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UbuntuHelp:SwapFaq

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  1. title Swap FAQ

Introduction

  • This FAQ is aimed at Linux novices.
  • People always wonder how much swap they should create at install time, or after installing they may think, "have I made a large enough swap? Should I reinstall with a larger swap?"
  • This FAQ will tell you how much swap you need and how to add more swap after installation.
  • You will be given very simple answers (to prevent losing too much time reading this FAQ) and some explanations that may help you form your own opinion. For a more detailed explanation of memory and swap, see here.

What is swap?

Swap space is the area on a hard disk which is part of the Virtual Memory of your machine, which is a combination of accessible physical memory (RAM) and the swap space. Swap space temporarily holds memory pages that are inactive. Swap space is used when your system decides that it needs physical memory for active processes and there is insufficient unused physical memory available. If the system happens to need more memory resources or space, inactive pages in physical memory are then moved to the swap space therefore freeing up that physical memory for other uses. Note that the access time for swap is slower therefore do not consider it to be a complete replacement for the physical memory. Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files.

Why do I need swap?

Memory consuming programs Sometimes, a large program (like OpenOffice, Neverwinter Nights, or a video editor) make the entire system need extra memory. A significant number of the pages used by these large programs during its startup may only be used for initialization and then never used again. The system can swap out those pages and free the memory for other programs or even for the disk cache. In these cases, swap will be used to help the system handle any extra load. Hibernation (suspend-to-disk) The hibernation feature (suspend-to-disk) writes out the contents of RAM to the swap partition before turning off the machine. Therefore, your swap partition should be at least as big as your RAM size. The hibernation implementation currently used in Ubuntu, swsusp, needs a swap or suspend partition. It cannot use a swap file on an active file system. Unforeseeable Circumstances Unforeseeable events can and will happen (a program going crazy, some action needing much more space than you thought, or any other unpredictable combination of events). In these cases, swap can give you an extra delay to figure out what happened, or to finish what you are working on. Optimizing memory usage Since mechanical hard drives are considerably slower than RAM (SSD - Solid State Drive - storage is not as slow as physical drives, but still slower than RAM), when you need a file (be it a data file like a video, executables like Firefox, or libraries), the Linux kernel reads the file into RAM and keeps it there, so that the next time you need it, it is already in RAM and data access is much faster. The portions of RAM that accelerate disk read are called "cached memory." You will notice that they make a huge difference in terms of responsiveness. The Linux kernel automatically moves RAM reserved by programs--but not really used--into swap, so that it can serve the better purpose of extending cached memory. Optimizing Swap performance Because swap space uses a disk device, this can cause performance issues in any system that uses swap space significantly because the system itself may also be using the same disk device at the same time that it is required for swap operations. One way to reduce this problem is to have swap space on a different physical drive so that the competition for that resource is either reduced or eliminated.

How much swap do I need?

As a base minimum, it's highly recommended that the swap space should be equal to the amount of physical memory (RAM). Also, it's recommended that the swap space is twice the amount of physical memory (RAM) depending upon the amount of hard disk space available for the system (although this "recommendation" dates back from a time when physical RAM was very expensive and most Unix systems ran with many processes in swap space - a situation that hardly applies in most situations these days, but ancient Unix/Linux myths like this "recommendation" tend to survive well past their "use by" dates). In reality, if you use hibernation you need what was outlined the relevant paragraph above, otherwise you need as much swap space as your system will use - which may be actually be very little in a modern hardware setup. The only downside to having more swap space than you will actually use is the disk space you will be reserving for it.

Example Scenarios

Low RAM and low disk space With 512 MB RAM and 30 GB hard disk, use 512 MB for swap since RAM is very low. Low RAM and high disk space With 512 MB RAM and 100 GB hard disk, use 1 GB for swap since RAM is very low and hard disk space is in plenty. High RAM and low disk space With 2 GB RAM and 30 GB hard disk, use 1 GB for swap since hard disk space is very low. High RAM and high disk space With 2 GB RAM and 100 GB hard disk, use 2 GB for swap since hard disk space is plentiful.

How do I add more swap?

<<Anchor(addswap)>> Swap is generally associated with a swap partition, perhaps because the user is prompted to create a swap partition at the time of installation. In fact, any file can be used as a swapping device, be it a partition or a conventional file. Although, to improve the responsiveness, it's recommended to have a good sized amount of RAM available. Swap can be added by adding a swap file.

Four-step Process to Add Swap

  • Creating a file the size you want.
  • Formatting that file to create a swapping device.
  • Adding the swap to the running system.
  • Making the change permanent.

For Adding a 512 MB swap

  • Creating a file for 512 MB size you want:

We will create a /mnt/512Mb.swap swap file.

sudo dd if=/dev/zero of=/mnt/512Mb.swap bs=1M count=512

Here count=512, means we want our file to contain 512 blocks of bs=1M, which means block size = 1 mebibyte (1 048 576 bytes). Be careful *not* to do this dd of=/mnt/512Mb.swap bs=1M seek=512 count=0 Though the file grows to 512Mb immediately, it will have holes that makes it unusable.

