Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Andres Freund <andres@anarazel.de>

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: 2026-04-07T17:59:48Z

Lists: pgsql-hackers

Hi,

On 2026-04-07 00:04:51 -0700, Lukas Fittl wrote:
> Attached v22 with documentation updates. I've also marked the ARM
> patch "nocfbot" for now, so its clear we're doing that one later.

- changed comment in InitProcessGlobals to be consistent with other cases
- added a comment to the fallthrough to the generic logic in x86_tsc_frequency_khz
- started to wonder if we ought to report
	 * The similar __get_cpuid_count function does not work as expected since
	 * it contains a check for __get_cpuid_max, which has been observed to be
  as a bug.  That's obviously independent of committing this.

- a few typos (s/possibly/possible/, s/its/it's/, s/\. are/. Are/,
  INSTR_TIME_ADD_NANOSEC comment variable names) found with modern spell
  checking tools (i.e. an ai review)

- fixed include order in guc_tables.c

- slight rephrasing of pg_initialize_timing() comment about
  pg_set_timing_clock_source()

- Moved the indexterm to the relevant paragraphs. I don't think it really
  matters, but it seemed a tad clearer

- shortened some commit message titles ;)

- Added a comment about why always_inline is needed for pg_get_ticks, and as
  part of that swapped _fast() and plain versions around.

- I'm somewhat tired, because I was wondering whether TSC_CALIBRATION_SKIPS
  should be a power of 2, to make the check faster. But uh, IT DOESN'T MATTER
  :)

I've pushed 0001, 0002, 0003.

There's one minor documentation issue in 0004 that I wanted to look at before
pushing (and I need to switch to something else for a bit).  The rephrasing
gets rid of

-   [...] , with the worst case somewhere between 32768 and
-   65535 nanoseconds.  In the second block, we can see that typical loop
-   time is 16 nanoseconds, and the readings appear to have full nanosecond
-   precision.

I don't mind loosing the first sentence, but the second one might be useful to
somebody?


> > Now I wonder if we should rename 'tsc' to 'cpu'...
> 
> Yeah, I had proposed making it more generic in an earlier email, but I
> don't think there are great names available (and nobody jumped at the
> suggestion). I think "cpu" is a bit too unspecific, vs "tsc" is more
> clearly referencing the kind of instruction being used directly, and
> is unique enough that it makes people read on vs jumping to a
> conclusion. I was previously thinking of something like "hwtimer" or
> "hwclock", but I don't think those are great.
> 
> I think the worst case here is that we need to add a "Note this is
> named after the TSC instructions on x86-64, but utilizes the similarly
> functioning cntvct_el0 instruction on ARM." or something to the
> documentation if/when we expand support to ARM.

Yea, I think it's ok.

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 →

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