Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

On Tue, 3 Jan 2023 at 14:08, David Geier <geidav.pg@gmail.com> wrote:
>
> Hi Lukas,
>
> On 1/2/23 20:50, Lukas Fittl wrote:
> > Thanks for continuing to work on this patch, and my apologies for
> > silence on the patch.
>
> It would be great if you could review it.
> Please also share your thoughts around exposing the used clock source as
> GUC and renaming INSTR_TIME_GET_DOUBLE() to _SECS().
>
> I rebased again on master because of [1]. Patches attached.
>
> >
> > Its been hard to make time, and especially so because I typically
> > develop on an ARM-based macOS system where I can't test this directly
> > - hence my tests with virtualized EC2 instances, where I ran into the
> > timing oddities.
> That's good and bad. Bad to do the development and good to test the
> implementation on more virtualized setups; given that I also encountered
> "interesting" behavior on VMWare (see my previous mails).
> >
> > On Mon, Jan 2, 2023 at 5:28 AM David Geier <geidav.pg@gmail.com> wrote:
> >
> >     The INSTR_TIME_GET_MICROSEC() returns a uint64 while the other
> >     variants
> >     return double. This seems error prone. What about renaming the
> >     function
> >     or also have the function return a double and cast where necessary at
> >     the call site?
> >
> >
> > Minor note, but in my understanding using a uint64 (where we can) is
> > faster for any simple arithmetic we do with the values.
>
> That's true. So the argument could be that for seconds and milliseconds
> we want the extra precision while microseconds are precise enough.
> Still, we could also make the seconds and milliseconds conversion code
> integer only and e.g. return two integers with the value before and
> after the comma. FWICS, the functions are nowhere used in performance
> critical code, so it doesn't really make a difference performance-wise.
>
> >
> > +1, and feel free to carry this patch forward - I'll try to make an
> > effort to review my earlier testing issues again, as well as your
> > later improvements to the patch.
> Moved to the current commit fest. Will you become reviewer?
> >
> > Also, FYI, I just posted an alternate idea for speeding up EXPLAIN
> > ANALYZE with timing over in [0], using a sampling-based approach to
> > reduce the timing overhead.
>
> Interesting idea. I'll reply with some thoughts on the corresponding thread.
>
> [1]
> https://www.postgresql.org/message-id/flat/CALDaNm3kRBGPhndujr9JcjjbDCG3anhj0vW8b9YtbXrBDMSvvw%40mail.gmail.com

CFBot shows some compilation errors as in [1], please post an updated
version for the same:
09:08:12.525] /usr/bin/ld:
src/bin/pg_test_timing/pg_test_timing.p/pg_test_timing.c.o: warning:
relocation against `cycles_to_sec' in read-only section `.text'
[09:08:12.525] /usr/bin/ld:
src/bin/pg_test_timing/pg_test_timing.p/pg_test_timing.c.o: in
function `pg_clock_gettime_ref_cycles':
[09:08:12.525] /tmp/cirrus-ci-build/build/../src/include/portability/instr_time.h:119:
undefined reference to `use_rdtsc'
[09:08:12.525] /usr/bin/ld:
src/bin/pg_test_timing/pg_test_timing.p/pg_test_timing.c.o: in
function `test_timing':
[09:08:12.525] /tmp/cirrus-ci-build/build/../src/bin/pg_test_timing/pg_test_timing.c:135:
undefined reference to `pg_clock_gettime_initialize_rdtsc'
[09:08:12.525] /usr/bin/ld:
/tmp/cirrus-ci-build/build/../src/bin/pg_test_timing/pg_test_timing.c:137:
undefined reference to `cycles_to_us'
[09:08:12.525] /usr/bin/ld:
/tmp/cirrus-ci-build/build/../src/bin/pg_test_timing/pg_test_timing.c:146:
undefined reference to `cycles_to_us'
[09:08:12.525] /usr/bin/ld:
/tmp/cirrus-ci-build/build/../src/bin/pg_test_timing/pg_test_timing.c:169:
undefined reference to `cycles_to_us'
[09:08:12.525] /usr/bin/ld:
/tmp/cirrus-ci-build/build/../src/bin/pg_test_timing/pg_test_timing.c:176:
undefined reference to `cycles_to_sec'
[09:08:12.525] /usr/bin/ld: warning: creating DT_TEXTREL in a PIE
[09:08:12.525] collect2: error: ld returned 1 exit status

[1] - https://cirrus-ci.com/task/5375312565895168

Regards,
Vignesh