by April 09, 2009

Does your Linux-based virtualized hosting feel a little slow? Does it suffer from preemptive swapping? Here's a quick tip on getting some more performance out of your current setup. This is especially useful if you have a significant ratio of physical RAM to in-memory programs.


The Linux 2.6 kernel has a parameter called vm.swappiness that regulates the kernel's likelihood to swap memory to disk (e.g., to free up memory for disk/content caching, load other programs, etc). Valid values are between 0 and 100. On the current release of Ubuntu (Intrepid) this value has a default value of 60 that you can investigate with the following command:

$ cat /proc/sys/vm/swappiness

This value is generally fine, but if your physical memory is significantly higher than what you need it's worth investigating other choices.I personally find a pretty significant performance increase in Rails behavior with a value of 0; essentially telling the kernel to not swap anything out until it becomes absolutely necessary.

The value can be changed either with:

$ sysctl vm.swappiness=0


$ echo 0 > /proc/sys/vm/swappiness

A persistent lack of swappiness

If you restart the server, then your change will be lost. If you want the change to remain persistent across restarts you'll either need to create a script that calls one of these commands or edit /etc/sysctl.conf to specify the swappiness. As always, exercise caution playing with kernel parameters :-).

Swapping it back in

If you change vm.swappiness and want to force the kernel to swap everything back in you can temporarily disable the swap partition and then immediately re-enable it.

$ swapon -s

Filename         Type      Size  Used  Priority
/dev/sda2         partition    524280 33948  -2

$ swapoff /dev/sda2
$ swapon /dev/sda2

If the machine doesn't have enough memory to accommodate the swap-in the command will fail with an error.

Let us know if this helps you out!

Curtis Miller

Curtis Miller

Managing Partner

Startup junkie, Rubyist and gamer. Loves to brainstorm about new ideas.

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