 | operations).</para> |
275 | operations).</para> |
250 | 276 | ||
251 | <formalpara> |
277 | <formalpara> |
252 | <title>Allocation</title> |
278 | <title>Allocation</title> |
253 | 279 | ||
254 | <para><emphasis>Step 1.</emphasis> When an allocation request |
280 | <para><emphasis>Step 1.</emphasis> When an allocation request |
255 | comes, the slab allocator checks availability of memory in the |
281 | comes, the slab allocator checks availability of memory in the |
256 | current magazine of the local processor magazine cache. If the |
282 | current magazine of the local processor magazine cache. If the |
257 | available memory is there, the allocator just pops the magazine |
283 | available memory is there, the allocator just pops the magazine |
258 | and returns pointer to the object.</para> |
284 | and returns pointer to the object.</para> |
259 | 285 | ||
260 | <para><emphasis>Step 2.</emphasis> If the current magazine in the |
286 | <para><emphasis>Step 2.</emphasis> If the current magazine in the |
261 | processor magazine cache is empty, the allocator will attempt to |
287 | processor magazine cache is empty, the allocator will attempt to |
262 | swap it with the last magazine from the cache and return to the |
288 | swap it with the last magazine from the cache and return to the |
263 | first step. If also the last magazine is empty, the algorithm will |
289 | first step. If also the last magazine is empty, the algorithm will |
264 | fall through to Step 3.</para> |
290 | fall through to Step 3.</para> |
265 | 291 | ||
266 | <para><emphasis>Step 3.</emphasis> Now the allocator is in the |
292 | <para><emphasis>Step 3.</emphasis> Now the allocator is in the |
267 | situation when both magazines in the processor magazine cache are |
293 | situation when both magazines in the processor magazine cache are |
268 | empty. The allocator reloads one magazine from the shared list of |
294 | empty. The allocator reloads one magazine from the shared list of |
269 | full magazines. If the reload is successful (i.e. there are full |
295 | full magazines. If the reload is successful (i.e. there are full |
270 | magazines in the list), the algorithm continues with Step |
296 | magazines in the list), the algorithm continues with Step |
271 | 1.</para> |
297 | 1.</para> |
272 | 298 | ||
273 | <para><emphasis>Step 4.</emphasis> In this fail-safe step, an |
299 | <para><emphasis>Step 4.</emphasis> In this fail-safe step, an |
274 | object is allocated from the conventional slab layer and a pointer |
300 | object is allocated from the conventional slab layer and a pointer |
275 | to it is returned. If also the last magazine is full,</para> |
301 | to it is returned. If also the last magazine is full,</para> |
276 | </formalpara> |
302 | </formalpara> |
277 | 303 | ||
278 | <formalpara> |
304 | <formalpara> |
279 | <title>Deallocation</title> |
305 | <title>Deallocation</title> |
280 | 306 | ||
281 | <para><emphasis>Step 1.</emphasis> During a deallocation request, |
307 | <para><emphasis>Step 1.</emphasis> During a deallocation request, |
282 | the slab allocator checks if the current magazine of the local |
308 | the slab allocator checks if the current magazine of the local |
283 | processor magazine cache is not full. If yes, the pointer to the |
309 | processor magazine cache is not full. If yes, the pointer to the |
284 | objects is just pushed into the magazine and the algorithm |
310 | objects is just pushed into the magazine and the algorithm |
285 | returns.</para> |
311 | returns.</para> |
286 | 312 | ||
287 | <para><emphasis>Step 2.</emphasis> If the current magazine is |
313 | <para><emphasis>Step 2.</emphasis> If the current magazine is |
288 | full, the allocator will attempt to swap it with the last magazine |
314 | full, the allocator will attempt to swap it with the last magazine |
289 | from the cache and return to the first step. If also the last |
315 | from the cache and return to the first step. If also the last |
290 | magazine is empty, the algorithm will fall through to Step |
316 | magazine is empty, the algorithm will fall through to Step |
291 | 3.</para> |
317 | 3.</para> |
292 | 318 | ||
293 | <para><emphasis>Step 3.</emphasis> Now the allocator is in the |
319 | <para><emphasis>Step 3.</emphasis> Now the allocator is in the |
294 | situation when both magazines in the processor magazine cache are |
320 | situation when both magazines in the processor magazine cache are |
295 | full. The allocator moves one magazine to the shared list of full |
