Re: shared_buffers performance

Gregory Stark wrote:
> "Gaetano Mendola" <mendola@gmail.com> writes:
>
>> The following graph reports the results:
>>
>> http://img84.imageshack.us/my.php?image=totalid7.png
>
> That's a *fascinating* graph.

It is, isn't it? Thanks Gaetano.

> It seems there are basically three domains.
>
> The small domain where the database fits in shared buffers -- though actually
> this domain seems to hold until the accounts table is about 1G so maybe it's
> more that the *indexes* fit in memory. Here larger shared buffers do clearly
> win.

I think this is actually in two parts - you can see it clearly on the
red trace (64MB), less so on the green (256MB) and not at all on the
blue (512MB). Presumably the left-hand steeper straight-line decline
starts with the working-set in shared-buffers, and the "knee" is where
we're down to just indexes in shared-buffers.

With the blue I guess you just get the first part, because by the time
you're overflowing shared-buffers, you've not got enough disk-cache to
take up the slack for you.

I wonder what difference 8.3 makes to this?

--
   Richard Huxton
   Archonet Ltd