On 3/19/13 7:13 PM, Daniel Farina wrote:
I'm confused. Postgres includes a CRC32 implementation for WAL, does it not? Are you referring to something else?
I'm just pointing out that zlib includes one, too, and they might be more motivated/able as a project to chase after Intel's hardware acceleration for CRCs one day. They already have code switching from C to assembly to get extra performance out of their longest_match() function. The PostgreSQL CRC code is unlikely to go into twiddling assembly code, but zlib--which is usually linked in anyway--will.
And Adler-32 isn't just an option, it's named after a dude who works on zlib, and I can see he's already playing with the Intel acceleration by some of his recent answers at http://stackoverflow.com/users/1180620/mark-adler
I just re-discovered Ross Williams' CRC guide, which was already referenced in pg_crc_tables.h, so I think I'm getting close to being caught up on all the options here. Simon suggested the other day that we should make the exact checksum mechanism used pluggable at initdb time, just some last minute alternatives checking on the performance of the real server code. I've now got the WAL CRC32, the zlib CRC32, and the Intel-derived versions Ants hacked on to compare.
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