On Wed, 4 Mar 2015 14:05:09 +0400 Alexander Shutyaev <shutyaev@xxxxxxxxx> wrote: > Thanks for the answer. Now, given this info I've calculated that our > postgresql should occupy approx. 30,53 GB while the server has 125 GB of > RAM. However we often see in top that there is very little free memory and > even swap is used. What could be the reason of postgres using so much > memory? Memory usage is much more dependent on the OS than Postgres than you might realize. I don't see where you state the OS, but I'll assume it's Linux for now. Linux default NUMA policy seems to be tuned toward applications that don't use a lot of RAM. If your 128G server has 8 CPUs, then Linux will allow a single process to use 16G of RAM before deciding that it has to use swap for that process. This is one of the advantantages I find with FreeBSD. Read up on how NUMA works a bit, and do some research into how to tune the NUMA policies ... assuming, of course, that you _are_ using Linux. Or switch to FreeBSD where the default NUMA policy is more friendly to programs that use a lot of RAM. > 2015-03-03 14:26 GMT+03:00 Andres Freund <andres@xxxxxxxxxxxxxxx>: > > > On 2015-03-03 15:06:54 +0400, Alexander Shutyaev wrote: > > > Recently we've been having problems with swap on our postgresql server. > > It > > > has 125GB of RAM. We've decided to calculate it's memory consumption. To > > do > > > this we've used the formulas from the official docs [1]. > > > > Note that I think those formulas have been removed from the docs for a > > while now (9.2?). > > > > > However there is > > > one parameter that seems strange - Shared disk buffers. According to the > > > formula it occupies the following space: > > > > > > (block_size + 208) * shared_buffers > > > > > Our values are > > > > > > block_size=8192 > > > shared_buffers=30GB > > > > > > > The block_size has the default value and shared_buffers was calculated by > > > pgtune. According to the formula the product will be around 252 000 GB > > > which doesn't make any sense. > > > > The problem with your calculation is that the shared_buffers referenced > > in the formula is the number of buffers - whereas when you specify it > > using a size unit (like MB, GB,...) that amount of memory is divided by > > the size of a page. So you're off by a factor of 8192. > > > > Greetings, > > > > Andres Freund > > > > -- > > Andres Freund http://www.2ndQuadrant.com/ > > PostgreSQL Development, 24x7 Support, Training & Services > > -- Bill Moran -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
