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From: Andreas Karlsson <[email protected]>
To: Hannu Krosing <[email protected]>
To: Lukas Fittl <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: David Geier <[email protected]>
Cc: Jakub Wartak <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Pavel Stehule <[email protected]>
Cc: vignesh C <[email protected]>
Cc: Michael Paquier <[email protected]>
Cc: Ibrar Ahmed <[email protected]>
Cc: Maciek Sakrejda <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?
Date: Fri, 13 Feb 2026 17:47:16 +0100
Message-ID: <[email protected]> (raw)
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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

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