On Mar 10, 2010, at 6:22 PM, Paul McGarry wrote: > Hi there, > > I'm after a little bit of advice on the shared_buffers setting (I have > read the various docs on/linked from the performance tuning wiki page, > some very helpful stuff there so thanks to those people). > > I am setting up a 64bit Linux server running Postgresql 8.3, the > server has 64gigs of memory and Postgres is the only major application > running on it. (This server is to go alongside some existing 8.3 > servers, we will look at 8.4/9 migration later) > > I'm basically wondering how the postgresql cache (ie shared_buffers) > and the OS page_cache interact. The general advice seems to be to > assign 1/4 of RAM to shared buffers. > > I don't have a good knowledge of the internals but I'm wondering if > this will effectively mean that roughly the same amount of RAM being > used for the OS page cache will be used for redundantly caching > something the Postgres is caching as well? > > IE when Postgres reads something from disk it will go into both the OS > page cache and the Postgresql shared_buffers and the OS page cache > copy is unlikely to be useful for anything. > > If that is the case what are the downsides to having less overlap > between the caches, IE heavily favouring one or the other, such as > allocating shared_buffers to a much larger percentage (such as 90-95% > of expected 'free' memory). Cache isn't all you have to worry about. There's also work_mem and the number of concurrent queries that you expect, and those may end up leaving you less than 25% of ram for shared_buffers - though probably not in your case. Also, I've read that 10GB is the upper end of where shared_buffers becomes useful, though I'm not entirely sure why. I think that rule of thumb has its roots in some heuristics around the double buffering effects you're asking about. I *can* say a 10GB shared_buffer value is working "well" with my 128GB of RAM..... whether or not it's "optimal," I couldn't say without a lot of experimentation I can't afford to do right now. You might have a look at the pg_buffercache contrib module. It can tell you how utilized your shared buffers are. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
