| 144,11 → 144,11 |
|---|
| <title>Time source for userspace</title> |
| |
| <para>In HelenOS, userspace tasks don't communicate with the kernel in |
| order to read system time. Instead, a mechanism that shares kernel time of |
| the day counters with userspace address spaces is deployed. On the kernel |
| side, during system initialization, HelenOS allocates a frame of physical |
| memory and stores the time of the day counters there. The counters have |
| the following structure:</para> |
| order to read the system time. Instead, a mechanism that shares kernel |
| time of the the day counters with userspace address spaces is deployed. On |
| the kernel side, during system initialization, HelenOS allocates a frame |
| of physical memory and stores the time of the day counters there. The |
| counters have the following structure:</para> |
| |
| <itemizedlist> |
| <listitem> |
| 165,36 → 165,37 |
|---|
| </itemizedlist> |
| |
| <para>One of the userspace tasks with capabilities of memory manager (e.g. |
| ns) asks the kernel to map this frame into its address space. Other, |
| non-privileged, tasks then use IPC to communicate read-only sharing of |
| this memory. Reading time in a userspace task is therefore just a matter |
| of reading memory.</para> |
| ns) asks the kernel to map this frame into its address space. Other |
| non-privileged tasks then use IPC to receive read-only share of this |
| memory. Reading time in a userspace task is therefore just a matter of |
| reading memory.</para> |
| |
| <para>There are two interesting points about this. First, the counters are |
| 32-bit even on 64-bit machines. The goal is to provide subsecond precision |
| with the possibility to span roughly 136 years. Note that a single 64-bit |
| microsecond counter could not be usually read atomically on 32-bit |
| platforms. Now the second point is that 32-bit platforms cannot atomically |
| read two 32-bit counters either. However, a generic protocol is used to |
| guarantee that sequentially read times will create a non-decreasing |
| sequence.</para> |
| platforms. Unfortunately, on 32-bit platforms it is usually impossible to |
| read atomically two 32-bit counters either. However, a generic protocol is |
| used to guarantee that sequentially read times will create a |
| non-decreasing sequence.</para> |
| |
| <para>The problematic part is updating both seconds and microseconds once |
| in a second. Seconds must be incremented and microseconds must be reset. |
| However, without any synchronization, the two kernel stores and the two |
| userspace reads can arbitrarily interleave. Furthemore, the reader has no |
| chance to detect that the counters were updated only from half. Therefore |
| three counters are used in HelenOS.</para> |
| <para>The problematic part is incrementing seconds counter and clearing |
| microseconds counter together once every second. Seconds must be |
| incremented and microseconds must be reset. However, without any |
| synchronization, the two kernel stores and the two userspace reads can |
| arbitrarily interleave. Furthemore, the reader has no chance to detect |
| that the counters were updated only paritally. Therefore three counters |
| are used in HelenOS.</para> |
| |
| <para>If seconds need to be updated, the kernel increments the first |
| second counter, issues a write memory barrier operation, updates the |
| microsecond counter, issues another write memory barrier operation and |
| increments the second second counter. When only microseconds need to be |
| increments the second second counter. When only microseconds needs to be |
| updated, no special action is taken by the kernel. On the other hand, the |
| userspace task must always read all three counters and in reversed order. |
| A read memory barrier operation must be issued between each two reads. A |
| userspace task must always read all three counters in reversed order. A |
| read memory barrier operation must be issued between each two reads. A |
| non-atomic read is detected when the two second counters differ. The |
| userspace library solves this situation by returning zero instead of the |
| value read from the microsecond counter.</para> |
| userspace library solves this situation by returning higher of them with |
| microseconds set to zero.</para> |
| </section> |
| </chapter> |