Postgresql performance profiling
kris at obsecurity.org
Sun Jun 11 20:31:58 UTC 2006
On Sun, Jun 11, 2006 at 01:45:28PM -0400, Kris Kennaway wrote:
> I set up supersmack against postgresql 8.1 from ports (default config)
> on a 12 CPU E4500. It scales and performs somewhat better than mysql
> on this machine (which is heavily limited by contention between
> threads in a process), but there are a number of obvious performance
FYI, on a dual p4 + HTT, mysql significantly outperforms pgsql (by
>55% peak performance, probably more if I was using libthr which I
cannot on this machine for technical reasons) on select-key.smack when
configured the same way (i.e. transport over IPv4 instead of local
socket, which supersmack prefers for mysql).
Contention is still a big issue here (only listing mutexes contended
more than 10% of acquisitions):
0 0 142969 0 1996 14458 .101 kern/kern_synch.c:218 (Giant)
0 0 199028 0 11649 27944 .140 kern/kern_condvar.c:208 (sellck)
0 0 400103 0 111216 91336 .228 kern/kern_sysctl.c:1317 (Giant)
0 0 303147 0 108735 131237 .432 i386/i386/trap.c:1005 (Giant)
I turned off process title setting and got an 8% performance boost.
Contention is now a bit better but still serious:
0 0 22952 0 2067 2521 .109 vm/vm_fault.c:987 (vm object)
0 0 199153 0 12589 31512 .158 kern/kern_condvar.c:208 (sellck)
0 0 361305 0 124766 130901 .362 i386/i386/trap.c:1005 (Giant)
i.e. semop() (the Giant-locked syscall) is contending with itself a
lot, and select() is a secondary problem.
Actually rwatson noticed that semop() is marked MPSAFE, so it's not
clear (but nevertheless true) why Giant is acquired here. OK, pjd
worked out that it's because SYSCALL_MODULE_HELPER() *never* sets the
mpsafe flag, so all such syscalls registered that way (i.e. those
which are part of subsystems that may be loaded from kld) are
Giant-locked regardless of what syscalls.master says.
I removed the SYSCALL_MODULE_HELPERs from sysv_sem.c but now
postgresql hangs when trying to start; possibly the locking in
sysv_sem.c is just broken since it was never in fact tested.
> * The postgres processes seem to change their proctitle hundreds or
> thousands of times per second. This is currently done via a
> Giant-locked sysctl (kern.proc.args) so there is enormous contention
> for Giant. Even when this is fixed (thanks to a patch from csjp@),
> each of them requires a syscall and syscalls ain't free. This is not
> a clever thing to be doing from a performance standpoint.
> * pgsql uses select() and this seems to be a major choke point. I bet
> you'd see fairly impressive performance gains (especially on SMP) if
> it was modified to use kqueue instead of select.
> * You really want to avoid using IPv6 for transport (since it's
> Giant-locked). This was an issue at first since I was running against
> localhost, which maps to ::1 by default. We should reconsider the
> preference for IPv6 over IPv4 until IPv6 is Giant-free - there are
> probably many other situations where IPv6 is being secretly used
> "because it is there" and costing performance.
> * The sysv IPC code is still giant-locked. pgsql makes a lot of
> semop() calls which grab Giant, and it also msleep()s on the Giant
> lock in the semwait channel.
> * When semop() wants to wake up some sleeping processes because
> semaphores have been released, it does a wakeup() and wakes them all
> up. This means a thundering herd (I see up to 11 CPUs being woken
> here). Since we know exactly how many resources are available, it
> would be better to only wakeup_one() that number of times instead.
> Here are what seem to be the relevant heavily-contended mutex
> acquisitions (ratio = cnt_lock/count measures how many times this lock
> was contended by something else while held by this code line):
> count cnt_hold cnt_lock ratio name
> 106080 7420 19238 .181 kern/kern_synch.c:222 (lockbuilder mtxpool) <-- vfs
> 175435 13952 42365 .241 kern/kern_condvar.c:113 (lockbuilder mtxpool) <-- vfs
> 1075841 271138 419862 .390 kern/kern_synch.c:220 (Giant) <-- msleep with Giant
> 734613 248249 291969 .397 kern/sys_generic.c:1140 (sellck) <-- select
> 800332 379020 326324 .407 kern/sys_generic.c:944 (sellck) <-- select
> 401751 19731 175305 .436 kern/sys_generic.c:1092 (sellck) <-- select
> 400280 198880 176623 .441 kern/sys_generic.c:935 (sellck) <-- select
> 1361163 695637 624171 .458 sparc64/sparc64/trap.c:586 (Giant) <-- semop
> 400190 193112 238578 .596 kern/kern_condvar.c:208 (sellck) <-- select
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