path: root/src/thread/sem_timedwait.c
diff options
authorRich Felker <>2014-08-25 16:38:25 -0400
committerRich Felker <>2014-08-25 16:38:25 -0400
commit2ff714c6138da8abb50fd532503fd8d68a18811a (patch)
treed962f59e257148b70dbfd60284738e004d5b0d02 /src/thread/sem_timedwait.c
parent8b3d7d0d35f97051bc994800c928a7db53316dd2 (diff)
spin in sem_[timed]wait before performing futex wait
empirically, this increases the maximum rate of wait/post operations between two threads by 20-150 times on machines I tested, including x86 and arm. conceptually, it makes sense to do some spinning because semaphores are intended to be usable as a notification mechanism between threads, not just as locks, and low-latency notification is a valuable property to have.
Diffstat (limited to 'src/thread/sem_timedwait.c')
1 files changed, 5 insertions, 0 deletions
diff --git a/src/thread/sem_timedwait.c b/src/thread/sem_timedwait.c
index bfcb6dcd..df5f3a6c 100644
--- a/src/thread/sem_timedwait.c
+++ b/src/thread/sem_timedwait.c
@@ -8,6 +8,11 @@ static void cleanup(void *p)
int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict at)
+ if (!sem_trywait(sem)) return 0;
+ int spins = 100;
+ while (spins-- && sem->__val[0] <= 0) a_spin();
while (sem_trywait(sem)) {
int r;