futex
contentions.
futex
contention occurs when multiple processes are trying to access the same region of memory. In some cases, this can result in a deadlock between the processes in contention, thereby appearing as an application hang.
futex
system call.
#! /usr/bin/env stap # This script tries to identify contended user-space locks by hooking # into the futex system call. global thread_thislock # short global thread_blocktime # global FUTEX_WAIT = 0 /*, FUTEX_WAKE = 1 */ global lock_waits # long-lived stats on (tid,lock) blockage elapsed time global process_names # long-lived pid-to-execname mapping probe syscall.futex { if (op != FUTEX_WAIT) next # don't care about WAKE event originator t = tid () process_names[pid()] = execname() thread_thislock[t] = $uaddr thread_blocktime[t] = gettimeofday_us() } probe syscall.futex.return { t = tid() ts = thread_blocktime[t] if (ts) { elapsed = gettimeofday_us() - ts lock_waits[pid(), thread_thislock[t]] <<< elapsed delete thread_blocktime[t] delete thread_thislock[t] } } probe end { foreach ([pid+, lock] in lock_waits) printf ("%s[%d] lock %p contended %d times, %d avg us\n", process_names[pid], pid, lock, @count(lock_waits[pid,lock]), @avg(lock_waits[pid,lock])) }
[...] automount[2825] lock 0x00bc7784 contended 18 times, 999931 avg us synergyc[3686] lock 0x0861e96c contended 192 times, 101991 avg us synergyc[3758] lock 0x08d98744 contended 192 times, 101990 avg us synergyc[3938] lock 0x0982a8b4 contended 192 times, 101997 avg us [...]