Date: Wed, 9 Apr 2014 07:46:27 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Alexey Klyukin <alexk@hintbits.com>
Cc: Alexey Vasiliev <leopard_ne@inbox.ru>, pgsql-performance@postgresql.org
Subject: Re: Why shared_buffers max is 8GB?
Message-ID: <20140409114627.GA8686@momjian.us>
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On Wed, Apr  2, 2014 at 11:38:57AM +0200, Alexey Klyukin wrote:
> In most cases 8GB should be enough even for the servers with hundreds of GB of
> data, since the FS uses the rest of the memory as a cache (make sure you give a
> hint to the planner on how much memory is left for this with the
> effective_cache_size), but the exact answer is a matter of performance testing.
> 
> Now, the last question would be what was the initial justification for the 8GB
> barrier, I've heard that there were a lock congestion when dealing with huge
> pool of buffers, but I think that was fixed even in the pre-9.0 era.

The issue in earlier releases was the overhead of managing more then 1
million 8k buffers.  I have not seen any recent tests to confirm that
overhead is still significant.

A larger issue is that going over 8GB doesn't help unless you are
accessing more than 8GB of data in a short period of time.  Add to that
the problem if potentially dirtying all the buffers and flushing it to a
now-smaller kernel buffer cache, and you can see why the 8GB limit is
recommended.

I do think this merits more testing against the current Postgres source
code.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +


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