Horton

Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

David Geier <geidav.pg@gmail.com>

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From: David Geier <geidav.pg@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Michael Paquier <michael@paquier.xyz>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Maciek Sakrejda <m.sakrejda@gmail.com>, Lukas Fittl <lukas@fittl.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-01-18T13:02:48Z
Lists: pgsql-hackers

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

  2. Allow retrieving x86 TSC frequency/flags from CPUID

  3. instrumentation: Standardize ticks to nanosecond conversion method

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

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

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

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

On 1/16/23 18:37, Andres Freund wrote:
> Hi,
>
> On 2023-01-02 14:28:20 +0100, David Geier wrote:
>
> I'm doubtful this is worth the complexity it incurs. By the time we convert
> out of the instr_time format, the times shouldn't be small enough that the
> accuracy is affected much.
I don't feel strong about it and you have a point that we most likely 
only convert ones we've accumulated a fair amount of cycles.
> Looking around, most of the existing uses of INSTR_TIME_GET_MICROSEC()
> actually accumulate themselves, and should instead keep things in the
> instr_time format and convert later. We'd win more accuracy / speed that way.
>
> I don't think the introduction of pg_time_usec_t was a great idea, but oh
> well.
Fully agreed. Why not replacing pg_time_usec_t with instr_time in a 
separate patch? I haven't looked carefully enough if all occurrences 
could sanely replaced but at least the ones that accumulate time seem 
good starting points.
>> Additionally, I initialized a few variables of type instr_time which
>> otherwise resulted in warnings due to use of potentially uninitialized
>> variables.
> Unless we decide, as I suggested downthread, that we deprecate
> INSTR_TIME_SET_ZERO(), that's unfortunately not the right fix. I've a similar
> patch that adds all the necesarry INSTR_TIME_SET_ZERO() calls.
I don't feel strong about it, but like Tom tend towards keeping the 
initialization macro.
Thanks that you have improved on the first patch and fixed these issues 
in a better way.
>> What about renaming INSTR_TIME_GET_DOUBLE() to INSTR_TIME_GET_SECS() so that
>> it's consistent with the _MILLISEC() and _MICROSEC() variants?
>> The INSTR_TIME_GET_MICROSEC() returns a uint64 while the other variants
>> return double. This seems error prone. What about renaming the function or
>> also have the function return a double and cast where necessary at the call
>> site?
> I think those should be a separate discussion / patch.

OK. I'll propose follow-on patches ones we're done with the ones at hand.

I'll then rebase the RDTSC patches on your patch set.

-- 
David Geier
(ServiceNow)