Re: Performance Bottleneck

On Fri, 2004-08-06 at 17:24, Gaetano Mendola wrote:
> Martin Foster wrote:
>
> > Gaetano Mendola wrote:
> >
> >>
> >>
> >> Let start from your postgres configuration:
> >>
> >> shared_buffers = 8192    <==== This is really too small for your
> >> configuration
> >> sort_mem = 2048
> >>
> >> wal_buffers = 128    <==== This is really too small for your
> >> configuration
> >>
> >> effective_cache_size = 16000
> >>
> >> change this values in:
> >>
> >> shared_buffers = 50000
> >> sort_mem = 16084
> >>
> >> wal_buffers = 1500
> >>
> >> effective_cache_size = 32000
> >>
> >>
> >> to bump up the shm usage you have to configure your OS in order to be
> >> allowed to use that ammount of SHM.
> >>
> >> This are the numbers that I feel good for your HW, the second step now is
> >> analyze your queries
> >>
> >
> > These changes have yielded some visible improvements, with load averages
> > rarely going over the anything noticeable.   However, I do have a
> > question on the matter, why do these values seem to be far higher then
> > what a frequently pointed to document would indicate as necessary?
> >
> > http://www.varlena.com/GeneralBits/Tidbits/perf.html
> >
> > I am simply curious, as this clearly shows that my understanding of
> > PostgreSQL is clearly lacking when it comes to tweaking for the hardware.
>
> Unfortunately there is no a "wizard tuning" for postgres so each one of
> us have a own "school". The data I gave you are oversized to be sure
> to achieve improvements. Now you can start to decrease these values
> ( starting from the wal_buffers ) in order to find the good compromise
> with your HW.

FYI, my school of tuning is to change one thing at a time some
reasonable percentage (shared_buffers from 1000 to 2000) and measure the
change under simulated load.  Make another change, test it, chart the
shape of the change line.  It should look something like this for most
folks:

shared_buffers | q/s (more is better)
100 | 20
200 | 45
400 | 80
1000 | 100
... levels out here...
8000 | 110
10000 | 108
20000 | 40
30000 | 20

Note it going back down as we exceed our memory and start swapping
shared_buffers.  Where that happens on your machine is determined by
many things like your machine's memory, memory bandwidth, type of load,
etc... but it will happen on most machines and when it does, it often
happens at the worst times, under heavy parallel load.

Unless testing shows it's faster, 10000 or 25% of mem (whichever is
less) is usually a pretty good setting for shared_buffers.  Large data
sets may require more than 10000, but going over 25% on machines with
large memory is usually a mistake, especially servers that do anything
other than just PostgreSQL.

You're absolutely right about one thing, there's no automatic wizard for
tuning this stuff.