Re: Performance Bottleneck

Martin Foster wrote:

> Gaetano Mendola wrote:
>
>> Martin Foster wrote:
>>
>>> I run a Perl/CGI driven website that makes extensive use of
>>> PostgreSQL (7.4.3) for everything from user information to formatting
>>> and display of specific sections of the site. The server itself, is
>>> a dual processor AMD Opteron 1.4Ghz w/ 2GB Ram and 2 x 120GB hard
>>> drives mirrored for redundancy running under FreeBSD 5.2.1 (AMD64).
>>>
>>> Recently loads on the site have increased during peak hours to the
>>> point of showing considerable loss in performance. This can be
>>> observed when connections move from the 120 concurrent connections to
>>> PostgreSQL to roughly 175 or more. Essentially, the machine seems
>>> to struggle to keep up with continual requests and slows down
>>> respectively as resources are tied down.
>>>
>>> Code changes have been made to the scripts to essentially back off in
>>> high load working environments which have worked to an extent.
>>> However, as loads continue to increase the database itself is not
>>> taking well to the increased traffic taking place.
>>>
>>> Having taken a look at 'Tuning PostgreSQL for Performance'
>>> (http://www.varlena.com/GeneralBits/Tidbits/perf.html) using it as
>>> best I could in order to set my settings. However, even with
>>> statistics disabled and ever setting tweaked things still consider to
>>> deteriorate.
>>>
>>> Is there anything anyone can recommend in order to give the system a
>>> necessary speed boost? It would seem to me that a modest dataset of
>>> roughly a Gig combined with that type of hardware should be able to
>>> handle substantially more load then what it is. Can anyone provide
>>> me with clues as where to pursue? Would disabling 'fsync' provide
>>> more performance if I choose that information may be lost in case of
>>> a crash?
>>>
>>> If anyone needs access to logs, settings et cetera. Please ask, I
>>> simply wish to test the waters first on what is needed. Thanks!
>>
>>
>>
>> Tell us about your tipical queries, show us your configuration file.
>> The access are only in read only mode or do you have concurrent writers
>> and readers ? During peak hours your processors are tied to 100% ?
>> What say the vmstat and the iostat ?
>>
>> May be you are not using indexes some where, or may be yes but the
>> planner is not using it... In two words we needs other informations
>> in order to help you.
>>
>>
>>
>> Regards
>> Gaetano Mendola
>>
>>
>
> I included all the files in attachments, which will hopefully cut down
> on any replied to Emails. As for things like connection pooling, the
> web server makes use of Apache::DBI to pool the connections for the Perl
> scripts being driven on that server. For the sake of being thorough,
> a quick 'apachectl status' was thrown in when the database was under a
> good load.

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

> The queries themselves are simple, normally drawing information from one
> table with few conditions or in the most complex cases using joins on
> two table or sub queries. These behave very well and always have, the
> problem is that these queries take place in rather large amounts due to
> the dumb nature of the scripts themselves.

Show us the explain analyze on that queries, how many rows the tables are
containing, the table schema could be also usefull.

regards
Gaetano Mendola