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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: Jakub Wartak <jakub.wartak@enterprisedb.com>
Cc: Lukas Fittl <lukas@fittl.com>, David Geier <geidav.pg@gmail.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>
Date: 2026-02-03T17:44:21Z
Lists: pgsql-hackers 
Hi,

On 2026-02-02 10:22:37 +0100, Jakub Wartak wrote:
> When trying to understand this code I was thinking how it could be
> made smaller or less dependent on such low-level intrinsics, the only
> thing that came to my mind was launching systemd-detect-virt(1) via
> fork+execve, as after all we do have USE_SYSTEMD (for sd_notify(2) already
> consumed in backend/postmaster/postmaster.c) anyway.
> 
> Sadly this path for checking VM-types seems like opening can of worms
> - they evolved lots of code to cover various other products,
> see e.g. in detect_vm() and that thing is not exported.
> 
> Another way would be probably inquiring their D-Bus API, something like
> below command seems to work:
>    busctl get-property org.freedesktop.systemd1
> /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager
> Virtualization
> 
> (that seems to be sd_bus_get_property_string(3)).
> 
> It's not that I'm recommending usage of any of those (which is linked
> to us most of the time?) or fan of D-Bus (I'm not). I've just thought
> it might be less code to use it for autodetection of VM type, but
> apparently not (?) See their detect_vm_cpuid() with that vm_table[]
> and memcmp() seems to be a more elegant way of writing this.

Yea, I doubt this is the right path... Particularly because I really think we
ought to eventually use this not just on linux but other OSs as well.


> BTW, -1 to fast_clock_source, +1 to clock_source or maybe
> explain_clock_source(?)

Hm. I think it'd make sense to use eventually use this clock source for other
timing tasks too, not just explain.  E.g. pg_stat_statements could benefit
quite a bit from reducing the timing overhead.

Whereas it'll not make sense for anything that needs wall clock times - which
imo makes a "clock_source" GUC misnamed.  Maybe "clock_source_timing" or such?

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