Swappiness – Why Default is 60 and Effects of Lowering It

Actually there is no dphys-swapfile program on a default Ubuntu installation CD and it is not used to calculate the swap size.

What happens is that the ubiquity installer uses the partman-auto scripts and configuration files (called recipes) to determine the sizes of all partitions. It works like this:

• Partman finds the right recipe file according to the type of the computer and the option the user has chosen.
• There it finds the minimum and maximum size of the partition and its priority. For swap it can be 96 (min - in MB) 512 (priority) 300% (max).
• Then it gets the RAM size (via /proc/meminfo).
• It uses its own algorithm to calculate the sizes.

Details:

A recipe file can look like this:

1 1 1 free
    $iflabel{ gpt }
    method{ biosgrub } .

500 10000 -1 $default_filesystem
    $lvmok{ }
    method{ format }
    format{ }
    mountpoint{ / } .

96 512 300% linux-swap
    $lvmok{ }
    method{ swap }
    format{ } .

The algorithm to calculate the sizes:

for(i=1;i<=N;i++) {
   factor[i] = priority[i] - min[i];
}
ready = FALSE;
while (! ready) {
   minsum = min[1] + min[2] + ... + min[N];
   factsum = factor[1] + factor[2] + ... + factor[N];
   ready = TRUE;
   for(i=1;i<=N;i++) {
      x = min[i] + (free_space - minsum) * factor[i] / factsum;
      if (x > max[i])
         x = max[i];
      if (x != min[i]) {
         ready = FALSE;
         min[i] = x;
      }
   }
}

For more see:

• partman-auto-recipe.txt^{Way Back Machine}

• http://dev.blankonlinux.or.id/browser/pattimura/ubiquity/d-i/source/partman-auto/lib/recipes.sh?rev=pattimura%2Cubiquity%2C261 ^{DEAD LINK}

If you want to set the swapiness without rebooting you could execute the following command:

echo 10 > /proc/sys/vm/swappiness

This will set the swapiness to 10 in this case. You can only echo a number because all this will do is call a kernel function to set the swapiness to 10 and this function only accepts numbers (this explains why you get a invalid argument error: you're trying to write other characters than numbers).

However, this is only temporary. The value will be reset at the next reboot to the default value.

To make the change permanent:

1. Edit /etc/sysctl.conf as root

   sudo nano /etc/sysctl.conf
   

2. Add the following line to the file:

   vm.swappiness = 10
   

3. Save the file using CTRL + X

The changes made to sysctl.conf will apply on reboot or you can reload them using sudo sysctl -p (so sudo sysctl -p will have the same effect as echo 10 > /proc/sys/vm/swappiness in this case).