Re: Better LWLocks with compare-and-swap (9.4)

On Wed, May 15, 2013 at 3:08 PM, Daniel Farina <daniel(at)heroku(dot)com> wrote:
> On Mon, May 13, 2013 at 5:50 AM, Heikki Linnakangas
> <hlinnakangas(at)vmware(dot)com> wrote:
>> pgbench -S is such a workload. With 9.3beta1, I'm seeing this profile, when
>> I run "pgbench -S -c64 -j64 -T60 -M prepared" on a 32-core Linux machine:
>>
>> - 64.09% postgres postgres [.] tas
>> - tas
>> - 99.83% s_lock
>> - 53.22% LWLockAcquire
>> + 99.87% GetSnapshotData
>> - 46.78% LWLockRelease
>> GetSnapshotData
>> + GetTransactionSnapshot
>> + 2.97% postgres postgres [.] tas
>> + 1.53% postgres libc-2.13.so [.] 0x119873
>> + 1.44% postgres postgres [.] GetSnapshotData
>> + 1.29% postgres [kernel.kallsyms] [k] arch_local_irq_enable
>> + 1.18% postgres postgres [.] AllocSetAlloc
>> ...
>>
>> So, on this test, a lot of time is wasted spinning on the mutex of
>> ProcArrayLock. If you plot a graph of TPS vs. # of clients, there is a
>> surprisingly steep drop in performance once you go beyond 29 clients
>> (attached, pgbench-lwlock-cas-local-clients-sets.png, red line). My theory
>> is that after that point all the cores are busy, and processes start to be
>> sometimes context switched while holding the spinlock, which kills
>> performance.

I accidentally some important last words from Heikki's last words in
his mail, which make my correspondence pretty bizarre to understand at
the outset. Apologies. He wrote:

>> [...] Has anyone else seen that pattern?

> I have, I also used linux perf to come to this conclusion, and my
> determination was similar: a system was undergoing increasingly heavy
> load, in this case with processes >> number of processors. It was
> also a phase-change type of event: at one moment everything would be
> going great, but once a critical threshold was hit, s_lock would
> consume enormous amount of CPU time. I figured preemption while in
> the spinlock was to blame at the time, given the extreme nature.