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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Lukas Fittl <lukas@fittl.com>
Cc: John Naylor <johncnaylorls@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Hannu Krosing <hannuk@google.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>, David Geier <geidav.pg@gmail.com>
Date: 2026-03-06T19:22:27Z
Lists: pgsql-hackers 
Hi,

On 2026-03-03 10:22:42 -0800, Lukas Fittl wrote:
> > But if we read files anyway, wouldn't just using
> >   /sys/devices/system/cpu/cpu0/cpufreq/base_frequency
> > work?
>
> I tested this just now on an Azure VM (Standard D2s v3), and its
> close, but unfortunately CPU frequency doesn't match the TSC frequency
> (cpuinfo_max_freq is 2800000, scaling_cur_freq is 2496279, and TSC
> frequency via MSR is 2793438 -- note that I didn't have base_frequency
> on this VM). My understanding is that the TSC clock is virtualized in
> HyperV and does not directly match the CPU frequency.

:(

It seems quite ridiculous that there's no cpuid to get the frequency of both
virtualized and "real" tsc.


> I'm also happy to take this out again - maybe we can get the
> HyperV/Azure Linux folks to improve the Kernel side here to pass down
> the TSC frequency without needing the MSR, and just not support it for
> now.

Yea, this doesn't seem worth it, it won't get used this way, I think.


> An alternate idea could be to allow overriding the TSC frequency via a
> GUC - then one could use the root user (or a setuid program) to get
> the TSC frequency on Azure/HyperV via the MSR and pass it to Postgres
> at start. But not sure that's worth the trouble, since it won't help
> with environments that don't have a reliable TSC (e.g. Virtualbox, I
> think).

I don't think manually specifying it makes sense either.


But maybe we should just do the stupid thing and figure out the multiplier as
such:

  ns_to_cycles = tsc_via_rdtsc / to_ns(clock_gettime(CLOCK_BOOTTIME))

in some quick experiments that ends up with a very good estimate.  There would
have to be an awful long gap between the rdtsc and clock_gettime() computation
for the frequency to be meaningfully inaccurate.


I was worried for a moment that the there would be issues with the tsc counter
overflowing after a long uptime, but that doesn't seem a real issue if I did
the math right (at a 10GHz tsc freq the time to overflow would be ~58 years).

Greetings,

Andres Freund

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