321 | full. The allocator moves one magazine to the shared list of full |
296 | magazines. The algoritm continues with Step 1.</para> |
322 | magazines. The algoritm continues with Step 1.</para> |
297 | </formalpara> |
323 | </formalpara> |
298 | </section> |
324 | </section> |
299 | </section> |
325 | </section> |
300 | </section> |
326 | </section> |
301 | </section> |
327 | </section> |
302 | 328 | ||
303 | <section> |
329 | <section> |
304 | <title>Virtual memory management</title> |
330 | <title>Virtual memory management</title> |
305 | 331 | ||
306 | <section> |
332 | <section> |
307 | <title>Introduction</title> |
333 | <title>Introduction</title> |
308 | 334 | ||
309 | <para>Virtual memory is a special memory management technique, used by |
335 | <para>Virtual memory is a special memory management technique, used by |
310 | kernel to achieve a bunch of mission critical goals. <itemizedlist> |
336 | kernel to achieve a bunch of mission critical goals. <itemizedlist> |
311 | <listitem> |
337 | <listitem> |
312 | Isolate each task from other tasks that are running on the system at the same time. |
338 | Isolate each task from other tasks that are running on the system at the same time. |
313 | </listitem> |
339 | </listitem> |
314 | 340 | ||
315 | <listitem> |
341 | <listitem> |
316 | Allow to allocate more memory, than is actual physical memory size of the machine. |
342 | Allow to allocate more memory, than is actual physical memory size of the machine. |
317 | </listitem> |
343 | </listitem> |
318 | 344 | ||
319 | <listitem> |
345 | <listitem> |
320 | Allowing, in general, to load and execute two programs that are linked on the same address without complicated relocations. |
346 | Allowing, in general, to load and execute two programs that are linked on the same address without complicated relocations. |
321 | </listitem> |
347 | </listitem> |
322 | </itemizedlist></para> |
348 | </itemizedlist></para> |
323 | 349 | ||
324 | <para><!-- |
350 | <para><!-- |
325 | - | ||
326 | TLB shootdown ASID/ASID:PAGE/ALL. |
- | |
327 | TLB shootdown requests can come in asynchroniously |
- | |
328 | so there is a cache of TLB shootdown requests. Upon cache overflow TLB shootdown ALL is executed |
- | |
329 | - | ||
330 | - | ||
331 | <para> |
351 | <para> |
332 | Address spaces. Address space area (B+ tree). Only for uspace. Set of syscalls (shrink/extend etc). |
352 | Address spaces. Address space area (B+ tree). Only for uspace. Set of syscalls (shrink/extend etc). |
333 | Special address space area type - device - prohibits shrink/extend syscalls to call on it. |
353 | Special address space area type - device - prohibits shrink/extend syscalls to call on it. |
334 | Address space has link to mapping tables (hierarchical - per Address space, hash - global tables). |
354 | Address space has link to mapping tables (hierarchical - per Address space, hash - global tables). |
335 | </para> |
355 | </para> |
336 | 356 | ||
337 | --></para> |
357 | --></para> |
338 | </section> |
358 | </section> |
339 | 359 | ||
340 | <section> |
360 | <section> |
341 | <title>Paging</title> |
- | |
342 | - | ||
343 | <para>Virtual memory is usually using paged memory model, where virtual |
- | |
344 | memory address space is divided into the <emphasis>pages</emphasis> |
- | |
345 | (usually having size 4096 bytes) and physical memory is divided into the |
- | |
346 | frames (same sized as a page, of course). Each page may be mapped to |
- | |
347 | some frame and then, upon memory access to the virtual address, CPU |
- | |
348 | performs <emphasis>address translation</emphasis> during the instruction |
- | |
349 | execution. Non-existing mapping generates page fault exception, calling |
- | |
350 | kernel exception handler, thus allowing kernel to manipulate rules of |
- | |
351 | memory access. Information for pages mapping is stored by kernel in the |
- | |
352 | <link linkend="page_tables">page tables</link></para> |
- | |
353 | - | ||
354 | <para>The majority of the architectures use multi-level page tables, |
- | |
355 | which means need to access physical memory several times before getting |
- | |
356 | physical address. This fact would make serios performance overhead in |
- | |
357 | virtual memory management. To avoid this <link linkend="tlb">Traslation |
- | |
358 | Lookaside Buffer (TLB)</link> is used.</para> |
- | |
359 | </section> |
- | |
360 | - | ||
361 | <section> |
- | |
362 | <title>Address spaces</title> |
361 | <title>Address spaces</title> |
363 | 362 | ||
364 | <section> |
