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Re: max_connections / shared_buffers / effective_cache_size

From: John A Meinel <john(at)arbash-meinel(dot)com>
To: Puddle <rotaecho(at)yahoo(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: max_connections / shared_buffers / effective_cache_size
Date: 2005-06-24 19:16:02
Message-ID: 42BC5BF2.9070900@arbash-meinel.com (view raw or flat)
Thread:
Lists: pgsql-performance
Puddle wrote:

>Hello, I'm a Sun Solaris sys admin for a start-up
>company.  I've got the UNIX background, but now I'm
>having to learn PostgreSQL to support it on our
>servers :)
>
>Server Background:
>
>Solaris 10 x86
>PostgreSQL 8.0.3
>Dell PowerEdge 2650 w/4gb ram.
>This is running JBoss/Apache as well (I KNOW the bad
>juju of running it all on one box, but it's all we
>have currently for this project). I'm dedicating 1gb
>for PostgreSQL alone.
>
>So, far I LOVE it compared to MySQL it's solid.
>
>The only things I'm kind of confused about (and I've
>been searching for answers on lot of good perf docs,
>but not too clear to me) are the following:
>
>1.) shared_buffers I see lot of reference to making
>this the size of available ram (for the DB).  However,
>I also read to make it the size of pgdata directory.
>
>I notice when I load postgres each daemon is using the
>amount of shared memory (shared_buffers).  Our current
>dataset (pgdata) is 85mb in size.  So, I'm curious
>should this size reflect the pgdata or the 'actual'
>memory given?
>
>I currently have this at 128mb
>
>
You generally want shared_buffers to be no more than 10% of available
ram. Postgres expects the OS to do it's own caching. 128M/4G = 3% seems
reasonable to me. I would certainly never set it to 100% of ram.

>2.) effective_cache_size - from what I read this is
>the 'total' allowed memory for postgresql to use
>correct? So, if I am willing to allow 1GB of memory
>should I make this 1GB?
>
>
This is the effective amount of caching between the actual postgres
buffers, and the OS buffers. If you are dedicating this machine to
postgres, I would set it to something like 3.5G. If it is a mixed
machine, then you have to think about it.

This does not change how postgres uses RAM, it changes how postgres
estimates whether an Index scan will be cheaper than a Sequential scan,
based on the likelihood that the data you want will already be cached in
Ram.

If you dataset is only 85MB, and you don't think it will grow, you
really don't have to worry about this much. You have a very small database.

>3.) max_connections, been trying to figure 'how' to
>determine this #.  I've read this is buffer_size+500k
>per a connection.
>
>ie.  128mb(buffer) + 500kb = 128.5mb per connection?
>
>
Max connections is just how many concurrent connections you want to
allow. If you can get away with lower, do so.  Mostly this is to prevent
connections * work_mem to get bigger than your real working memory and
causing you to swap.

>I was curious about 'sort_mem' I can't find reference
>of it in the 8.0.3 documentation, has it been removed?
>
>
sort_mem changed to work_mem in 8.0, same thing with vacuum_mem ->
maintenance_work_mem.

>work_mem and max_stack_depth set to 4096
>maintenance_work_mem set to 64mb
>
>
Depends how much space you want to give per connection. 4M is pretty
small for a machine with 4G of RAM, but if your DB is only 85M it might
be plenty.
work_mem is how much memory a sort/hash/etc will use before it spills to
disk. So look at your queries. If you tend to sort most of your 85M db
in a single query, you might want to make it a little bit more. But if
all of your queries are very selective, 4M could be plenty.

I would make maintenance_work_mem more like 512M. It is only used for
CREATE INDEX, VACUUM, etc. Things that are not generally done by more
than one process at a time. And it's nice for them to have plenty of
room to run fast.

>Thanks for any help on this.  I'm sure bombardment of
>newbies gets old :)
>
>-William
>
>
Good luck,
John
=:->

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