Re: spinlocks on HP-UX

On Tue, Aug 30, 2011 at 4:05 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> This suggests that (1) an unlocked test in TAS_SPIN might be a good idea
> on x86_64 after all, and (2) this test scenario may not be pushing the
> system hard enough to expose limitations of the spinlock implementation.
>
> I am now thinking that the reason we saw clear differences in spinlock
> implementations years ago, and now are not seeing them except on insane
> hardware, is mainly that we've managed to reduce contention at higher
> levels of the system.  That doesn't mean spinlocks have become
> uninteresting, just that "pgbench -S" isn't the ideal test case for
> stressing them.  I'm thinking maybe we need a test scenario that
> generates sinval traffic, for example, or forces snapshots to be taken
> more often.  Ideas anyone?

On current sources, with a workload that fits into shared_buffers,
pgbench -S hammers the spinlock protecting ProcArrayLock extremely
hard. I'm sure it's possible to come up with a test case that
hammers them harder, but using a real workload can expose issues (like
aggregate memory bandwidth) that you might not see otherwise.

I am a bit surprised by your test results, because I also tried x86_64
with an unlocked test, also on pgbench -S, and I am pretty sure I got
a regression. Maybe I'll try rerunning that. It seems possible that
the x86_64 results depend on the particular sub-architecture and/or
whether HT is in use, which would be kind of a nuisance.

Also, did you happen to measure the amount of user time vs. system
time that your test runs used? If this is on Linux, I am surprised
that you didn't get killed by the lseek() contention problem on a
machine with that many cores. I found it to be visible at 32 and
crippling at 64, so I can't even imagine what it would be like at 160.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company