|From:||Andres Freund <andres(at)anarazel(dot)de>|
|To:||"Zidenberg, Tsahi" <tsahee(at)amazon(dot)com>|
|Subject:||Re: [PATCH] audo-detect and use -moutline-atomics compilation flag for aarch64|
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On 2020-07-01 15:40:38 +0000, Zidenberg, Tsahi wrote:
> Outline-atomics is a gcc compilation flag that adds runtime detection
> of weather or not the cpu supports atomic instructions. CPUs that
> don't support atomic instructions will use the old
> load-exclusive/store-exclusive instructions. If a different
> compilation flag defined an architecture that unconditionally supports
> atomic instructions (e.g. -march=armv8.2), the outline-atomic flag
> will have no effect.
> The patch was tested to improve pgbench simple-update by 10% and
> sysbench write-only by 3% on a 64-core armv8.2 machine (AWS
> m6g.16xlarge). Select-only and read-only benchmarks were not
> significantly affected, and neither was performance on a 16-core
> armv8.0 machine that does not support atomic instructions (AWS
What does "not significantly affected" exactly mean? Could you post the
raw numbers? I'm a bit concerned that the additional conditional
branches on platforms without non ll/sc atomics could hurt noticably.
I'm surprised that read-only didn't benefit - with ll/sc that ought to
have pretty high contention on a few lwlocks.
Could you post the numbers?
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