363 | <section> |
- | 364 | <indexterm> |
|
- | 365 | <primary>address space</primary> |
|
- | 366 | ||
- | 367 | <secondary>area</secondary> |
|
- | 368 | </indexterm> |
|
- | 369 | ||
365 | <title>Address space areas</title> |
370 | <title>Address space areas</title> |
366 | 371 | ||
367 | <para>Each address space consists of mutually disjunctive continuous |
372 | <para>Each address space consists of mutually disjunctive continuous |
368 | address space areas. Address space area is precisely defined by its |
373 | address space areas. Address space area is precisely defined by its |
369 | base address and the number of frames/pages is contains.</para> |
374 | base address and the number of frames/pages is contains.</para> |
370 | 375 | ||
371 | <para>Address space area , that define behaviour and permissions on |
376 | <para>Address space area , that define behaviour and permissions on |
372 | the particular area. <itemizedlist> |
377 | the particular area. <itemizedlist> |
373 | <listitem> |
- | |
374 | - | ||
375 | - | ||
376 | <emphasis>AS_AREA_READ</emphasis> |
378 | <listitem><emphasis>AS_AREA_READ</emphasis> flag indicates reading |
377 | - | ||
378 | flag indicates reading permission. |
- | |
379 | </listitem> |
- | |
380 | - | ||
381 | <listitem> |
- | |
382 | - | ||
383 | - | ||
384 | <emphasis>AS_AREA_WRITE</emphasis> |
- | |
385 | - | ||
386 | flag indicates writing permission. |
- | |
387 | </listitem> |
379 | permission.</listitem> |
388 | - | ||
389 | <listitem> |
- | |
390 | - | ||
391 | 380 | ||
392 | <emphasis>AS_AREA_EXEC</emphasis> |
381 | <listitem><emphasis>AS_AREA_WRITE</emphasis> flag indicates |
- | 382 | writing permission.</listitem> |
|
393 | 383 | ||
- | 384 | <listitem><emphasis>AS_AREA_EXEC</emphasis> flag indicates code |
|
394 | flag indicates code execution permission. Some architectures do not support execution persmission restriction. In this case this flag has no effect. |
385 | execution permission. Some architectures do not support execution |
395 | </listitem> |
386 | persmission restriction. In this case this flag has no |
396 | - | ||
397 | <listitem> |
387 | effect.</listitem> |
398 | - | ||
399 | 388 | ||
400 | <emphasis>AS_AREA_DEVICE</emphasis> |
389 | <listitem><emphasis>AS_AREA_DEVICE</emphasis> marks area as mapped |
401 | - | ||
402 | marks area as mapped to the device memory. |
- | |
403 | </listitem> |
390 | to the device memory.</listitem> |
404 | </itemizedlist></para> |
391 | </itemizedlist></para> |
405 | 392 | ||
406 | <para>Kernel provides possibility tasks create/expand/shrink/share its |
393 | <para>Kernel provides possibility tasks create/expand/shrink/share its |
407 | address space via the set of syscalls.</para> |
394 | address space via the set of syscalls.</para> |
408 | </section> |
395 | </section> |
409 | 396 | ||
410 | <section> |
397 | <section> |
- | 398 | <indexterm> |
|
- | 399 | <primary>address space</primary> |
|
- | 400 | ||
- | 401 | <secondary>ASID</secondary> |
|
- | 402 | </indexterm> |
|
- | 403 | ||
411 | <title>Address Space ID (ASID)</title> |
404 | <title>Address Space ID (ASID)</title> |
412 | 405 | ||
413 | <para>When switching to the different task, kernel also require to |
406 | <para>When switching to the different task, kernel also require to |
414 | switch mappings to the different address space. In case TLB cannot |
407 | switch mappings to the different address space. In case TLB cannot |
415 | distinguish address space mappings, all mapping information in TLB |
408 | distinguish address space mappings, all mapping information in TLB |
416 | from the old address space must be flushed, which can create certain |
409 | from the old address space must be flushed, which can create certain |
417 | uncessary overhead during the task switching. To avoid this, some |
410 | uncessary overhead during the task switching. To avoid this, some |
418 | architectures have capability to segregate different address spaces on |
411 | architectures have capability to segregate different address spaces on |
419 | hardware level introducing the address space identifier as a part of |
412 | hardware level introducing the address space identifier as a part of |
420 | TLB record, telling the virtual address space translation unit to |
413 | TLB record, telling the virtual address space translation unit to |
421 | which address space this record is applicable.</para> |
414 | which address space this record is applicable.</para> |
422 | 415 | ||
423 | <para>HelenOS kernel can take advantage of this hardware supported |
