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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Andreas Karlsson <andreas@proxel.se>

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From: Andreas Karlsson <andreas@proxel.se>
To: Hannu Krosing <hannuk@google.com>, Lukas Fittl <lukas@fittl.com>
Cc: Andres Freund <andres@anarazel.de>, David Geier <geidav.pg@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Robert Haas <robertmhaas@gmail.com>, 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: 2026-02-13T16:47:16Z
Lists: pgsql-hackers 
On 2/13/26 3:42 PM, Hannu Krosing wrote:
> I haven't looked at the code here yet, but when using plain rdtsc on
> modern CPUs one sees much more overhead from just the fact that the
> code is there than from calling the rdtsc instruction, and the
> overhead can vary by orders of magnitude based on how complex the work
> is that is timed.
> 
> I discovered this when I timed the (then-)new dead tid lookups in the
> Vacuum in Pg 17 and saw significantly larger overhead per lookup when
> the lookups themselves were slower, i.e. a case where the lookups were
> done in random order (inded was on  created on a column filled with
> random())
> 
> So while just a tight loop of N million rtdsc calls will give you the
> lower limit, it is likely not very representative of actual overhead.

Isn't the same issue still there if you call clock_gettime() but that it 
is just less noticeable due to the high cost of clock_gettime()?

Andreas

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: Show additional TSC clock source debug info

  2. instrumentation: Avoid CPUID 0x15/0x16 for Hypervisor TSC frequency

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

  4. Allow retrieving x86 TSC frequency/flags from CPUID

  5. instrumentation: Standardize ticks to nanosecond conversion method

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

  7. Check for __cpuidex and __get_cpuid_count separately

  8. pg_test_timing: Reduce per-loop overhead

  9. Refactor handling of x86 CPUID instructions

  10. instrumentation: Drop INSTR_TIME_SET_CURRENT_LAZY macro

  11. Rename pg_crc32c_sse42_choose.c for general purpose

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

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

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