| From: | Jim Nasby <jim(at)nasby(dot)net> |
|---|---|
| To: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
| Cc: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Mel Gorman <mgorman(at)suse(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <kgrittn(at)ymail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Joshua Drake <jd(at)commandprompt(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Magnus Hagander <magnus(at)hagander(dot)net>, "lsf-pc(at)lists(dot)linux-foundation(dot)org" <lsf-pc(at)lists(dot)linux-foundation(dot)org> |
| Subject: | Re: Linux kernel impact on PostgreSQL performance |
| Date: | 2014-01-16 00:14:18 |
| Message-ID: | 52D7245A.1030707@nasby.net |
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| Lists: | pgsql-hackers |
On 1/15/14, 12:00 AM, Claudio Freire wrote:
> My completely unproven theory is that swapping is overwhelmed by
> near-misses. Ie: a process touches a page, and before it's actually
> swapped in, another process touches it too, blocking on the other
> process' read. But the second process doesn't account for that page
> when evaluating predictive models (ie: read-ahead), so the next I/O by
> process 2 is unexpected to the kernel. Then the same with 1. Etc... In
> essence, swap, by a fluke of its implementation, fails utterly to
> predict the I/O pattern, and results in far sub-optimal reads.
>
> Explicit I/O is free from that effect, all read calls are accountable,
> and that makes a difference.
>
> Maybe, if the kernel could be fixed in that respect, you could
> consider mmap'd files as a suitable form of temporary storage. But
> that would depend on the success and availability of such a fix/patch.
Another option is to consider some of the more "radical" ideas in this thread, but only for temporary data. Our write sequencing and other needs are far less stringent for this stuff.
--
Jim C. Nasby, Data Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net
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