Horton

Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Maciek Sakrejda <m.sakrejda@gmail.com>

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From: Maciek Sakrejda <m.sakrejda@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Lukas Fittl <lukas@fittl.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-01-02T20:44:42Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pg_test_timing: Also test RDTSC[P] timing, report time source, TSC frequency

  2. Allow retrieving x86 TSC frequency/flags from CPUID

  3. instrumentation: Standardize ticks to nanosecond conversion method

  4. instrumentation: Use Time-Stamp Counter on x86-64 to lower overhead

  5. Zero initialize uses of instr_time about to trigger compiler warnings

  6. instr_time: Represent time as an int64 on all platforms

  7. Add 250c8ee07ed to git-blame-ignore-revs

On Fri, Jul 15, 2022 at 11:21 AM Maciek Sakrejda <m.sakrejda@gmail.com> wrote:
> On Fri, Jul 1, 2022 at 10:26 AM Andres Freund <andres@anarazel.de> wrote:
> > On 2022-07-01 01:23:01 -0700, Lukas Fittl wrote:
> >...
> > > Known WIP problems with this patch version:
> > >
> > > * There appears to be a timing discrepancy I haven't yet worked out, where
> > >   the \timing data reported by psql doesn't match what EXPLAIN ANALYZE is
> > >   reporting. With Andres' earlier test case, I'm seeing a consistent ~700ms
> > >   higher for \timing than for the EXPLAIN ANALYZE time reported on the
> > > server
> > >   side, only when rdtsc measurement is used -- its likely there is a problem
> > >   somewhere with how we perform the cycles to time conversion
> >
> > Could you explain a bit more what you're seeing? I just tested your patches
> > and didn't see that here.
>
> I did not see this either, but I did see that the execution time
> reported by \timing is (for this test case) consistently 0.5-1ms
> *lower* than the Execution Time reported by EXPLAIN. I did not see
> that on master. Is that expected?

For what it's worth, I can no longer reproduce this. In fact, I went
back to master-as-of-around-then and applied Lukas' v2 patches again,
and I still can't reproduce that. I do remember it happening
consistently across several executions, but now \timing consistently
shows 0.5-1ms slower, as expected. This does not explain the different
timing issue Lukas was seeing in his tests, but I think we can assume
what I reported originally here is not an issue.