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Re: effective_cache_size less than shared_buffers

From: Greg Stark <stark(at)enterprisedb(dot)com>
To: jd(at)commandprompt(dot)com
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: effective_cache_size less than shared_buffers
Date: 2009-02-26 08:27:40
Message-ID: 4136ffa0902260027l202e2df9xe55591ba8d9eecc8@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Feb 26, 2009 at 1:33 AM, Joshua D. Drake <jd(at)commandprompt(dot)com> wrote:
>
> On Wed, 2009-02-25 at 17:04 -0800, Josh Berkus wrote:
>
>> Joshua D. Drake wrote:
>>
>> > I would say no. Although I could see an argument for the default
>> > effective_cache_size always being the same size as shared_buffers.
>>
>> That's certainly not what we've meant historically by ECS.

> Since we are already using X amount
> of shared_buffers we know we have at least X amount of cache.

That's not what you wrote, at least how it was understood. It sounds
like you're in violent agreement.

> We can't determine the size of the FS cache.

Hence why we have the parameter.

> We can determine the size
> of the shared_buffers. The idea here is to eliminate one of those by
> default PostgreSQL is slow issues.

Well we won't eliminate any problems unless we actually override the
effective_cache_size setting by clipping it to shared_buffers. I don't
really see much of a problem doing that. The only case where that
would annoy someone was if they're intentionally understating
effective_cache_size to push the planner into avoiding nested loops
and I doin't think it's a powerful enough knob to be very likely used
that way.

-- 
greg

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