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ehsanul
Asked: 2011-01-27 12:20:23 +0800 CST

Too much memory being swapped out, while cache uses it all on Centos 5.4

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I've set swappiness on my DB server to zero, but during a large sort operation the swap space used grew even as the cache was still using a lot of memory, and then cache grew but swap was left alone. This makes no sense to me. Here's free -m as of now:

             total       used       free     shared    buffers     cached
Mem:         16000      15979         20          0         24      10520
-/+ buffers/cache:       5434      10565
Swap:         6111       5478        633

This is happening in the middle of a large sort in PostgreSQL. The issue is that the swapped out memory will probably be called upon as soon as the sort is over (it's a big UPDATE on a table I put into tmpfs), resulting in a huge slowdown as it will be random access.

linux centos cache postgresql swap

1 Answers

  1. Best Answer

    How much space are you using in your tmpfs?

    Bear in mind that tmpfs is backed by cache, and shows up in that statistic. So, the normal advice of "ignore cached and use the +/- line" doesn't necessarily apply.

    In fact, tmpfs is swappable, so it's very possible that that's what's getting swapped out as your real memory pressure increases.

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