On Mon, 2020-12-21 at 09:17 -0400, frank picabia wrote: > This error occurred a couple of days ago and it is unlikely to repeat as > it was a series of exams being done in moodle with over 500 students. > However I'd like to tune Postgres to see if it can be prevented. > > First of all, we did not run out of memory within the OS. This system is monitored by cacti and > Real Used memory never exceeded 30 GB on a server having 64 GB ram plus some swap. > > I can also see in the messages log, the Linux kernel OOM killer was never triggered. > > The postgres user running the DB has no ulimits on memory which might apply. > > Here is the entry in the Postgres log: > > <2020-12-19 13:19:29 GMT>LOG: could not fork new process for connection: Cannot allocate memory > TopMemoryContext: 136704 total in 13 blocks; 6688 free (4 chunks); 130016 used > smgr relation table: 24576 total in 2 blocks; 13904 free (4 chunks); 10672 used > TopTransactionContext: 8192 total in 1 blocks; 6152 free (5 chunks); 2040 used > <2020-12-19 13:19:29 GMT>LOG: could not fork new process for connection: Cannot allocate memory > TransactionAbortContext: 32768 total in 1 blocks; 32728 free (0 chunks); 40 used > Portal hash: 8192 total in 1 blocks; 1672 free (0 chunks); 6520 used > PortalMemory: 0 total in 0 blocks; 0 free (0 chunks); 0 used > > My conclusion is this memory error must be related to limits within Postgres tunables, or perhaps > something like SHM memory, which we have not set up in the sysctl.conf , so this is just a default amount. > > The values we have in postgres.conf are mostly suggested by pgtune script. The one exception being > max_connections=500, which was increased to try to avoid bottlenecks being reached with Apache client > numbers, and gradually over time as we saw we still had lots of memory available with a higher connection limit. > > Which tunables do you need to see? You probably have to increase the operating system limit for open files for the "postgres" user (ulimit -n). Yours, Laurenz Albe -- Cybertec | https://www.cybertec-postgresql.com
