From: | Abhijit Menon-Sen <ams(at)2ndQuadrant(dot)com> |
---|---|
To: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: What exactly is our CRC algorithm? |
Date: | 2014-11-20 04:22:01 |
Message-ID: | 20141120042201.GA6345@toroid.org |
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Lists: | pgsql-hackers |
At 2014-11-19 19:12:22 +0200, hlinnakangas(at)vmware(dot)com wrote:
>
> But pg_xlogdump's way of using the CRC isn't necessarily
> representative of how the backend uses it. It's probably pretty close
> to WAL replay in the server, but even there the server might be hurt
> more by the extra cache used by the lookup tables.
Sure. As Robert said, my initial benchmark was designed to show the CRC
improvements in isolation. I would be happy to conduct other tests and
post the numbers.
If I understand correctly, I need to demonstrate two things that are
"probably fine", but we don't have proof of:
(a) that the improvements in pg_xlogdump performance translate to an
improvement in the server when reading WAL.
(b) that the slice-by-8 code doesn't hurt performance for writing WAL.
To address (a), I am timing a standby restoring the same 11GB of WAL via
restore_command with and without the CRC patch. My earlier tests showed
that this time can vary quite a bit between runs even with no changes,
but I expect to see an improvement anyway.
Suggestions for how to address (b) are welcome.
-- Abhijit
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