Re: was there a change in FreeBSD SHM implementation from

Curt Sampson wrote:
> On Thu, 11 Jul 2002, Vivek Khera wrote:
>
> > Actually performance went down. Way down. I disagree with your
> > argument that increasing the cache will help, since the cache is not
> > needed if you don't pushd out your SHM pages in the first place and
> > need to reload them quickly.
>
> Ah, so your entire working data set can fit into your shared memory? In
> that case, yes, you are better off not decreasing it. I guess we should
> add a note to the FAQ for people using very small databases....

Curt, I haven't heard anyone else who agrees with your idea of making
PostgreSQL shared buffers smaller and kernel cache bigger.

Now, you have potent argument that having 3X of kernel buffer and 1X of
PostgreSQL buffers is better than having 2X of kernel buffers and 2X of
PostgreSQL buffers. However, you are assuming the extra 1X of kernel
buffer size will be used often _and_ that the copying from kernel buffer to
PostgreSQL is minimal.

I understand your idea that the disk I/O is clearly slower than the copy
from kernel to PostgreSQL buffer, but with the 3X/1X solution, is moving
more buffers from kernel to PostgreSQL slower than the number of times
you are doing more I/O in the 2X/2X case. My guess is that the 2X/2X
case is the winner, but it is an interesting idea.

The case of this user was going from XXX megabytes to 25MB, which is a
drastic change, and there the buffer copying is probably much more of a
performance hit than the larger kernel buffer.

--
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