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By Gary Sims
Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.
Swapping is necessary for two important reasons. First, when the system requires more memory than is physically available, the kernel swaps out less used pages and gives memory to the current application (process) that needs the memory immediately. Second, a significant number of the pages used by an application during its startup phase may only be used for initialization and then never used again. The system can swap out those pages and free the memory for other applications or even for the disk cache.
However, swapping does have a downside. Compared to memory, disks are very slow. Memory speeds can be measured in nanoseconds, while disks are measured in milliseconds, so accessing the disk can be tens of thousands times slower than accessing physical memory. The more swapping that occurs, the slower your system will be. Sometimes excessive swapping or thrashing occurs where a page is swapped out and then very soon swapped in and then swapped out again and so on. In such situations the system is struggling to find free memory and keep applications running at the same time. In this case only adding more RAM will help.
Linux has two forms of swap space: the swap partition and the swap file. The swap partition is an independent section of the hard disk used solely for swapping; no other files can reside there. The swap file is a special file in the filesystem that resides amongst your system and data files.
To see what swap space you have, use the command swapon -s. The output will look something like this:
Filename Type Size Used Priority
/dev/sda5 partition 859436 0 -1
Each line lists a separate swap space being used by the system. Here, the 'Type' field indicates that this swap space is a partition rather than a file, and from 'Filename' we see that it is on the disk sda5. The 'Size' is listed in kilobytes, and the 'Used' field tells us how many kilobytes of swap space has been used (in this case none). 'Priority' tells Linux which swap space to use first. One great thing about the Linux swapping subsystem is that if you mount two (or more) swap spaces (preferably on two different devices) with the same priority, Linux will interleave its swapping activity between them, which can greatly increase swapping performance.
To add an extra swap partition to your system, you first need to prepare it. Step one is to ensure that the partition is marked as a swap partition and step two is to make the swap filesystem. To check that the partition is marked for swap, run as root:
fdisk -l /dev/hdb
Replace /dev/hdb with the device of the hard disk on your system with the swap partition on it. You should see output that looks like this:
Device Boot Start End Blocks Id System
/dev/hdb1 2328 2434 859446 82 Linux swap / Solaris
If the partition isn't marked as swap you will need to alter it by running fdisk and using the 't' menu option. Be careful when working with partitions -- you don't want to delete important partitions by mistake or change the id of your system partition to swap by mistake. All data on a swap partition will be lost, so double-check every change you make. Also note that Solaris uses the same ID as Linux swap space for its partitions, so be careful not to kill your Solaris partitions by mistake.
Once a partition is marked as swap, you need to prepare it using the mkswap (make swap) command as root:
mkswap /dev/hdb1
If you see no errors, your swap space is ready to use. To activate it immediately, type:
swapon /dev/hdb1
You can verify that it is being used by running swapon -s. To mount the swap space automatically at boot time, you must add an entry to the /etc/fstab file, which contains a list of filesystems and swap spaces that need to be mounted at boot up. The format of each line is:
Since swap space is a special type of filesystem, many of these parameters aren't applicable. For swap space, add:
/dev/hdb1 none swap sw 0 0
where /dev/hdb1 is the swap partition. It doesn't have a specific mount point, hence none. It is of type swap with options of sw, and the last two parameters aren't used so they are entered as 0.
To check that your swap space is being automatically mounted without having to reboot, you can run the swapoff -a command (which turns off all swap spaces) and then swapon -a (which mounts all swap spaces listed in the /etc/fstab file) and then check it with swapon -s.
Swap file
As well as the swap partition, Linux also supports a swap file that you can create, prepare, and mount in a fashion similar to that of a swap partition. The advantage of swap files is that you don't need to find an empty partition or repartition a disk to add additional swap space.
To create a swap file, use the dd command to create an empty file. To create a 1GB file, type:
dd if=/dev/zero of=/swapfile bs=1024 count=1048576
/swapfile is the name of the swap file, and the count of 1048576 is the size in kilobytes (i.e. 1GB).
Prepare the swap file using mkswap just as you would a partition, but this time use the name of the swap file:
mkswap /swapfile
And similarly, mount it using the swapon command: swapon /swapfile.
The /etc/fstab entry for a swap file would look like this:
/swapfile none swap sw 0 0
How big should my swap space be?
It is possible to run a Linux system without a swap space, and the system will run well if you have a large amount of memory -- but if you run out of physical memory then the system will crash, as it has nothing else it can do, so it is advisable to have a swap space, especially since disk space is relatively cheap.
