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Re: : bg_writer overloaded ?

From: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
To: Venkat Balaji <venkat(dot)balaji(at)verse(dot)in>
Cc: PGSQL Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: : bg_writer overloaded ?
Date: 2011-11-09 14:46:10
Message-ID: CAOR=d=0QTB=jPU0rqJTVo_PNiMmwZKv9LOa1c4TUjUgJnaa32g@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Wed, Nov 9, 2011 at 2:25 AM, Venkat Balaji <venkat(dot)balaji(at)verse(dot)in> wrote:
> Hello Everyone,
> I could see the following in the production server (result of the "top" M
> command) -
>  PID    USER     PR  NI  VIRT    RES   SHR   S  %CPU   %MEM       TIME+
> COMMAND
> 25265 postgres  15   0  3329m   2.5g   1.9g   S     0.0          4.0
>  542:47.83   postgres: writer process
> The "writer process" refers to bg_writer ? and we have shared_buffers set to
> 1920 MB (around 1.9 GB).

So it is using 2.5G of mem of which 1.9G is shared memory (i.e. shared
buffers) so the actual amount of RAM it's using is ~600Megs.

I see no problem.

> In an other similar situation, we have "postgres writer process" using up 7
> - 8 GB memory constantly.

I doubt it.  Sounds more like you're misreading the output of top.

> pg_tune is suggesting to increase the shared_buffers to 8 GB.

Reasonable.

> If the shared_buffer is not enough, Postgres uses OS cache ?

Not really how things work.  The OS uses all spare memory as cache.
PostgreSQL uses shared_buffers as a cache.  The OS is much more
efficient about caching in dozens of gigabytes than pgsql is.

> We have a 64 GB RAM.
> We have decided the following -
> 1. We have 20 databases running in one cluster and all are more or less
> highly active databases.
> 2. We will be splitting across the databases across multiple clusters to
> have multiple writer processes working across databases.
> Please help us if you have any other solutions around this.

You have shown us no actual problem.

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Next:From: Kevin GrittnerDate: 2011-11-09 15:15:35
Subject: Re: Subquery in a JOIN not getting restricted?
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Subject: : bg_writer overloaded ?

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