Message-ID: <4F8838EB.7040400@dunslane.net>
Date: Fri, 13 Apr 2012 10:32:11 -0400
From: Andrew Dunstan <andrew@dunslane.net>
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To: Eyal Wilde <eyal@impactsoft.co.il>
CC: pgsql-performance@postgresql.org
Subject: Re: scale up (postgresql vs mssql)
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On 04/11/2012 06:11 PM, Eyal Wilde wrote:
> hi,
>
> i had a stored procedure in ms-sql server. this stored procedure gets 
> a parameter (account-id), dose about 20 queries, fills some temporary 
> tables, and finally, returns a few result-sets. this stored procedure 
> converted to stored function in postgresql (9.1). the result-sets are 
> being returned using refcursors. this stored function is logically, 
> almost identical to the ms-sql stored procedure.  a LOT of work had 
> been done to make postgresql getting close to ms-sql speed (preparing 
> temp-tables in advance, using "analyze" in special places inside the 
> stored function in order to hint the optimizer that the temp-tables 
> have very few records, thus eliminating unnecessary and expansive 
> hash-join, and a lot more..). after all that, the stored function is 
> running in a reasonable speed (normally ~60 milliseconds).
>
> now, i run a test that simulates 20 simultaneous clients, asking for 
> "account-id" randomly. once a client get a result, it immediately asks 
> for another one. the test last 5 seconds.  i use a connection pool 
> (with Tomcat web-server). the pool is automatically increased to ~20 
> connections (as expected). the result is postgresql dose ~60 
> "account-id"s, whereas ms-sql dose ~330 "account-id"s. postgresql 
> shows that each "account-id" took about 400-1000 msec ,which is so 
> much slower than the ~60 msec of a single execution.
>
> in a single execution postgresql may be less the twice slower than 
> ms-sql, but in 20 simultaneous clients, it's about 6 times worse. why 
> is that?
>
> the hardware is one 4-core xeon. 8GB of ram. the database size is just 
> a few GB's. centos-6.2.
>
> do you think the fact that postgresql use a process per connection 
> (instead of multi-threading) is inherently a weakness of postgrsql, 
> regarding scale-up?
> would it be better to limit the number of connections to something 
> like 4, so that executions don't interrupt each other?
>
> thanks in advance for any help!


I doubt that the process-per-connection has much effect, especially on 
Linux where process creation is extremely cheap, and you're using a 
connection pooler anyway. The server is pretty modest, though. If you 
can add enough RAM that you can fit the whole db into Postgres shared 
buffers you might find things run a whole lot better. You should show us 
your memory settings, among other things - especially shared_buffers, 
temp_buffers and work_mem.

cheers

andrew