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Re: scale up (postgresql vs mssql)

From: Merlin Moncure <mmoncure(at)gmail(dot)com>
To: Eyal Wilde <eyal(at)impactsoft(dot)co(dot)il>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: scale up (postgresql vs mssql)
Date: 2012-04-16 13:10:18
Message-ID: CAHyXU0z6S+n77oat3Gvt6oUA3UrA_Sk0i_xzLfjZA9eptkVFTg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Wed, Apr 11, 2012 at 5:11 PM, Eyal Wilde <eyal(at)impactsoft(dot)co(dot)il> 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!

let's see the procedure.  I bet that the temp tables are the issue
here -- while they are speeding single user the i/o is stacking during
high concurrency (you are also writing to system catalogs which is not
good).

I'm sure we can get it fast but it's hard to do that without seeing the code.

merlin

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