Re: Limited performance on multi core server - Mailing list pgsql-performance

Hi Sven,

Yes I have done a reasonable amount of query tuning.  The application is a web service using an apache/resin combination at the front end, we have thought about using resin threads to limit the number of connections but are worried about backing up connections in apache and getting some overflow here.  But some kind of limiting of connections is probably required.

Thanks
Matthew

Sven Geisler wrote:

Hi Matthew,

The context switching isn't the issue. This is an indicator which is
useful to identify your problem.

What kind of application do you running? Can you limit the database clients?

We have a web application based on apache running. We have a limit
number of apache processes which are able to connect the database.
We use that to reduce the number of concurrent queries.
The apache does the rest for us - the apache does queue incoming http
request if all workers are busy. The configuration helps us to solve theperformance issue with to much concurrent queries.

I assume that you already checked you application and each sql query is
necessary and tuned as best as you can.

Regards
Sven.

Matthew Lunnon schrieb: 

Limiting the queries was our initial thought but we then hit a problem
with connection pooling which didn't implement a fifo algorithm.  Looks
like I'll have to look deeper into the connection pooling.

So you think the problem might be context switching on the server, I'll
take a closer look at the this

Thanks

Matthew

Sven Geisler wrote:   

Hi Matthew,

I know exactly what you experience.
We had a 4-way DC Opteron and Pg 7.4 too.
You should monitor context switches.


First suggest upgrade to 8.2.5 because the scale up is much better with 8.2.

You need to limit the number of concurrent queries to less than 8 (8
cores) if you need to stay with Pg 7.4.

The memory setting is looking good to me. I would increase sort_mem and
effective_cache_size, but this would solve your problem.

Best regards
Sven.



Matthew Lunnon schrieb:      

Hi,

I have a 4 * dual core 64bit AMD OPTERON server with 16G of RAM, running
postgres 7.4.3.  This has been recompiled on the server for 64 stored
procedure parameters, (I assume this makes postgres 64 bit but are not
sure).  When the server gets under load from database connections
executing reads, lets say 20 - 40 concurrent reads, the CPU's seem to
limit at about 30-35% usage with no iowait reported. If I run a simple
select at this time it takes 5 seconds, the same query runs in 300
millis when the server is not under load so it seems that the database
is not performing well even though there is plenty of spare CPU.  There
does not appear to be large amounts of disk IO and my database is about
5.5G so this should fit comfortably in RAM.

changes to postgresql.sql:

max_connections = 500
shared_buffers = 96000
sort_mem = 10240
effective_cache_size = 1000000

Does anyone have any ideas what my bottle neck might be and what I can
do about it?

Thanks for any help.

Matthew.

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-- 
Matthew Lunnon
Technical Consultant
RWA Ltd.
mlunnon@rwa-net.co.ukTel: +44 (0)29 2081 5056www.rwa-net.co.uk
--
   

-- 
Matthew Lunnon
Technical Consultant
RWA Ltd.
mlunnon@rwa-net.co.ukTel: +44 (0)29 2081 5056www.rwa-net.co.uk
--