416 | <para>HelenOS kernel can take advantage of this hardware supported |
424 | identifier by having an ASID abstraction which is somehow related to |
417 | identifier by having an ASID abstraction which is somehow related to |
425 | the corresponding architecture identifier. I.e. on ia64 kernel ASID is |
418 | the corresponding architecture identifier. I.e. on ia64 kernel ASID is |
426 | derived from RID (region identifier) and on the mips32 kernel ASID is |
419 | derived from RID (region identifier) and on the mips32 kernel ASID is |
427 | actually the hardware identifier. As expected, this ASID information |
420 | actually the hardware identifier. As expected, this ASID information |
428 | record is the part of <emphasis>as_t</emphasis> structure.</para> |
421 | record is the part of <emphasis>as_t</emphasis> structure.</para> |
429 | 422 | ||
430 | <para>Due to the hardware limitations, hardware ASID has limited |
423 | <para>Due to the hardware limitations, hardware ASID has limited |
431 | length from 8 bits on ia64 to 24 bits on mips32, which makes it |
424 | length from 8 bits on ia64 to 24 bits on mips32, which makes it |
432 | impossible to use it as unique address space identifier for all tasks |
425 | impossible to use it as unique address space identifier for all tasks |
433 | running in the system. In such situations special ASID stealing |
426 | running in the system. In such situations special ASID stealing |
434 | algoritm is used, which takes ASID from inactive task and assigns it |
427 | algoritm is used, which takes ASID from inactive task and assigns it |
435 | to the active task.</para> |
428 | to the active task.</para> |
436 | 429 | ||
- | 430 | <indexterm> |
|
- | 431 | <primary>address space</primary> |
|
- | 432 | ||
- | 433 | <secondary>ASID stealing</secondary> |
|
- | 434 | </indexterm> |
|
- | 435 | ||
- | 436 | <para> |
|
437 | <para><classname>ASID stealing algoritm here.</classname></para> |
437 | <classname>ASID stealing algoritm here.</classname> |
- | 438 | </para> |
|
438 | </section> |
439 | </section> |
439 | </section> |
440 | </section> |
440 | 441 | ||
441 | <section> |
442 | <section id="paging"> |
442 | <title>Virtual address translation</title> |
443 | <title>Virtual address translation</title> |
443 | 444 | ||
444 | <section id="page_tables"> |
445 | <section> |
445 | <title>Page tables</title> |
446 | <title>Introduction</title> |
- | 447 | ||
- | 448 | <para>Virtual memory is usually using paged memory model, where |
|
- | 449 | virtual memory address space is divided into the |
|
- | 450 | <emphasis>pages</emphasis> (usually having size 4096 bytes) and |
|
- | 451 | physical memory is divided into the frames (same sized as a page, of |
|
- | 452 | course). Each page may be mapped to some frame and then, upon memory |
|
- | 453 | access to the virtual address, CPU performs <emphasis>address |
|
- | 454 | translation</emphasis> during the instruction execution. Non-existing |
|
- | 455 | mapping generates page fault exception, calling kernel exception |
|
- | 456 | handler, thus allowing kernel to manipulate rules of memory access. |
|
- | 457 | Information for pages mapping is stored by kernel in the <link |
|
- | 458 | linkend="page_tables">page tables</link></para> |
|
- | 459 | ||
- | 460 | <indexterm> |
|
- | 461 | <primary>page tables</primary> |
|
- | 462 | </indexterm> |
|
- | 463 | ||
- | 464 | <para>The majority of the architectures use multi-level page tables, |
|
- | 465 | which means need to access physical memory several times before |
|
- | 466 | getting physical address. This fact would make serios performance |
|
- | 467 | overhead in virtual memory management. To avoid this <link |
|
- | 468 | linkend="tlb">Traslation Lookaside Buffer (TLB)</link> is used.</para> |
|
446 | 469 | ||
447 | <para>HelenOS kernel has two different approaches to the paging |
470 | <para>HelenOS kernel has two different approaches to the paging |
448 | implementation: <emphasis>4 level page tables</emphasis> and |
471 | implementation: <emphasis>4 level page tables</emphasis> and |
449 | <emphasis>global hash tables</emphasis>, which are accessible via |
472 | <emphasis>global hash table</emphasis>, which are accessible via |
450 | generic paging abstraction layer. Such different functionality was |
473 | generic paging abstraction layer. Such different functionality was |
451 | caused by the major architectural differences between supported |
474 | caused by the major architectural differences between supported |
