tree 8fd7a9adcdc981c7785d157d852cded18d4e9034
parent fed20a70e39bb9385020bdc4e8839d95326df8e2
author Paolo Bonzini <pbonzini@redhat.com> 1487009560 +0100
committer Stefan Hajnoczi <stefanha@redhat.com> 1487677180 +0000

coroutine-lock: add limited spinning to CoMutex

Running a very small critical section on pthread_mutex_t and CoMutex
shows that pthread_mutex_t is much faster because it doesn't actually
go to sleep.  What happens is that the critical section is shorter
than the latency of entering the kernel and thus FUTEX_WAIT always
fails.  With CoMutex there is no such latency but you still want to
avoid wait and wakeup.  So introduce it artificially.

This only works with one waiters; because CoMutex is fair, it will
always have more waits and wakeups than a pthread_mutex_t.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170213181244.16297-3-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>