The key question is how much? Older versions of Unix-type operating systems (such as Sun OS and Ultrix) demanded a swap space of two to three times that of physical memory. Modern implementations (such as Linux) don't require that much, but they can use it if you configure it. A rule of thumb is as follows: 1) for a desktop system, use a swap space of double system memory, as it will allow you to run a large number of applications (many of which may will be idle and easily swapped), making more RAM available for the active applications; 2) for a server, have a smaller amount of swap available (say half of physical memory) so that you have some flexibility for swapping when needed, but monitor the amount of swap space used and upgrade your RAM if necessary; 3) for older desktop machines (with say only 128MB), use as much swap space as you can spare, even up to 1GB.
The Linux 2.6 kernel added a new kernel parameter called swappiness to let administrators tweak the way Linux swaps. It is a number from 0 to 100. In essence, higher values lead to more pages being swapped, and lower values lead to more applications being kept in memory, even if they are idle. Kernel maintainer Andrew Morton has said that he runs his desktop machines with a swappiness of 100, stating that "My point is that decreasing the tendency of the kernel to swap stuff out is wrong. You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine. Get it out on the disk, use the memory for something useful."
One downside to Morton's idea is that if memory is swapped out too quickly then application response time drops, because when the application's window is clicked the system has to swap the application back into memory, which will make it feel slow.
The default value for swappiness is 60. You can alter it temporarily (until you next reboot) by typing as root:
echo 50 > /proc/sys/vm/swappiness
If you want to alter it permanently then you need to change the vm.swappiness parameter in the /etc/sysctl.conf file.
Conclusion
Managing swap space is an essential aspect of system administration. With good planning and proper use swapping can provide many benefits. Don't be afraid to experiment, and always monitor your system to ensure you are getting the results you need.
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written by Tony, November 16, 2009
You see, while you never want to use swap space, the system has some interesting issues that it has to deal with when handing out memory, and the default choice Linux makes is very bad for reliable environments.
Here is a summary:
When a program asks for memory (via malloc, or indirectly via fork(), which has to make a copy of the running process), Linux assumes that the request is larger than it really needs. This is a reasonable assumption in a lot of cases. For example, in a shell, every time you run a command, the data set of the shell itself is marked copy-on-write, and is therefore a potential memory sink if that new shell were to continue running. In reality, the next thing that happens in 99% of the cases is an exec(2), so that those potential memory drains are immediately thrown out.
Basically, in order to run with little or no swap, the kernel has to lie and tell all comers that memory is available. If that turns out to be wrong, then it KILLS whatever program ends up actually needing the memory at some future time when none is available.
The problem is that there is no way to make a memory "guarantee" with this policy. It's a bit like selling too many tickets for an airplane: you don't know who is going to get bumped. So, say your Java VM calls a native method that does a fork() of a 1GB process, gets a bunch of copy-on-write pages, and then starts to to touch them like made...something is going to break, and it may not be the JVM. That is the problem. If the timing is wrong, it could be your production database that gets the axe!
People talk about "old" or "vintage" UNIX systems that made you have twice the amount of swap as RAM.
This had nothing to do with small memory systems. It had to do with guarantees.
Those UNIX systems were written to be very predictable in their runtime behavior, and the only way to do that is to cause memory allocation to fail in cases where there is nowhere to store the data. So, the idea is this: "Only tell me I can have the memory if it is really there (via RAM + swap)", but the performance assumption is that programs will ask for more than they need (which is common because of the whole fork()/exec()/copy-on-write model).
So, as an administrator of these other UNIX system, dealing with memory management goes like this: you expect to see swap in "use"...it means that allocations have been made against it. But it but does not mean any disk IO has happened!.
What you have to do for monitoring is watch the actual swapping stats via sar or some other utility. If you are seeing swap I/O, then you need more RAM or you need fewer processes. The amount of swap "in use" is an almost useless number (other than if you run out, new memory allocations will fail on request).
Linux can be tuned to work in a fashion that is closer to this "more reliable" technique as of the 2.6 kernel line. See Google on "Linux Memory Overcommit". The basics are:
- Have at least 2MB + size of RAM. I'd recommend RAM*2. This is independent of RAM size, since what you are using swap for is guarantees, not actual storage.
- Change the Kernel parameter vm.overcommit_memory to 2 in sysctl.conf
As your systems run, watch for swap I/O, which is a little difficult and misleading.
sar -B has numbers about paging, but unfortunately, paging is how all I/O happens. The &#xvm;eff is a good indicator (you want it to be high).
If you use a swap partition then you can directly monitor the reads/writes from/to that partition itself via sar -d.
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