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Improve our mechanism for controlling the Linux out-of-memory killer.

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Arrange for postmaster child processes to respond to two environment
variables, PG_OOM_ADJUST_FILE and PG_OOM_ADJUST_VALUE, to determine whether
they reset their OOM score adjustments and if so to what.  This is superior
to the previous design involving #ifdef's in several ways.  The behavior is
now available in a default build, and both ends of the adjustment --- the
original adjustment of the postmaster's level and the subsequent
readjustment by child processes --- can now be controlled in one place,
namely the postmaster launch script.  So it's no longer necessary for the
launch script to act on faith that the server was compiled with the
appropriate options.  In addition, if someone wants to use an OOM score
other than zero for the child processes, that doesn't take a recompile
anymore; and we no longer have to cater separately to the two different
historical kernel APIs for this adjustment.

Gurjeet Singh, somewhat revised by me
This commit is contained in:
Tom Lane
2014-06-18 20:12:47 -04:00
parent 960661980b
commit df8b7bc9ff
3 changed files with 45 additions and 55 deletions
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26
doc/src/sgml/runtime.sgml
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@ -1275,7 +1275,7 @@ sysctl -w vm.overcommit_memory=2
<para>
Another approach, which can be used with or without altering
<varname>vm.overcommit_memory</>, is to set the process-specific
<varname>oom_score_adj</> value for the postmaster process to
<firstterm>OOM score adjustment</> value for the postmaster process to
<literal>-1000</>, thereby guaranteeing it will not be targeted by the OOM
killer. The simplest way to do this is to execute
<programlisting>
@ -1284,20 +1284,28 @@ echo -1000 > /proc/self/oom_score_adj
in the postmaster's startup script just before invoking the postmaster.
Note that this action must be done as root, or it will have no effect;
so a root-owned startup script is the easiest place to do it. If you
do this, you may also wish to build <productname>PostgreSQL</>
with <literal>-DLINUX_OOM_SCORE_ADJ=0</> added to <varname>CPPFLAGS</>.
That will cause postmaster child processes to run with the normal
<varname>oom_score_adj</> value of zero, so that the OOM killer can still
target them at need.
do this, you should also set these environment variables in the startup
script before invoking the postmaster:
<programlisting>
export PG_OOM_ADJUST_FILE=/proc/self/oom_score_adj
export PG_OOM_ADJUST_VALUE=0
</programlisting>
These settings will cause postmaster child processes to run with the
normal OOM score adjustment of zero, so that the OOM killer can still
target them at need. You could use some other value for
<envar>PG_OOM_ADJUST_VALUE</> if you want the child processes to run
with some other OOM score adjustment. (<envar>PG_OOM_ADJUST_VALUE</>
can also be omitted, in which case it defaults to zero.) If you do not
set <envar>PG_OOM_ADJUST_FILE</>, the child processes will run with the
same OOM score adjustment as the postmaster, which is unwise since the
whole point is to ensure that the postmaster has a preferential setting.
</para>
<para>
Older Linux kernels do not offer <filename>/proc/self/oom_score_adj</>,
but may have a previous version of the same functionality called
<filename>/proc/self/oom_adj</>. This works the same except the disable
value is <literal>-17</> not <literal>-1000</>. The corresponding
build flag for <productname>PostgreSQL</> is
<literal>-DLINUX_OOM_ADJ=0</>.
value is <literal>-17</> not <literal>-1000</>.
</para>
<note>