Re: Newbie Question About Swap Partition

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Date: Tue, 24 Aug 2004 17:25:15 -0600

On Tue, 2004-08-24 at 12:33 -0700, Google Mike wrote:
> I have standard RH9 Linux with standard ext3 filesystem.
>
> I have 512 MB of RAM and plan to go to 1GB of RAM tomorrow with a new
> RAM chip. My swap partition is set to approximately double the RAM
> size.
>
> 1. Do I need to type some command after adding RAM to manually
> increase the size of the swap partition?

The swap partition will never, ever just change size automatically; you
will always need to manually trigger any changes to it.

> 2. Will the swap partition automatically increase and I won't need to
> type a command manually?

No, it will not increase, or decrease; see above.

> 3. Can I increase it without risk to whatever else is on the disk, if
> I have available space?

That depends on exactly how your disk is laid out; if you're at all
unsure, I'd say no. Otherwise, find a local guru and ask them to show
you (not 'do it for you'). Changing the size of the swap partition
generally involves pulling out a partitioning tool, backing up your
entire disk, and repartitioning the drive; unless you really need to, I
wouldn't bother. It sounds like you already have a swap partition that
is easily big enough for anything you'd probably need to do.

> 4. What size should it be? Should it be RAM x2 + 64MB, or is there a
> more preferrable setting?

The magic formula used to be 'RAMx2', but that's not really a good way
of judging it. The real question you want to ask now is, "Do I need more
than 2GB of memory?", because that's what you've got with 1GB RAM and
1GB swap. Remember, though, that accessing swap is many, many times
slower than RAM. Chances are good you could probably reduce your swap
partition and nothing would even notice, even now before you add the
extra 512MB. Think of swap as 'emergency memory' which the system can
use when it runs out of RAM so that it doesn't completely flake out.

Assuming typical desktop use, an average Linux system, today, generally
requires at least 256MB of total memory (that's both RAM and swap
combined), with at least half of that being real RAM instead of swap;
real RAM is always better than swap, when it can be afforded. With 512MB
or more of RAM, you probably won't ever need more than 256MB of swap.
However, this all depends on exactly what you do.

If you run a lot of applications at once, or applications that consume a
lot of RAM (eg: GIMP, editing a 4000x4000x32bit image), you'll want to
set up a relatively large swap partition, to ensure that the system has
plenty of combined memory for those times that it needs it.

As a general rule, however, don't ever set up a swap partition that is
more than double your available RAM; it's not that it will hurt
anything, but it is pointless. If you have 1GB of RAM, and (therefore,
following the swap=RAMx2 rule) set up a 2GB swap, you have a combined
total of 3GB of memory. Before your system comes anywhere near consuming
that 3GB, it's performance will have slowed to a crawl, as it spends
most of its time paging memory in and out of the swap partition
(remember, swap is hundreds, if not thousands of times slower than RAM).
In fact, chances are good that by the time your hypothetical system has
consumed a mere 1GB of its swap space, it will have slowed down to
unusability, perhaps even acting like it has frozen.

The general rule of thumb I'm seeing today is: set your swap partition
to be equal to your RAM, assuming you have 512MB or more; also, don't
ever bother making your swap partition larger than 1GB, especially if
you have more than 1GB of RAM. And this is only if you're a hard-core
power user who runs applications that consume memory like water. If not,
a swap partition no larger than 512MB is probably more than sufficient.

If you have less than 512MB, the old RAMx2 formula is probably good
enough for most uses.

Personally, I've got my swap set to 1GB, but that's because hard drive
space is dirt cheap nowadays, so it doesn't really hurt. With 1GB of
RAM, though, I've rarely seen it use more than 200Kb of my swap space.
Ever. Even when I *was* editing those huge 4000x4000 images in the GIMP
(to be fair, though, GIMP 2 does a pretty good job of paging itself out
to disk without relying on the swap partition, so this may not be a good
example).

Remember though, real RAM is *always* better than swap, when you can
afford it.

Finally, I'd recommend using swap partitions over swap files, but that's
just me. Swap files just strike me as being too easy to screw with,
although that might just be my Windows bias seeping through.

>
> Thank you.

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