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:
1 2 $ cat /proc/sys/vm/swappiness 60
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:
1 $ sysctl vm.swappiness=0
1 $ 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.
1 2 3 4 5 6 7 $ 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!
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