Scheduling

POSIX specifies three scheduling policies: first-in-first-out (SCHED_FIFO), round-robin (SCHED_RR), and custom (SCHED_OTHER). SCHED_FIFO is a queue-based scheduler with different queues for each priority level. SCHED_RR is like FIFO except that each thread has an execution time quota.

Both SCHED_FIFO and SCHED_RR are POSIX Realtime extensions. SCHED_OTHER is the default scheduling policy.

See "LWPs and Scheduling Classes" for information about the SCHED_OTHER policy, and about emulating some properties of the POSIX SCHED_FIFO and SCHED_RR policies.

Two scheduling scopes are available: process scope for unbound threads and system scope for bound threads. Threads with differing scope states can coexist on the same system and even in the same process. In general, the scope sets the range in which the threads scheduling policy is in effect.

Process Scope (Unbound Threads)

Unbound threads are created PTHREAD_SCOPE_PROCESS. These threads are scheduled in user space to attach and detach from available LWPs in the LWP pool. LWPs are available to threads in this process only; that is threads are scheduled on these LWPs.

In most cases, threads should be PTHREAD_SCOPE_PROCESS. This allows the threads to float among the LWPs, and this improves threads performance (and is equivalent to creating a Solaris thread in the THR_UNBOUND state). The threads library decides, with regard to other threads, which threads get serviced by the kernel.

System Scope (Bound Threads)

Bound threads are created PTHREAD_SCOPE_SYSTEM. A boundthread is permanently attached to an LWP.

Each bound thread is bound to an LWP for the lifetime of the thread. This is equivalent to creating a Solaris thread in the THR_BOUND state. You can bind a thread to give it an alternate signal stack or to use special scheduling attributes with Realtime scheduling. All scheduling is done by the operating environment.

In neither case, bound or unbound, can a thread be directly accessed by or moved to another process.