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How Big Should My Swap Partition Be?
For the last 10 years, I have been asking people more knowledgeable than I, "How big should my swap be?" and the answer has always been "Just set it to twice your RAM and forget about it." In the old days, it wasn't much to think about — 128 megs of RAM means 256 megs of swap. Now that I have 4 gigs of RAM in my laptop, I find myself wondering, "Is 8 gigs of swap really necessary?" How much swap does the average desktop user really need? Does the whole "twice your RAM" rule still apply? If so, for how much longer will it likely apply? Or will it always apply? Or have I been consistently misinformed over the last 10 years?
'Is 8 gigs of swap really necessary?'
With a 750GB hard drive selling under $100, what has changed?
... and 8GB of space is still trivial with a 750GB hard drive.
Yeah, your 256MB of space was trivial when you had a 30GB hard drive
That said, I'll forward you some common information on paging.
Linux and other Unix-like operating systems use the term "swap" to describe both the act of moving memory pages between RAM and disk, and the region of a disk the pages are stored on. It is common to use a whole partition of a hard disk for swapping. However, with the 2.6 Linux kernel, swap files are just as fast as swap partitions, although Red Hat recommends using a swap partition. The administrative flexibility of swap files outweighs that of partitions; since modern high capacity hard drives can remap physical sectors, no partition is guaranteed to be contiguous.
I'm no expert but the short answer to this is to look at your swap partition as your extended virtual memory. By saying that your swap partition should be 2x your main memory is like saying that you will never use 3x of what your main memory is (in this case 12GB). While that rule of thumb is a good one, there may in fact be applications today in the graphics and processing world that require insane amounts of memory. While Firefox is probably never going to reach that critical mass (nor will most average programs) it's probable that a few years from now it will be common place. I know it's insane to think of but 'ought to be enough for anybody' is not the phrase you want to throw around in the digital information world.
It's those days when I'm playing Warcraft through wine, listening to streaming radio through Amarok, have 20 windows open behind it, idling a LAMP server for my development projects, running a vent client, some form of news aggregater, pidgin & an e-mail client hooked up to several POP3/IMAP accounts that I am happy I erred on the side of a whole ton of swap space.
My work here is dung.
The origin of the 'twice real RAM' came in the early days of windows, in which windows could not use any swap unless you had at least as much as real RAM. That's been gone for ages now - and you should actively avoid too much swap.
If you allocate, say, 8G of swap for 4G of RAM, most of the time almost all of it will go unused. If it actually /is/ used, your machine has probably spent the past hour or so frantically swapping to try to accomidate this 12G request; ie, your system is completely unresponsive due to every program being mostly swapped out. The additional swap merely delays the out of memory event, and in the meantime you can't control the machine.
Swap is still useful for holding data that's not part of the working set, in order to free memory for cache; but this shouldn't be very much RAM (256-512mb should be enough). It's also useful for software suspend on linux - if you have a laptop, make it a little bit larger than physical RAM. And always have /some/ - linux's memory manager doesn't like having none.
If you were running Oracle - here is what they recommend:
RAM -> Swap Space
1 GB - 2 GB -> 1.5 times the size of RAM
2 GB - 8 GB -> Equal to the size of RAM
more than 8GB -> 0.75 times the size of RAM
I don't know if this would carry across to general computing - it seems to me if it's enough for an Oracle RDBMS server, it ought to do it for most things.
It's hard to believe that's how Micronians are made. Why don't we see it right now by having you both kiss one another?
Reading through OpenBSD's FAQ:
"The 'b' partition of your root drive automatically becomes your system swap partition. Many people follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. This rule is nonsense. On a modern system, that's a LOT of swap, most people prefer that their systems never swap. You don't want your system to ever run out of RAM+swap, but you usually would rather have enough RAM in the system so it doesn't need to swap. If you are using a flash device for disk, you probably want no swap partition at all. Use what is appropriate for your needs. If you guess wrong, you can add another swap partition in /etc/fstab or swap to a file later."
