| From: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
| Cc: | Merlin Moncure <mmoncure(at)gmail(dot)com>, Peter Geoghegan <pg(at)heroku(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Bruce Momjian <bruce(at)momjian(dot)us>, Magnus Hagander <magnus(at)hagander(dot)net>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Claudio Freire <klaussfreire(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: proposal: Set effective_cache_size to greater of .conf value, shared_buffers |
| Date: | 2014-05-07 20:51:57 |
| Message-ID: | CAMkU=1yktwAOLCHic6jcOF5vTCtZ=V5JRpePonxhXCP32CMUQA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, May 7, 2014 at 11:38 AM, Andres Freund <andres(at)2ndquadrant(dot)com>wrote:
> On 2014-05-07 13:32:41 -0500, Merlin Moncure wrote:
> >
> > *) raising shared buffers does not 'give more memory to postgres for
> > caching' -- it can only reduce it via double paging
>
> That's absolutely not a necessary consequence. If pages are in s_b for a
> while the OS will be perfectly happy to throw them away.
>
Is that an empirical observation? I've run some simulations a couple years
ago, and also wrote some instrumentation to test that theory under
favorably engineered (but still plausible) conditions, and couldn't get
more than a small fraction of s_b to be so tightly bound in that the kernel
could forget about them. Unless of course the entire workload or close to
it fits in s_b.
Cheers,
Jeff
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