Re: Caching by Postgres

Donald Courtney wrote:
> in that even if you ran postgreSQL on a 64 bit address space
> with larger number of CPUs you won't see much of a scale up
> and possibly even a drop. I am not alone in having the *expectation*

What's your basis for believing this is the case? Why would PostgreSQL's
dependence on the OS's caching/filesystem limit scalability? I know when
I went from 32bit to 64bit Linux, I got *HUGE* increases in performance
using the same amount of memory. And when I went from 2x1P to 2xDC, my
average cpu usage % dropped almost in half.

> that a database should have some cache size parameter and
> the option to skip the file system. If I use oracle, sybase, mysql
> and maxdb they all have the ability to size a data cache and move
> to 64 bits.

Josh Berkus has already mentioned this as conventional wisdom as written
by Oracle. This may also be legacy wisdom. Oracle/Sybase/etc has been
around for a long time; it was probably a clear performance win way back
when. Nowadays with how far open-source OS's have advanced, I'd take it
with a grain of salt and do my own performance analysis. I suspect the
big vendors wouldn't change their stance even if they knew it was no
longer true due to the support hassles.

My personal experience with PostgreSQL. Dropping shared buffers from 2GB
to 750MB improved performance on my OLTP DB a good 25%. Going down from
750MB to 150MB was another +10%.