452 | platforms. This abstraction is implemented with help of the global |
475 | platforms. This abstraction is implemented with help of the global |
453 | structure of pointers to basic mapping functions |
476 | structure of pointers to basic mapping functions |
454 | <emphasis>page_mapping_operations</emphasis>. To achieve different |
477 | <emphasis>page_mapping_operations</emphasis>. To achieve different |
455 | functionality of page tables, corresponding layer must implement |
478 | functionality of page tables, corresponding layer must implement |
456 | functions, declared in |
479 | functions, declared in |
457 | <emphasis>page_mapping_operations</emphasis></para> |
480 | <emphasis>page_mapping_operations</emphasis></para> |
458 | 481 | ||
- | 482 | <para>Thanks to the abstract paging interface, there was a place left |
|
- | 483 | for more paging implementations (besides already implemented |
|
- | 484 | hieararchical page tables and hash table), for example B-Tree based |
|
459 | <formalpara> |
485 | page tables.</para> |
460 | <title>4-level page tables</title> |
486 | </section> |
461 | 487 | ||
462 | <para>4-level page tables are the generalization of the hardware |
- | |
463 | capabilities of several architectures.<itemizedlist> |
- | |
464 | <listitem> |
- | |
465 | ia32 uses 2-level page tables, with full hardware support. |
- | |
466 | </listitem> |
488 | <section id="page_tables"> |
467 | - | ||
468 | <listitem> |
- | |
469 | amd64 uses 4-level page tables, also coming with full hardware support. |
- | |
470 | </listitem> |
- | |
471 | - | ||
472 | <listitem> |
489 | <indexterm> |
473 | mips and ppc32 have 2-level tables, software simulated support. |
- | |
474 | </listitem> |
- | |
475 | </itemizedlist></para> |
490 | <primary>page tables</primary> |
476 | </formalpara> |
- | |
477 | 491 | ||
478 | <formalpara> |
492 | <secondary>hierarchical</secondary> |
479 | <title>Global hash tables</title> |
493 | </indexterm> |
480 | 494 | ||
- | 495 | <title>Hierarchical 4-level page tables</title> |
|
- | 496 | ||
481 | <para>- global page hash table: existuje jen jedna v celem systemu |
497 | <para>Hierarchical 4-level page tables are the generalization of the |
482 | (vyuziva ji ia64), pozn. ia64 ma zatim vypnuty VHPT. Pouziva se |
498 | hardware capabilities of most architectures. Each address space has |
483 | genericke hash table s oddelenymi collision chains. ASID support is |
499 | its own page tables.<itemizedlist> |
484 | required to use global hash tables.</para> |
500 | <listitem>ia32 uses 2-level page tables, with full hardware |
485 | </formalpara> |
501 | support.</listitem> |
- | 502 | ||
- | 503 | <listitem>amd64 uses 4-level page tables, also coming with full |
|
- | 504 | hardware support.</listitem> |
|
486 | 505 | ||
487 | <para>Thanks to the abstract paging interface, there is possibility |
506 | <listitem>mips and ppc32 have 2-level tables, software simulated |
488 | left have more paging implementations, for example B-Tree page |
507 | support.</listitem> |
489 | tables.</para> |
508 | </itemizedlist></para> |
490 | </section> |
509 | </section> |
491 | 510 | ||
492 | <section id="tlb"> |
511 | <section> |
- | 512 | <indexterm> |
|
493 | <title>Translation Lookaside buffer</title> |
513 | <primary>page tables</primary> |
494 | 514 | ||
495 | <para>Due to the extensive overhead during the page mapping lookup in |
- | |
496 | the page tables, all architectures has fast assotiative cache memory |
- | |
497 | built-in CPU. This memory called TLB stores recently used page table |
515 | <secondary>hashing</secondary> |
498 | entries.</para> |
516 | </indexterm> |
499 | 517 | ||
500 | <section id="tlb_shootdown"> |
518 | <title>Global hash table</title> |
501 | <title>TLB consistency. TLB shootdown algorithm.</title> |
- | |
502 | 519 | ||
503 | <para>Operating system is responsible for keeping TLB consistent by |
520 | <para>Implementation of the global hash table was encouraged by the |
504 | invalidating the contents of TLB, whenever there is some change in |
521 | ia64 architecture support. One of the major differences between global |
505 | page tables. Those changes may occur when page or group of pages |
522 | hash table and hierarchical tables is that global hash table exists |
506 | were unmapped, mapping is changed or system switching active address |
- | |
507 | space to schedule a new system task (which is a batch unmap of all |
- | |
508 | address space mappings). Moreover, this invalidation operation must |
- | |
509 | be done an all system CPUs because each CPU has its own independent |
523 | only once in the system and the hierarchical tables are maintained per |
510 | TLB cache. Thus maintaining TLB consistency on SMP configuration as |
- | |
511 | not as trivial task as it looks at the first glance. Naive solution |
- | |
512 | would assume remote TLB invalidatation, which is not possible on the |
- | |
513 | most of the architectures, because of the simple fact - flushing TLB |
- | |
514 | is allowed only on the local CPU and there is no possibility to |
- | |
515 | access other CPUs' TLB caches.</para> |
524 | address space.</para> |
516 | 525 | ||
517 | <para>Technique of remote invalidation of TLB entries is called "TLB |
526 | <para>Thus, hash table contains information about all address spaces |
518 | shootdown". HelenOS uses a variation of the algorithm described by |
527 | mappings in the system, so, the hash of an entry must contain |
519 | D. Black et al., "Translation Lookaside Buffer Consistency: A |
- | |
520 | Software Approach," Proc. Third Int'l Conf. Architectural Support |
- | |
521 | for Programming Languages and Operating Systems, 1989, pp. |
528 | information of both address space pointer or id and the virtual |
522 | 113-122.</para> |
- | |
523 | - | ||
524 | <para>As the situation demands, you will want partitial invalidation |
- | |
525 | of TLB caches. In case of simple memory mapping change it is |
529 | address of the page. Generic hash table implementation assumes that |
526 | necessary to invalidate only one or more adjacent pages. In case if |
530 | the addresses of the pointers to the address spaces are likely to be |
527 | the architecture is aware of ASIDs, during the address space |
531 | on the close addresses, so it uses least significant bits for hash; |
528 | switching, kernel invalidates only entries from this particular |
532 | also it assumes that the virtual page addresses have roughly the same |
529 | address space. Final option of the TLB invalidation is the complete |
533 | probability of occurring, so the least significant bits of VPN compose |
530 | TLB cache invalidation, which is the operation that flushes all |
- | |
531 | entries in TLB.</para> |
534 | the hash index.</para> |
532 | 535 | ||
533 | <para>TLB shootdown is performed in two phases. First, the initiator |
536 | <para>- global page hash table: existuje jen jedna v celem systemu |
534 | process sends an IPI message indicating the TLB shootdown request to |
537 | (vyuziva ji ia64), pozn. ia64 ma zatim vypnuty VHPT. Pouziva se |
535 | the rest of the CPUs. Then, it waits until all CPUs confirm TLB |
538 | genericke hash table s oddelenymi collision chains. ASID support is |
536 | invalidating action execution.</para> |
539 | required to use global hash tables.</para> |
537 | </section> |
- | |
538 | </section> |
540 | </section> |
539 | </section> |
541 | </section> |
540 | 542 | ||
541 | <section> |
543 | <section id="tlb"> |
- | 544 | <indexterm> |
|
- | 545 | <primary>TLB</primary> |
|
542 | <title>---</title> |
546 | </indexterm> |
- | 547 | ||
- | 548 | <title>Translation Lookaside buffer</title> |
|
- | 549 | ||
- | 550 | <para>Due to the extensive overhead during the page mapping lookup in |
|
- | 551 | the page tables, all architectures has fast assotiative cache memory |
|
- | 552 | built-in CPU. This memory called TLB stores recently used page table |
|
- | 553 | entries.</para> |
|
- | 554 | ||
- | 555 | <section id="tlb_shootdown"> |
|
- | 556 | <indexterm> |
|
- | 557 | <primary>TLB</primary> |
|
- | 558 | ||
- | 559 | <secondary>TLB shootdown</secondary> |
|
- | 560 | </indexterm> |
|
- | 561 | ||
- | 562 | <title>TLB consistency. TLB shootdown algorithm.</title> |
|
- | 563 | ||
- | 564 | <para>Operating system is responsible for keeping TLB consistent by |
|
- | 565 | invalidating the contents of TLB, whenever there is some change in |
|
- | 566 | page tables. Those changes may occur when page or group of pages were |
|
- | 567 | unmapped, mapping is changed or system switching active address space |
|
- | 568 | to schedule a new system task. Moreover, this invalidation operation |
|
- | 569 | must be done an all system CPUs because each CPU has its own |
|
- | 570 | independent TLB cache. Thus maintaining TLB consistency on SMP |
|
- | 571 | configuration as not as trivial task as it looks on the first glance. |
|
- | 572 | Naive solution would assume that is the CPU which wants to invalidate |
|
- | 573 | TLB will invalidate TLB caches on other CPUs. It is not possible on |
