Re: proposal: Set effective_cache_size to greater of .conf value, shared_buffers

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От Jeff Janes
Тема Re: proposal: Set effective_cache_size to greater of .conf value, shared_buffers
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Msg-id CAMkU=1yktwAOLCHic6jcOF5vTCtZ=V5JRpePonxhXCP32CMUQA@mail.gmail.com
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Ответ на Re: proposal: Set effective_cache_size to greater of .conf value, shared_buffers  (Andres Freund <andres@2ndquadrant.com>)
Ответы Re: proposal: Set effective_cache_size to greater of .conf value, shared_buffers
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On Wed, May 7, 2014 at 11:38 AM, Andres Freund <andres@2ndquadrant.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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