Sv: Re: Sv: Re: Sv: Re: data-checksums

På tirsdag 09. januar 2018 kl. 23:42:45, skrev Rob Sargent <
robjsargent(at)gmail(dot)com <mailto:robjsargent(at)gmail(dot)com>>:
 

  On 01/09/2018 03:30 PM, Andreas Joseph Krogh wrote:
På tirsdag 09. januar 2018 kl. 23:06:06, skrev Andres Freund <
andres(at)anarazel(dot)de <mailto:andres(at)anarazel(dot)de>>:
Hi,

On 2018-01-09 21:47:17 +0100, Andreas Joseph Krogh wrote:
> Does PG use HW-accellerated crc if CPU supports it[1]?

Yes we do, for WAL checksums. The page checksums are a different
algorithm though, one which has the advantage of being SIMD compatible.

The checksum computations have some impact, but if there's bigger impact
it's much more likely to be related to the fact that some hint bit
writes to a page now needs to be WAL logged.
 
But SIMD-instructions are also HW-accellerated by modern CPUs IIUC?
 
So, if these CRCs all are HW-accelerated the penalty chould be next to
neglishable?
 
Leading directly back to JD's proposed documentation update.  
Ducumentation mentioning "Depends on hardware" and "you should test" isn't
really helpfull imo. as it is too general and can be said about just about
anything.
 
Being explicit about usage of HW-accelerated (in CPU) instructions in
algorithms used is helpfull, and might help users choose enabling
data-checksums.
 
-- Andreas Joseph Krogh
CTO / Partner - Visena AS
Mobile: +47 909 56 963
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