Great discussion and illuminating for those of us who are still
learning the subtleties of postGres.


To be clear -
I built postgreSQL 8.1 64K bit on solaris 10 a few months ago
and side by side with the 32 bit postgreSQL build saw no improvement. In fact the 64 bit result was slightly lower.

I used  the *same 64 bit S10 OS* for both versions.  I think your
experience makes sense since your change was from 32 to 64 bit Linux.
From my experiment I am surmising that there will not be any file/os/buffer-cache scale up effect on the same OS with postgreSQL 64.
I was testing on a 4 core system in both cases.

William Yu wrote:

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%.

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