| From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
|---|---|
| To: | Peter Geoghegan <pg(at)heroku(dot)com> |
| Cc: | Merlin Moncure <mmoncure(at)gmail(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 18:49:58 |
| Message-ID: | 20140507184958.GM13397@awork2.anarazel.de |
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| Thread: | |
| Lists: | pgsql-hackers |
On 2014-05-07 11:45:04 -0700, Peter Geoghegan wrote:
> On Wed, May 7, 2014 at 11:38 AM, Andres Freund <andres(at)2ndquadrant(dot)com> 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.
>
> The biggest problem with double buffering is not that it wastes
> memory. Rather, it's that it wastes memory bandwidth.
Doesn't match my experience. Even with the current buffer manager
there's usually enough locality to keep important pages in s_b for a
meaningful time. I *have* seen workloads that should have fit into
memory not fit because of double buffering.
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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