POSIX Synchronisation
Mutex
In the uncontended case a mutex can be acquired entirely in user-space.
Contention
When a contended mutex needs to be acquired, some implementations (Solaris and HP-UX, but not Linux) try to spin for some time in user-space on SMP systems to try to avoid using potentially costly kernel synchronisation primitives. Particularly HP-UX tries very hard and even repeatedly calls sched_yield before finally resorting to ksleep.
Condition Variable
Broadcast
A straightforward implementation of pthread_cond_broadcast (one that wakes up all waiters at once) can be quite inefficient as all waiters will usually have to contend for the mutex associated with the condition variable (HP-UX is known to be implemented that way).
If the notifying thread owns the mutex (which is not mandated by the POSIX specification) can queue the waiting threads to be woken up one by one when the mutex is unlocked (and thus avoiding contention).
Resources
- Threads (The Open Group Base Specifications Issue 6)
- The Native POSIX Thread Library for Linux
- The Native POSIX Thread Library for Linux
- kernel/futex.c (Linux 2.6)
- nptl/ (glibc)
- usr/src/lib/libc/port/threads/synch.c (Open Solaris)
- Solaris: (Undocumented) Thread Environment Variables
- Multithreading in the Solaris Operating Environment (PDF)
- POSIX Threads on HP-UX 11i (PDF)
- Threads, Monitors and Resource Contention (PDF)
