| 60,11 → 60,13 |
|---|
| <para>Although the kernel does not do any floating point |
| arithmetics<footnote> |
| <para>Some architectures (e.g. ia64) inevitably use a fixed set of |
| floating point registers to carry out its normal operations.</para> |
| floating point registers to carry out their normal operations.</para> |
| </footnote>, it must protect FPU context of userspace threads against |
| destruction by other threads. Moreover, only a fraction of userspace |
| programs use the floating point unit. HelenOS contains a generic framework |
| for switching FPU context only when the switch is forced.</para> |
| for switching FPU context only when the switch is forced (i.e. a thread |
| uses a floating point instruction and its FPU context is not loaded in the |
| processor).</para> |
| |
| <para>The last member of the context family is the memory management |
| context. It includes memory management registers that identify address |
| 119,44 → 121,60 |
|---|
| preemption and thread migration during times when preemption is |
| possible.</para> |
| |
| <figure> |
| <title>Transitions among thread states.</title> |
| <formalpara> |
| <title>Thread states</title> |
| |
| <mediaobject id="thread_states" xreflabel=""> |
| <imageobject role="html"> |
| <imagedata fileref="images/thread_states.png" format="PNG" /> |
| </imageobject> |
| <para>In each moment, a thread exists in one of five possible thread |
| states. When the thread is created and first readied into the |
| scheduler's run queues or when a thread is migrated to a new processor, |
| it is put into the <code>Entering</code> state. After some time, the |
| scheduler picks up the thread and starts executing it. A thread being |
| currently executed on a processor is in the <code>Running</code> state. |
| From there, the thread has three possibilities. It either runs until it |
| is preemtped, in which case the state changes to <code>Ready</code>, |
| goes to the <code>Sleeping</code> state by going to sleep or enters the |
| <code>Exiting</code> state when it reaches termination.<figure> |
| <title>Transitions among thread states.</title> |
| |
| <imageobject role="fop"> |
| <imagedata fileref="images.vector/thread_states.svg" format="SVG" /> |
| </imageobject> |
| </mediaobject> |
| </figure> |
| <mediaobject id="thread_states" xreflabel=""> |
| <imageobject role="html"> |
| <imagedata fileref="images/thread_states.png" format="PNG" /> |
| </imageobject> |
| |
| <para>HelenOS userspace layer knows even smaller units of execution. Each |
| userspace thread can make use of an arbitrary number of pseudo threads. |
| These pseudo threads have their own synchronous register context, |
| userspace code and stack. They live their own life within the userspace |
| thread and the scheduler does not have any idea about them because they |
| are completely implemented by the userspace library. This implies several |
| things:</para> |
| <imageobject role="fop"> |
| <imagedata fileref="images.vector/thread_states.svg" |
| format="SVG" /> |
| </imageobject> |
| </mediaobject> |
| </figure></para> |
| </formalpara> |
| |
| <itemizedlist> |
| <listitem> |
| <para>pseudothreads schedule themselves cooperatively within the time |
| slice given to their userspace thread,</para> |
| </listitem> |
| <formalpara> |
| <title>Pseudo threads</title> |
| |
| <listitem> |
| <para>pseudothreads share FPU context of their containing thread |
| and</para> |
| </listitem> |
| <para>HelenOS userspace layer knows even smaller units of execution. |
| Each userspace thread can make use of an arbitrary number of pseudo |
| threads. These pseudo threads have their own synchronous register |
| context, userspace code and stack. They live their own life within the |
| userspace thread and the scheduler does not have any idea about them |
| because they are completely implemented by the userspace library. This |
| implies several things:<itemizedlist> |
| <listitem> |
| <para>pseudothreads schedule themselves cooperatively within the |
| time slice given to their userspace thread,</para> |
| </listitem> |
| |
| <listitem> |
| <para>all pseudothreads of one userspace thread block when one of them |
| goes to sleep.</para> |
| </listitem> |
| </itemizedlist> |
| <listitem> |
| <para>pseudothreads share FPU context of their containing thread |
| and</para> |
| </listitem> |
| |
| <listitem> |
| <para>all pseudothreads of one userspace thread block when one of |
| them goes to sleep.</para> |
| </listitem> |
| </itemizedlist></para> |
| </formalpara> |
| </section> |
| |
| <section> |