Re: RFC: replace pg_stat_activity.waiting with something more descriptive

On Fri, Sep 18, 2015 at 4:08 AM, Vladimir Borodin <root(at)simply(dot)name> wrote:
> For both scenarios on linux we got approximately the same results - version
> with timings was faster then version with sampling (sampling was done every
> 10 ms). Vanilla PostgreSQL from REL9_4_STABLE gave ~15500 tps and version
> with timings gave ~14500 tps while version with sampling gave ~13800 tps. In
> all cases processor was 100% utilized. Comparing vanilla PostgreSQL and
> version with timings on constant workload (12000 tps) gave the following
> results in latencies for queries:

If the timing is speeding things up, that's most likely a sign that
the spinlock contention on that workload is so severe that you are
spending a lot of time in s_lock. Adding more things for the system
to do that don't require that lock will speed the system up by
reducing the contention. Instead of inserting gettimeofday() calls,
you could insert a for loop that counts to some large number without
doing any useful work, and that would likely have a similar effect.

In any case, I think your experiment clearly proves that the presence
or absence of this instrumentation *is* performance-relevant and that
we *do* need to worry about what it costs. If the system gets 20%
faster when you call gettimeofday() a lot, does that mean we should
insert gettimeofday() calls all over the system in random places to
speed it up?

I do agree that if we're going to include support for timings, having
them be controlled by a GUC is a good idea.

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