|
- | 574 | the most of the architectures, because of the simple fact - flushing |
|
- | 575 | TLB is allowed only on the local CPU and there is no possibility to |
|
- | 576 | access other CPUs' TLB caches, thus invalidate TLB remotely.</para> |
|
- | 577 | ||
- | 578 | <para>Technique of remote invalidation of TLB entries is called "TLB |
|
- | 579 | shootdown". HelenOS uses a variation of the algorithm described by D. |
|
- | 580 | Black et al., "Translation Lookaside Buffer Consistency: A Software |
|
- | 581 | Approach," Proc. Third Int'l Conf. Architectural Support for |
|
- | 582 | Programming Languages and Operating Systems, 1989, pp. 113-122. <xref |
|
- | 583 | linkend="Black89" /></para> |
|
- | 584 | ||
- | 585 | <para>As the situation demands, you will want partitial invalidation |
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- | 586 | of TLB caches. In case of simple memory mapping change it is necessary |
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- | 587 | to invalidate only one or more adjacent pages. In case if the |
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- | 588 | architecture is aware of ASIDs, when kernel needs to dump some ASID to |
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- | 589 | use by another task, it invalidates only entries from this particular |
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- | 590 | address space. Final option of the TLB invalidation is the complete |
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- | 591 | TLB cache invalidation, which is the operation that flushes all |
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- | 592 | entries in TLB.</para> |
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543 | 593 | ||
544 | <para>At the moment HelenOS does not support swapping.</para> |
594 | <para>TLB shootdown is performed in two phases.</para> |
545 | 595 | ||
- | 596 | <formalpara> |
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- | 597 | <title>Phase 1.</title> |
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- | 598 | ||
546 | <para>- pouzivame vypadky stranky k alokaci ramcu on-demand v ramci |
599 | <para>First, initiator locks a global TLB spinlock, then request is |
- | 600 | being put to the local request cache of every other CPU in the |
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- | 601 | system protected by its spinlock. In case the cache is full, all |
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547 | as_area - na architekturach, ktere to podporuji, podporujeme non-exec |
602 | requests in the cache are replaced by one request, indicating global |
- | 603 | TLB flush. Then the initiator thread sends an IPI message indicating |
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- | 604 | the TLB shootdown request to the rest of the CPUs and waits actively |
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- | 605 | until all CPUs confirm TLB invalidating action execution by setting |
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- | 606 | up a special flag. After setting this flag this thread is blocked on |
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- | 607 | the TLB spinlock, held by the initiator.</para> |
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- | 608 | </formalpara> |
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- | 609 | ||
- | 610 | <formalpara> |
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- | 611 | <title>Phase 2.</title> |
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- | 612 | ||
- | 613 | <para>All CPUs are waiting on the TLB spinlock to execute TLB |
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- | 614 | invalidation action and have indicated their intention to the |
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- | 615 | initiator. Initiator continues, cleaning up its TLB and releasing |
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- | 616 | the global TLB spinlock. After this all other CPUs gain and |
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- | 617 | immidiately release TLB spinlock and perform TLB invalidation |
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548 | stranky</para> |
618 | actions.</para> |
- | 619 | </formalpara> |
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- | 620 | </section> |
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549 | </section> |
621 | </section> |
550 | </section> |
622 | </section> |
551 | </chapter> |
623 | </chapter> |