What is the default swap size?

Question

How does the installer calculate what size of swap partition to create? Presumably it's based on installed RAM size, but does that depend on how much RAM?

Answer 1

This is going to be rather technical but was fun to find out so here goes...

• I found a package dphys-swapfile and here is the Source code. Option setup is used to set it up:

setup - tells dphys-swapfile  to compute the optimal swap file size and
(re-)generate an fitting swap file. Default it 2 times RAM size.
This  can  be called at boot time, so the file allways stays the
right size for current RAM, or run by hand whenever RAM size has
changed.

• Inside dphys-swapfile is a setting conf_swapsize for a pre-defined size (in Natty this is empty):

size we want to force it to be, default (empty) gives 2*RAM
CONF_SWAPSIZE=

and a setting for a swapfactor...

this is what we want, 2 times RAM size
SWAPFACTOR=2

• The actual size is calculated a bit further on:

  if [ "${CONF_SWAPSIZE}" = "" ] ; then
      # compute automatic optimal size
      echo -n "computing size, "
      # this seems to be the nearest to physical RAM size, lacks about 60k
      KCORESIZE="`ls -l /proc/kcore | awk '{ print $5 }'`"
      # make MBytes which rounded down will be exactly 1 too few, so add 1
      MEMSIZE="`expr "${KCORESIZE}" / 1048576 + 1`"
      # default, without config file overwriding, swap=2*RAM
      CONF_SWAPSIZE="`expr "${MEMSIZE}" '*' "${SWAPFACTOR}"`"
    fi

As you can see the way they calculate it in this package depends on the size of /proc/kcore, then gets divided by 1048576, +1 to round it up and then gets multiplied by swapfactor. From my machine:

So the default for this system would be 1065349120 / 1048576 = 1015+1 = 1016 * 2 = 2032 MBytes.

Answer 2

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}