HTH.
Just make a note of your virtual memory use every hour or so (or just whenever you remember) for a few days/weeks. Then just give yourself maybe 2-3 times the peak usage.
I imagine different people will need different amounts of swap space, so use a size that's right for you.
If you're debugging your kernel or are helping people to debug your kernel, and are generating crashdumps either manually or as a result of kernel panic, you need your swap to be twice as big as the memory so it all fits comfortably (You can probably get away with X times bigger, where 1X2, but 2 is a safe number).
To my understanding that's always been the reason for the rule of thumb about doubling the memory. If you can afford the disk, go for it, because you never know when you might hit a panic and need crashdumps. If you are in a live environment and are sure you will never, ever need or even want crash dumps, and the disk space is at a premium, you can size it based on need.
Another thing to keep in mind is that as you have more ram, you have more pages, and the whole point of swap is to get pages to disk as well in case you need to free up physical ram quickly.
-bugg
Whatever you do, you need to remember to setup you swap partition to as large or bigger than your ram in order to be able to use the "suspend to disk" function in Linux. On older laptops suspend is sometimes handled by the bios. Then you need a special partition. But nowdays Linux just suspends to your swap. And if your memory was full ...
Forget the RAM X 2 rule. Capacity of drives are way up, base RAM load is way up. Drive transfer speed isn't up very much. Doesn't really matter how much ram you have, long before you get a Gig of swap utilized the system is going to be trashing to the point of being unusable under any but lab conditions.
Running with no swap can cause some problems, because it does help if the system can push out blocks of memory that aren't backed by a file and also haven't been used for awhile. Still on an all flash system with an adequate amount of RAM running without swap is probably the right move. On a machine with a spinning disc give it a 1GB swap and forget it.
The exception being in cases where the a system is doing suspend to disc into the swap. I don't have any Linux machines that will do suspend to disc so don't ask me about any details.
Democrat delenda est
In the end, it depends. If you are running several memory intensive applications you're going to want more swap space. At the very least, you should have as much as your RAM because when you hibernate it takes all the pages in your RAM and puts it into the swap space before powering off your computer. I wrote about this a while ago: http://www.bytetrap.com/blog/2008/06/02/swap-space-linux/
2X RAM was the standard rule of thumb at Sun, for SunOS long before Windows was around.
If anything, Microsoft ripped it off from Sun.
Well, I do occasionally need more than 2GB of RAM, without there being a memory leak. I've been running GIS programs, an IDE, a couple of RDBMSs, and then I fire up the old compression program...
Which brings me to my point. The question "how much swap do I need" is probably meaningless, even for a given amount of memory. There are people who find 2GB with no swap fine, and others, like me, who probably could get by with 2GB of RAM and maybe 512MB of swap, and others who might need more.
I think the 2x RAM rule of thumb has one virtue: excepting certain exotic kinds of systems, it's fairly safe that anybody who finds themselves needing more than that is probably feeling a world of pain that can only be fixed by getting more RAM. On the other hand, in most cases 2x RAM amounts to a trivial amount of disk. Probably most people could get by with 25% of RAM, but the value of thinking about whether that is true for you is very likely less than the cost of the disk space.
Common sense applies. If you have some kind of scientific computing device with a gazillion bytes of RAM, your swap requirements might not be related to your maximum RAM requirements at all. If you're running some kind of operating system that launches a bunch of rarely used garbage, you probably ought to think about your swap. I had awful problems with Vista until I figured out the page file Windows created had something like eight thousand fragments. I was actually better off getting rid of the page file
Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
I've been setting up machines with no swap partitions for a few years. Swap partitions have a bad habit if collecting secure info you may have assumed was just in ram. All modern operating systems allow to you use a file or other blank space as swap means you don't need a dedicated partition. There is also the issues that if your starting to swap, where does it end? If your swapping on a machine with 4 or 8 gig of ram, will an extra gig help fix what ever is broken or just make the machine very slow until it gets around to telling the runaway program that there is no more memory. In the case of no swap, that tends to happen much faster. The only reason I see for swap partitions is that the OS will need a place to dump debug info if it crashes and the swap partition has traditionally been used for that.
