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My database server is using 191MB of swap space - but also shows 182MB of free RAM

The box is a dedicated MySQL server, running Debian Lenny.

How can I determine who and why is filling the swap space instead of using the free RAM?

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4 Answers 4

up vote 11 down vote accepted

Enter top, then hit f to edit the visible fields, hit p to show swap usage, then hit Enter to get back to the display of programs. Use Shift+< and > to get it to sort by swap usage.

Linux swaps data from RAM based on how likely it is (in Linux's personified opinion) that it will need to be referred to any time soon. You can alter your swappiness to increase/decrease the likelihood of this happening.

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Good explanation. One additional point, though. It's not a bad thing that you're seeing some swap usage. As James said, the kernel will opportunistically swap out memory pages that are unlikely to be used. This frees up RAM for other apps that may need it, and will only in very rare occasions induce performance penalties. –  EEAA Nov 14 '10 at 22:02

That's not easy to answer - take a look here: https://help.ubuntu.com/community/SwapFaq.

You can use top command to see the swap usage, type man top or google for it.

As for why - one possible situation is the following. Say you have 1GB or RAM. Then you load 2GB worth of applications (including OS into this). You will have 1GB RAM used and 1GB swap used. Then you unload 1.5GB of applications. You would assume you would have 0.5GB RAM free and all swap free. However, it completely might not be the case. For example, if you loaded 3 applications:

  • A - 1 GB,
  • B - 900 MB
  • C - 100 MB

then used A actively, it's probable that both B and C would be swapped out to disk, to make A use RAM and thus fast. When you close A and switch to C, you would have 100 MB RAM used and B still swapped out (900 MB swap used), as it is not needed.

To say it simple - kernel tries to use RAM and swap the way it thinks it is going to be efficient. Swap usage while there is still free RAM doesn't necessarily mean your system is working slower then it could.

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tl;dr the page/swap is not simply a buffer for when all RAM is consumed. Memory manager utilizes it complex ways and having stuff in the swap file before RAM is filled is not a bad thing. –  Chris Thorpe Nov 15 '10 at 0:16
free -m

Look at the +/- buffers line for your true ram minus the filesystem buffers. Modern OS's load commonly used files into memory and the kernel will release them as needed.

For example

             total       used       free     shared    buffers     cached
Mem:          3483       1702       1780          0        299        778
-/+ buffers/cache:        623       2859
Swap:         3999          0       3999

Running top I would see I have 1.7gigs free of memory but if i look at free -m i see I really have 2.8gigs free. The other 1.1 gig is filesystem cache.

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Just a point about swapped memory, to support ErikA's comment to James' answer:

It's not a bad thing that you're seeing some swap usage. As James said, the kernel will opportunistically swap out memory pages that are unlikely to be used. This frees up RAM for other apps that may need it, and will only in very rare occasions induce performance penalties.

Data swapped out of RAM will exist both in RAM and on the swap disk until the RAM is claimed for another purpose, so are rather like mmap'ed files. As I understand it, if a process then decides to use this data before it is reclaimed, it therefore does not have to be swapped in again. Swapping is usually good, but there are two main reasons why you might not want it:

  1. You are trying to conserve power: eagerly swapping out will generally increase the amount of disk spinning, since some propertion of swapping out will not be needed;
  2. The swap might improve the zippiness of a process whose performance you might not care about, at the expense of a process whose latency is an issue.

Opportunistic swapping can be eliminated with echo 0 > /proc/pid/vm/swappiness.

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Excellent point - There is a common mis-understanding that pages are either "in RAM" or "Swapped" when they are often in both at the same time. Lots of swap used does not imply that more RAM could be used instead. –  Helvick Nov 19 '10 at 11:14

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