How much swap for 4GB RAM, if any?
 

Slackware 14.0

In the Slackware-HOWTO file, Volkerdi says: "If you machine doesn't have a lot of RAM, you'll want another partition for swap space". Does this mean that if you do (have a lot of RAM) then you do not need a swap partition? I'm installing 14.0 and the machine has 4GB RAM.

I have 4GB of ram.
I have a swap of 4GB to allow hibernating my laptop, but other than that, it doesn't get used.
Obviously, it depends how you use YOUR pc, so maybe set one up & if you find it is not useful, you can remove it later.
Or go without, and if it's a problem, add some later.

Choice is good :D

The two choices are in fact one and the same for, if I allocate only 1G thinking I have time later to add 1G more, where do I get that 1G extra once I partioned the whole disk? Unless I add a 2nd disk.

You can shrink partitions to make more space.

Its probably easier to just setup 4GB swap space from the start. Unless you've got a small HDD its what I would do.

You can also use swap files, they have a slight performance penalty but they do work.

Well, with 4GB RAM and 500GB disk, following a rule of thumb I found somewhere (2 * RAM + 1GB for exceptional situations than can slow down the machine while using GUI) I created 9GB swap. Now, 9 / 500 = 1.8 per cent of total disk, which seems to be steeling it not to much space.

Later on, I'll consider if I shrink it to a smaller size, a thing easily done with (c)fdisk by erasing first the whole partition and then creating it the new size. Thanks for giving your valuable opinion which, on the other hand, matches that of Volkerdi in the Slackware-HOWTO, when he speaks about swap size = RAM size.

The only reason for a swap partition is to suspend to disk / hibernate. There is no other reason, so if you don't suspend to disk there is no need for swap.

Those ancient rules of thumbs were useless in the beginning and are useless now. There is only one way to determine the amount of swap you need: You have to test it with your workload. On a modern machine with normal desktop use you are safe with just using the amount of RAM, if you want to use hibernate, or just use 1GB, which is enough for most cases and doesn't matter on modern disk sizes.

That is not really true. If you need swap still depends on your RAM. I have a laptop that has 2GB of RAM and it has a swap partition, that is used by the system to swap out RAM that is used by applications running in the background, but are almost always idling. This speeds up the system, since it makes more space for caching files in RAM. With systems in the range <4GB RAM, IMHO, this is useful and desired behavior.
I even have a 1GB swap partition on my main machine with 16GB RAM and it is occasionally used.

In short: If you need a swap partition and what its size should be can only be determined by your size of RAM and the workload you intent to run. General thumbs of rule are pretty worthless, but we can give you at least some recommendations from experience.

Is there a log that monitors swap usage at all?

Just for your info, a 500GB HDD isnt actually 500GB. Well, technically it is 500GB if you go for that MB/MiB 1000x1000x1000/1024x1024x1024 nonsense. I dont.

A 500GB HDD will actually be about 465GB. Or 465GiB if you like the whole MB/MiB thing.

http://en.wikipedia.org/wiki/Mebibyte
http://en.wikipedia.org/wiki/Megabyte

*edit- the 'swap = 2 x RAM' idea is very old, and there has been people pointing out that for years now-

http://www.cyberciti.biz/tips/linux-swap-space.html

Every time I see people say that 'swap size should be RAM x 2' I shudder. Its been repeated so many times that its even more annoying to me than seeing people post 'how tos' for debian using sudo commands....

That is what bothers me. The dependence of swap size on RAM size is nice, because this does not vary. But, I cannot guess the type of application I'll give my machine the day of tomorrow (= some day in the future). So I must provide for the worst scenario, i.e., maximum workload. Of course, it would be a good thing for me what are the kind of jobs which imposes great workloads. Perhaps my machine will be doing such jobs not even in its wildest dreams.

A valid question is, I think: can too much swap space conspire against system performance? If the answer is "no", what do I lose by using ten times the RAM if my disk has the space?

The only thing you lose is the disk space.

There shouldn't be a performance impact with having much swap space. But consider this: If you come up in the future with a workload that makes your machine use huge amounts of swap you would be better of with buying a more RAM, since using swap for large chunks of memory will have a serious impact on performance. RAM is cheap nowadays and there is only one thing that is better than RAM: more RAM.

The reason why I don't use swap anymore is because when the system runs out of RAM, because some program eats it up, I do NOT want my computer to slow to a crawl as it is swapping like mad. I just want it to kill the damn program and let me get back to doing what I was doing. I have messed with the various swap settings and I have decided that having swap in the first place is not very useful and in fact may be wasteful in most cases -- a waste of space and time waiting for your computer to stop swapping. I do not have enough applications running in the background and using any significant amount of RAM. If you run a server, then things may be different, but you will also have a lot more RAM.

My conclusion is for a desktop computer that does not suspend to disk, there is no use for swap.

I've always been told you takes the RAM size and Double it to get your Swap size.

4GB RAM = 8 GB Swap Partition