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<H1><A NAME="SECTION00650000000000000000">Allocating swap space</A></H1>
<A NAME="secswapalloc"> </A>
<P>
Some people will tell you that you should allocate twice as much
swap space as you have physical memory, but this is a bogus rule.
Here's how to do it properly:
<P>
<OL>
<LI>
<P>
Estimate your total memory needs. This is the largest amount of
memory you'll probably need at a time, that is the sum of the
memory requirements of all the programs you want to run at the
same time. This can be done by running at the same time all the
programs you are likely to ever be running at the same time.
<P>
For instance, if you want to run X, you should allocate
about 8 MB for it, gcc wants several megabytes (some
files need an unusually large amount, up to tens of
megabytes, but usually about four should do), and so on.
The kernel will use about a megabyte by itself, and the
usual shells and other small utilities perhaps a few
hundred kilobytes (say a megabyte together). There is
no need to try to be exact, rough estimates are fine,
but you might want to be on the pessimistic side.
<P>
Remember that if there are going to be several people
using the system at the same time, they are all going
to consume memory. However, if two people run the same
program at the same time, the total memory consumption
is usually not double, since code pages and shared
libraries exist only once.
<P>
The <tt>free</tt><A NAME="1924"> </A> and <tt>ps</tt><A NAME="1926"> </A> commands are useful for
estimating the memory needs.
<LI>
<P>
Add some security to the estimate in step 1. This is because
estimates of program sizes will probably be wrong, because
you'll probably forget some programs you want to run, and to
make certain that you have some extra space just in case. A
couple of megabytes should be fine. (It is better to allocate
too much than too little swap space, but there's no need to
over-do it and allocate the whole disk, since unused swap space
is wasted space; see later about adding more swap.) Also,
since it is nicer to deal with even numbers, you can round the
value up to the next full megabyte.
<LI>
<P>
Based on the computations above, you know how much memory
you'll be needing in total. So, in order to allocate
swap space, you just need to subtract the size of your
physical memory from the total memory needed, and you
know how much swap space you need. (On some versions
of UNIX, you need to allocate space for an image of the
physical memory as well, so the amount computed in step
2 is what you need and you shouldn't do the subtraction.)
<LI>
<P>
If your calculated swap space is very much larger than your
physical memory (more than a couple times larger), you should
probably invest in more physical memory, otherwise performance
will be too low.
<P>
</OL>
<P>
It's a good idea to have at least some swap space, even if
your calculations indicate that you need none. Linux uses
swap space somewhat aggressively, so that as much physical
memory as possible can be kept free. Linux will swap out
memory pages that have not been used, even if the memory
is not yet needed for anything. This avoids waiting for
swapping when it is needed--the swapping can be done
earlier, when the disk is otherwise idle.
<P>
Swap space can be divided among several disks. This
can sometimes improve performance, depending on the
relative speeds of the disks and the access patterns
of the disks. You might want to experiment with a few
schemes, but be aware that doing the experiments
properly is quite difficult. You should not believe
claims that any one scheme is superior to any other,
since it won't always be true.
<P>
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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