Re: More shared buffers causes lower performances

On Wed, 26 Dec 2007, Guillaume Smet wrote:

> It's not checkpointing either as using pgbench-tools, I can see that
> tps and latency are quite stable during the entire run. Btw, thanks
> Greg for these nice tools.

I stole the graph idea from Mark Wong's DBT2 code and one of these days
I'll credit him appropriately.

> Then I decided to perform read-only tests using -S option (pgbench -S
> -s 100 -c 16 -t 30000 -U postgres bench). And still the same
> behaviour:
> shared_buffers=64MB : 20k tps
> shared_buffers=1024MB : 8k tps

Ah, now this is really interesting, as it rules out all the write
components and should be easy to replicate even on a smaller server.  As
you've already dumped a bunch of time into this the only other thing I
would suggest checking is whether the same behavior also happens on 8.2 on
your server.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD