If there are 3 or more threads spin-waiting on the same mutex,
there will be excessive exclusive cacheline contention because
pthread_trylock() immediately tries to CAS in a new value, instead
of first checking if the lock is locked.
This diff adds a 'locked' hint flag, and we will only spin wait
without trylock()ing while set. I don't know of any other portable
way to get the same behavior as pthread_mutex_lock().
This is pretty easy to test via ttest, e.g.
./ttest1 500 3 10000 1 100
Throughput is nearly 3x as fast.
This blames to the mutex profiling changes, however, we almost never
have 3 or more threads contending in properly configured production
workloads, but still worth fixing.
b23336af96