8GB swap on a 120GB drive is 7%, not .07%. On a 200GB drive, it's 4%, not .04%, etc.
SirWired
Is there any point to separate partitions for / and /home? I mean, if you were running different file systems on each of them I could see the point.
I have gone through four different version of Linux on my laptop: mandrake/mandriva -> fedora -> knoppix -> ubuntu. Guess how many times I've thanked 8 lb 6 oz baby Jesus that I had the foresight to separate the two? All my data from my college days is still intact under /home.
For this simple reason, I heavily recommend it.
My work here is dung.
Oh dear FSM, Please for the sake of everyones sanity, NEVER LET WINDOWS GROW THE SWAPFILE! besides the fact it it will fragment the pagefile, it will also completely lock up the computer for X amount of time... right when you need it most! ...it ALWAYS happens at a bad time.
Modding Trolls +1 inciteful since 1999
I also agree that the old "2 x RAM" standard is outdated.
If you are a typical desktop user--browsing, email, games, etc, you will likely never swap. If you happen to edit photos a lot then you'll use a bit more. In these cases doing 4GB swap for 4GB RAM should be more than sufficient, and even then overkill.
If you are a serous 'power' desktop user, heavy graphics / video editing or similar heavy-duty tasks, you will likely have significantly more RAM. If you ever did swap things would become so slow your productivity would be severely hampered.
Were you talking about a server I'd say the same thing. Your swap space on an active server (thinking database or application server) is really just there to keep you operational should some process go haywire, long enough for you to fix it. If you are regularly swapping on a server then you need to upgrade your RAM or adjust your software on it.
It does hurt to allocate a couple of gigs of swap.
:). If you have lots of RAM and are too lazy to guess, set F=0.
I use swap only to tell me that I'm low on RAM. Basically once the machine starts using swap and getting slightly slow- it means I'm low, then I can try to shut down stuff (without it behaving otherwise strangely, or dying abruptly).
Here's how I suggest you figure out _roughly_ how much swap you need.
1) Figure out the amount of Virtual Memory your programs and services _allocate_ without really _using_ - call this F. There are some programs that allocate hundreds of MB of memory but never use it. But note that there are some programs that allocate lots of memory and may use it
2) Figure out your drive throughput for swap access (swap in + swap out)- this is often related to random access throughput - and for a typical hard drive it could be in the order of magnitude of 10MB/sec - call this M. Note that many flash drives have pathetic random write speeds of 4MB/sec (or even less!).
3) Figure out the time you are willing to wait for stuff to swap in and out (e.g. time to get an ssh prompt- call this T.
Swap = F + T * M.
So for example, if you have programs that allocate a total of 100MB and never use it, and your drive swap throughput is 10MB/sec and the amount of time you're willing to wait is 15 seconds.
Swap = 100MB + 15 sec * 10MB/sec = 250MB.
As you can see allocating gigabytes can hurt - since it'll take days to swap in and out processes that are using gigabytes of swap. You'll run out of time before you run out of swap, and when that happens somebody will do a hard shutdown of the machine - and that means ALL processes will be abnormally terminated, rather than just one.
Yes, there are cases where the offending program might not keep accessing all of that swap, but when a program misbehaves like that, you'd rather find out sooner rather than have to shutdown the whole computer (because it takes ages to respond).
Running programs from swap is best reserved for those who wish to experience the 1950s drum memory days. If you want to do retrocomputing keep in mind that memory speeds are now much faster than disk speeds, whereas in the 1950s memory speed = drum speed, and most modern programs assume modern memory speeds.
Why does everyone put their swap on a slow harddrive ? A Gentoo running mate of mine in the pub showed me how to map the swap file into RAM: runs much faster there.
(Although suspend does not seem to work now :-(