| 42,7 → 42,7 |
|---|
| architecture. To highlight some, the program counter and stack pointer |
| take part in the synchronous register context. These are the registers |
| that must be preserved across a procedure call and during synchronous |
| context switches. </para> |
| context switches.</para> |
| |
| <para>The next type of the context understood by the kernel is the |
| asynchronous register context. On an interrupt, the interrupted execution |
| 103,8 → 103,47 |
|---|
| </section> |
| |
| <section> |
| <title>Scheduler</title> |
| <title>Threads</title> |
| |
| <para>How scheduler designed and how it works.</para> |
| <para>A thread is the basic executable entity with some code and stack. |
| While the code, implemented by a C language function, can be shared by |
| several threads, the stack is always private to each instance of the |
| thread. Each thread belongs to exactly one task through which it shares |
| address space with its sibling threads. Threads that execute purely in the |
| kernel don't have any userspace memory allocated. However, when a thread |
| has ambitions to run in userspace, it must be allocated a userspace stack. |
| The distinction between the purely kernel threads and threads running also |
| in userspace is made by refering to the former group as to kernel threads |
| and to the latter group as to userspace threads. Both kernel and userspace |
| threads are visible to the scheduler and can become a subject of kernel |
| preemption and thread migration during times when preemption is |
| possible.</para> |
| |
| <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> |
| |
| <itemizedlist> |
| <listitem> |
| <para>pseudothreads schedule themselves cooperatively within the time |
| slice given to their userspace thread,</para> |
| </listitem> |
| |
| <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></para> |
| </section> |
| </chapter> |