Re: Avoid multiple calls to memcpy (src/backend/access/index/genam.c)

Hi.

Em seg., 9 de mar. de 2026 às 14:02, Bryan Green <dbryan(dot)green(at)gmail(dot)com>
escreveu:

> I performed a micro-benchmark on my dual epyc (zen 2) server and version 1
> wins for small values of n.
>
> 20 runs:
>
> n version min median mean max stddev noise%
> -----------------------------------------------------------------------
> n=1 version1 2.440 2.440 2.450 2.550 0.024 4.5%
> n=1 version2 4.260 4.280 4.277 4.290 0.007 0.7%
>
> n=2 version1 2.740 2.750 2.757 2.880 0.029 5.1%
> n=2 version2 3.970 3.980 3.980 4.020 0.010 1.3%
>
> n=4 version1 4.580 4.595 4.649 4.910 0.094 7.2%
> n=4 version2 5.780 5.815 5.809 5.820 0.013 0.7%
>
> But, micro-benchmarks always make me nervous, so I looked at the actual
> instruction cost for my
> platform given the version 1 and version 2 code.
>
> If we count cpu cycles using the AMD Zen 2 instruction latency/throughput
> tables: version 1 (loop body)
> has a critical path of ~5-6 cycles per iteration. version 2 (loop body)
> has ~3-4 cycles per iteration.
>
> The problem for version 2 is that the call to memcpy is ~24-30 cycles due
> to the stub + function call + return
> and branch predictor pressure on first call. This probably results in
> ~2.5 ns per iteration cost for version 2.
>
> So, no I wouldn't call it an optimization. But, it will be interesting to
> hear other opinions on this.
>
I made dirty and quick tests with two versions:
gcc 15.2.0
gcc -O2 memcpy1.c -o memcpy1

The first test was with keys 10000000 and 10000000 loops:
version1: on memcpy call
done in 1873 nanoseconds

version2: inlined memcpy
not finish

The second test was with keys 4 and 10000000 loops:
version1: one memcpy call
version2: inlined memcpy call

version1: done in 1519 nanoseconds
version2: done in 104981851 nanoseconds
(1.44692e-05 times faster)

version1: done in 1979 nanoseconds
version2: done in 110568901 nanoseconds
(1.78983e-05 times faster)

version1: done in 1814 nanoseconds
version2: done in 108555484 nanoseconds
(1.67103e-05 times faster)

version1: done in 1631 nanoseconds
version2: done in 109867919 nanoseconds
(1.48451e-05 times faster)

version1: done in 1269 nanoseconds
version2: done in 111639106 nanoseconds
(1.1367e-05 times faster)

Unless I'm doing something wrong, one call memcpy wins!
memcpy1.c attached.

best regards,
Ranier Vilela