Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

David Geier <geidav.pg@gmail.com>

From: David Geier <geidav.pg@gmail.com>

To: Hannu Krosing <hannuk@google.com>, Robert Haas <robertmhaas@gmail.com>

Cc: Lukas Fittl <lukas@fittl.com>, Andres Freund <andres@anarazel.de>, Pavel Stehule <pavel.stehule@gmail.com>, vignesh C <vignesh21@gmail.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: 2026-01-11T19:26:17Z

Lists: pgsql-hackers

Attachments

v3-0003-Add-GUC.patch (text/x-patch) patch v3-0003
v3-0002-pg_test_timing-Also-test-fast-timing-and-report-t.patch (text/x-patch) patch v3-0002
v3-0001-Use-time-stamp-counter-to-measure-time-on-Linux-x.patch (text/x-patch) patch v3-0001

> Based on Robert's suggestion I wanted to add a "fast_clock_source" enum
> GUC which can have the following values "auto", "rdtsc", "try_rdtsc" and
> "off". With that, at least no additional checks are needed and
> performance will remain as previously benchmarked in this thread.

The attached patch set is rebased on latest master and contains a commit
which adds a "fast_clock_source" GUC that can be "try", "off" and
"rdtsc" on Linux.

Alternatively, we could call the GUC "clock_source" with "auto",
"clock_gettime" and "rdtsc". Opinions?

I moved the call to INSTR_TIME_INITIALIZE() from InitPostgres() to
PostmasterMain(). In InitPostgres() it kept the database in a recovery
cycle.

> I'll still add unlikely() around the if (has_rdtsc).

Done.

--
David Geier

Commits

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

pg_test_timing: Show additional TSC clock source debug info
- 5ba34f6dc838 19 (unreleased) landed
instrumentation: Avoid CPUID 0x15/0x16 for Hypervisor TSC frequency
- 7fc36c5db550 19 (unreleased) landed
pg_test_timing: Also test RDTSC[P] timing, report time source, TSC frequency
- 16fca4825483 19 (unreleased) landed
Allow retrieving x86 TSC frequency/flags from CPUID
- bcb2cf41f964 19 (unreleased) landed
instrumentation: Standardize ticks to nanosecond conversion method
- 0022622c93d9 19 (unreleased) landed
instrumentation: Use Time-Stamp Counter on x86-64 to lower overhead
- 294520c44487 19 (unreleased) landed
Check for __cpuidex and __get_cpuid_count separately
- effaa464afd3 19 (unreleased) landed
pg_test_timing: Reduce per-loop overhead
- 82c0cb4e672d 19 (unreleased) landed
Refactor handling of x86 CPUID instructions
- be6a7494d2e3 19 (unreleased) landed
instrumentation: Drop INSTR_TIME_SET_CURRENT_LAZY macro
- 9d6294c09ed0 19 (unreleased) landed
Rename pg_crc32c_sse42_choose.c for general purpose
- b9278871f991 19 (unreleased) cited
Zero initialize uses of instr_time about to trigger compiler warnings
- 25b2aba0c3a5 16.0 landed
instr_time: Represent time as an int64 on all platforms
- 03023a2664f8 16.0 landed
Add 250c8ee07ed to git-blame-ignore-revs
- ff23b592ad66 16.0 cited