Thread Scheduling

 Thread Scheduling

• The process scheduler schedules only the kernel threads.
• User threads are mapped to kernel threads by the thread library - The OS ( and in particular the scheduler ) is unaware of them.

Contention Scope

• Contention scope refers to the scope in which threads compete for the use of physical CPUs.
• On systems implementing many-to-one and many-to-many threads, Process Contention Scope, PCS, occurs, because competition occurs between threads that are part of the same process. ( This is the management / scheduling of multiple user threads on a single kernel thread, and is managed by the thread library. )
• System Contention Scope, SCS, involves the system scheduler scheduling kernel threads to run on one or more CPUs. Systems implementing one-to-one threads ( XP, Solaris 9, Linux ), use only SCS.
• PCS scheduling is typically done with priority, where the programmer can set and/or change the priority of threads created by his or her programs. Even time slicing is not guaranteed among threads of equal priority.

Pthread Scheduling

• The Pthread library provides for specifying scope contention:
• PTHREAD_SCOPE_PROCESS schedules threads using PCS, by scheduling user threads onto available LWPs using the many-to-many model.
• PTHREAD_SCOPE_SYSTEM schedules threads using SCS, by binding user threads to particular LWPs, effectively implementing a one-to-one model.