Israel Brewster <israel@ravnalaska.net> writes:
> So just a bit ago I ran into a bit of excitement when the kernel decided
> to kill one of my postmaster processes due to an out-of-memory issue,
Fun :-(
> So a single postmaster process was using over 72GB of ram.
No, the kernel was blaming it for 72GB, which is an entirely different
statement. The Linux OOM killer makes some assumptions that are
ludicrously wrong for Postgres: not only does it blame a parent process
for the total memory consumption of all its children, but if the children
share a large shared memory segment, *it counts the shared memory segment
over again for each child*. At least this was true last I looked;
perhaps very recent kernels are a bit less insane about shared memory.
In any case, the core problem is blaming the parent process for the
sins of a child.
Now the PG postmaster itself consumes very little memory, and this is
quite unlikely to suddenly go wrong because it doesn't do very much.
A child backend process might go crazy, but what you want to happen then
is for the OOM killer to kill the child process not the postmaster.
That will still result in a database crash/restart scenario, but as long
as the postmaster is alive everything should recover automatically.
Your problem, then, is that the OOM killer is egregiously and with malice
aforethought killing the wrong process.
The usual fix for this is to configure things so that the postmaster is
excluded from OOM kill but its children aren't. See
https://www.postgresql.org/docs/current/static/kernel-resources.html#LINUX-MEMORY-OVERCOMMIT
(but be sure to consult the page for your PG version, as we've changed
the support mechanism for that in the past.)
If you're using a vendor-supplied packaging of PG and it doesn't have some
easy way to turn on this behavior, complain to the vendor ...
regards, tom lane
