Is SWAP really needed?

[d4m1r]

Hey guys, I am gearing up for the release of 12.04 where I will be nuking my 32 bit 11.10 install and going with a copy of Precise x64. Anyway, when I setup this partition, I specifically disabled any SWAP space on purpose before really understand what it does. While I kinda understand it better now, I still see it like the Windows paging file and would argue that letting my system use RAM (I have plenty, 8GB) is MUCH quicker than forcing it to use my HDDs....Anyone care to disagree and explain why?



Even when I am doing "heavy" stuff under Ubuntu, I rarely exceed 1GB/8GB usage and never notice slow downs. It's why I am inclined to disable SWAP space for 12.04 as well.

[MG&TL]

You're not forcing it to do anything...swap is more of a backup plan for when you run out of memory...I think it's a good idea to have about a GB, but opinions differ. Make your own mind up, unless you have a pitiful amount of RAM, you probably won't notice anyway.

[d4m1r]

You're not forcing it to do anything...swap is more of a backup plan for when you run out of memory.

From what I've understood though, it doesn't just use it when it runs out of RAM though, no? Someone previously asked how much swap they should use for Ubuntu partition and they also had 8GB of ram, and someone told them 8.5GB...Is it possible even to use that much RAM under Ubuntu?

I am also pretty sure that as long as any space is allocating for swap, the system will use it, regardless of if RAM is maxed out or not...

[MG&TL]

It will use a little, yes, and I'm not entirely sure why. But it will definitely not use ALL of it, probably only a tiny percentage.

[jerome1232]

Ram unused by applications will get used as file cache, if your box is doing a lot of file operations and unused application data is sitting in ram, sometimes it will opt to swap out that application data for more file cache, and yes file cache WILL fill up your ram, quickly.

That's about it to my knowledge, swap is good to have (with out it your system will crash if it ever did run out of ram, memory leaks do happen)

[Haneef Mubarak]

I think this ought to explain it to you the best:

Just turn off swap (by setting swappiness to zero), restart your system, and try to use it. You will notice it will be slower. Then go set swappiness back to the previous setting and restart. Now it feels faster. See?

[sammiev]

For what I do my swap is only used well I suspend. Every user can and maybe different.

[QIII]

Actually, Haneef, swappiness may or may not make much difference with as much memory as people have now. A DBA probably needs to tune it. Or someone administering servers. With 8GB on a run of the mill desktop, I don't suspect that memory will be aggressively moved to swap because there will probably be no need. When we were all trying to run 17 applications on 256MB it was different. The kernel doesn't have to worry so much about the competition for memory (well, not as much.) any more.

A memory leak that would gobble 8GB before you caught it would be a real issue and would probably take leaking by design. I've done that for demonstration purposes. But leaks do happen when people are forgetful, I suppose.

Swap is necessary if you want to suspend to disk. 1.1 - 1.25x memory is sufficient. All you need is enough to maintain what is held in memory. Back when memory was at a premium we used to say 2x memory so you could preserve both the memory state and what was already in swap.

With memory as plentiful as it is, many people simply don't use a swap and suspend to RAM when they need to. If you have 16GB of memory and only ever use 5 - 10% of it, why waste the space on a partition that is of no ther use?

[d4m1r]

I was hoping for a more technical explanation but;

a) I never suspend my system (and yes, this wouldn't work well without swap space, just like my Windows 7 partition wouldn't suspend nicely because of no paging file)

b) I noticed better performance under Windows 7 without the paging file, and it is proven you will too if you have 4GB+ RAM.

Anyway, I will be poking around for linux literature on swap vs non-swap performance but my guess is that my eyes don't deceive me, and disabling swap = better performance (as long as your RAM is faster than your HDD, which is most likely the case).

[QIII]

Five or six years ago this was a real bone of contention.

My feeling is this:

For a desktop, it can't hurt but it will probably never be used.

For a server or a machine that hosts a large database, you'd better.