Re: First set of OSDL Shared Mem scalability results, some

On 10/13/2004 11:52 PM, Greg Stark wrote:

> Jan Wieck <JanWieck(at)Yahoo(dot)com> writes:
>
>> On 10/8/2004 10:10 PM, Christopher Browne wrote:
>>
>> > josh(at)agliodbs(dot)com (Josh Berkus) wrote:
>> >> I've been trying to peg the "sweet spot" for shared memory using
>> >> OSDL's equipment. With Jan's new ARC patch, I was expecting that
>> >> the desired amount of shared_buffers to be greatly increased. This
>> >> has not turned out to be the case.
>> > That doesn't surprise me.
>>
>> Neither does it surprise me.
>
> There's been some speculation that having a large shared buffers be about 50%
> of your RAM is pessimal as it guarantees the OS cache is merely doubling up on
> all the buffers postgres is keeping. I wonder whether there's a second sweet
> spot where the postgres cache is closer to the total amount of RAM.

Which would require that shared memory is not allowed to be swapped out,
and that is allowed in Linux by default IIRC, not to completely distort
the entire test.

Jan

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