Sorry for late response, but may be you are still strugling.
It can be that some query(s) use a lot of work mem, either because of high work_mem setting or because of planner error. In this case the moment query runs it will need memory that will later be returned and become free. Usually this can be seen as active memory spike with a lot of free memory after.
2012/10/22 Shaun Thomas
<sthomas@optionshouse.com> Hey everyone!
This is pretty embarrassing, but I've never seen this before. This is our system's current memory allocation from 'free -m':
total used free buffers cached
Mem: 72485 58473 14012 3 34020
-/+ buffers/cache: 24449 48036
So, I've got 14GB of RAM that the OS is just refusing to use for disk or page cache. Does anyone know what might cause that?
Our uname -sir, for reference:
Linux 3.2.0-31-generic x86_64
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Best regards,
Vitalii Tymchyshyn