Re: Why shared_buffers max is 8GB?

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(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +