| From: | Andreas Karlsson <andreas(at)proxel(dot)se> |
|---|---|
| To: | Hannu Krosing <hannuk(at)google(dot)com>, Lukas Fittl <lukas(at)fittl(dot)com> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, David Geier <geidav(dot)pg(at)gmail(dot)com>, Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, vignesh C <vignesh21(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com>, Maciek Sakrejda <m(dot)sakrejda(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc? |
| Date: | 2026-02-13 16:47:16 |
| Message-ID: | 67931b8a-27a6-4d68-ae42-2bea87b56569@proxel.se |
| Views: | Whole Thread | Raw Message | Download mbox | Resend email |
| Thread: | |
| 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
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Alexandre Felipe | 2026-02-13 17:04:01 | Re: Incremental View Maintenance, take 2 |
| Previous Message | Lukas Fittl | 2026-02-13 16:39:42 | Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc? |