Horton

Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

David Geier <geidav.pg@gmail.com>

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From: David Geier <geidav.pg@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Pavel Stehule <pavel.stehule@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, vignesh C <vignesh21@gmail.com>, Lukas Fittl <lukas@fittl.com>, Michael Paquier <michael@paquier.xyz>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Maciek Sakrejda <m.sakrejda@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-02-14T11:11:01Z
Lists: pgsql-hackers

Commits

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  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

Attachments

Hi,

On 2/7/23 19:12, Andres Freund wrote:
> This fails to build on several platforms:
>
> https://cirrus-ci.com/github/postgresql-cfbot/postgresql/commitfest%2F42%2F3751

I think I fixed the compilation errors. It was due to a few variables 
being declared under

#if defined(__x86_64__) && defined(__linux__)

while being used also under non x86 Linux.

I also removed again the code to obtain the TSC frequency under 
hypervisors because the TSC is usually emulated and therefore no faster 
than clock_gettime() anyways. So we now simply fallback to 
clock_gettime() on hypervisors when we cannot obtain the frequency via 
leaf 0x16.

Beyond that I reviewed the first two patches a while ago in [1]. I hope 
we can progress with them to further reduce the size of this patch set.

[1] 
https://www.postgresql.org/message-id/3ac157f7-085d-e071-45fc-b87cd306360c%40gmail.com 


-- 
David Geier
(ServiceNow)