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?
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
