Re: Enabling Checksums

On Sat, Mar 23, 2013 at 3:10 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>> Andres showed that switching out the existing CRC for zlib's would
>> result in 8-30% increase in INSERT-SELECT speed
>> (http://www.postgresql.org/message-id/201005202227.49990.andres@anarazel.de)
>> with the speeded up CRC still showing up as 10% of the profile. So I
>> guess another 5% speedup by doing the CRC 8 bytes at a time instead of
>> the used 4. And another couple % by using Fletcher or SIMD.
>
> I am not sure the considerations for WAL are the same as for page checksums -
> the current WAL code only computes the CRCs in rather small chunks, so very
> pipelineable algorithms/implementations don't necessarly show the same benefit
> for WAL as they do for page checksums...

Sure, but I think that WAL checksums are not a big overhead in that case anyway.

I should point out that getting the SIMD algorithm to not be a loss
for small variable sized workloads will take considerable amount of
effort and code. Whereas it's quite easy for pipelined CRC32 and
Fletcher (or should I say Adler as we want to use mod 65521).

Regards,
Ants Aasma
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