Re: Enabling Checksums

On 2013-03-23 15:36:03 +0200, Ants Aasma wrote:
> On Sat, Mar 23, 2013 at 3:10 PM, Andres Freund <andres@2ndquadrant.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 have seen profiles that indicate rather the contrary... Even in the optimal
case of no FPWs a single heap_insert() results in the CRC computed in 5 steps
or so. 4 of them over potentially noncontiguous pointer addressed memory.
If you add an index or two where the situation is the same the slowdown is not
all that surprising.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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