  • Formatting that file to create a swapping device:
sudo mkswap /mnt/512Mb.swap
  • Adding the swap to the running system:
sudo swapon /mnt/512Mb.swap

The additional swap is now available and can be seen by "cat /proc/meminfo"

  • Making the change permanent:

Edit the /etc/fstab:

gksudo gedit /etc/fstab

Add this line at the end of the file:

/mnt/512Mb.swap  none  swap  sw  0 0

Save and reboot.

Example of making a swap file

This is an example of making and using a swap file on a computer with no swap partition.

user@computer:~$ sudo dd if=/dev/zero of=/mnt/512Mb.swap bs=1M count=512
Password:
512+0 records in
512+0 records out
536870912 bytes (537 MB) copied, 35.3802 seconds, 15.2 MB/s
user@computer:~$ sudo mkswap /mnt/512Mb.swap
Setting up swapspace version 1, size = 536866 kB
no label, UUID=dd6a01c8-93f0-41e0-9b7a-306956d8821b
user@computer:~$ sudo swapon /mnt/512Mb.swap
user@computer:~$ cat /proc/meminfo
MemTotal:       499496 kB
MemFree:          9156 kB
Buffers:          4748 kB
Cached:         233140 kB
SwapCached:        724 kB
Active:         254432 kB
Inactive:       157920 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:       499496 kB
LowFree:          9156 kB
SwapTotal:      524280 kB
SwapFree:       523556 kB
Dirty:             128 kB
Writeback:           0 kB
Mapped:         243420 kB
Slab:            20672 kB
CommitLimit:    774028 kB
Committed_AS:   648680 kB
PageTables:       2224 kB
VmallocTotal:   524280 kB
VmallocUsed:      5708 kB
VmallocChunk:   518176 kB
user@computer:~$ gksudo gedit /etc/fstab
user@computer:~$ free
             total       used       free     shared    buffers     cached
Mem:        499496     479488      20008          0       8256     215892
-/+ buffers/cache:     255340     244156
Swap:       524280       3856     520424
user@computer:~$ free
             total       used       free     shared    buffers     cached
Mem:        499496     492768       6728          0       1240     142336
-/+ buffers/cache:     349192     150304
Swap:       524280      53384     470896

user@computer:~$ free
             total       used       free     shared    buffers     cached
Mem:        499496     493136       6360          0       7528     174700
-/+ buffers/cache:     310908     188588
Swap:       524280      17148     507132

Undoing your changes : Undoing basically follows the same process in reverse.

gksudo gedit /etc/fstab

Remove the line

/mnt/512Mb.swap  none  swap  sw  0 0

Remove the swap from the running system and remove the swap file.

sudo swapoff /mnt/512Mb.swap && sudo rm /mnt/512Mb.swap

No need to reboot.

What is swappiness and how do I change it?

The swappiness parameter controls the tendency of the kernel to move processes out of physical memory and onto the swap disk. Because disks are much slower than RAM, this can lead to slower response times for system and applications if processes are too aggressively moved out of memory.

  • swappiness can have a value of between 0 and 100
  • swappiness=0 tells the kernel to avoid swapping processes out of physical memory for as long as possible
  • swappiness=100 tells the kernel to aggressively swap processes out of physical memory and move them to swap cache

The default setting in Ubuntu is swappiness=60. Reducing the default value of swappiness will probably improve overall performance for a typical Ubuntu desktop installation. A value of swappiness=10 is recommended, but feel free to experiment. Note: Ubuntu server installations have different performance requirements to desktop systems, and the default value of 60 is likely more suitable. To check the swappiness value

cat /proc/sys/vm/swappiness

To change the swappiness value A temporary change (lost on reboot) with a swappiness value of 10 can be made with

sudo sysctl vm.swappiness=10

To make a change permanent, edit the configuration file with your favorite editor:

gksudo gedit /etc/sysctl.conf

Search for vm.swappiness and change its value as desired. If vm.swappiness does not exist, add it to the end of the file like so:

vm.swappiness=10

Save the file and reboot.

What is the priority of swap containers?

The Linux kernel assigns priorities to all swap containers. To see the priorities that the Linux Kernel assigns to all the swap containers use this command.

cat /proc/swaps

Priorities can be changed by using the swapon command or defined in /etc/fstab. Consult the manual page of swapon for more info

man swapon

Should I reinstall with more swap?

Definitely not. With the 2.6 kernel, "a swap file is just as fast as a swap partition."(Wikipedia:Paging, LKML).

Why is my swap not being used?

My swap is not being used! When I issue the free command, it shows something like this:

tom@tom:~$ free
             total       used       free     shared    buffers     cached
Mem:        515980     448664      67316          0      17872     246348
-/+ buffers/cache:     184444     331536
Swap:       674688          0     674688

Note: This regards mainly swap on hard disk partitions, but it could help anyway. In these examples /dev/hda8 is considered as swap.

Swap may not be needed

Start many memory consuming applications (e.g. Gimp, web browsers, OpenOffice etc) and then issue the free command again. Is swap being used now?

Is there a swap partition at all?

Use this command to see all partitions

sudo fdisk -l

You should be able to see something like this in the output

/dev/hda8            4787        4870      674698+  82  Linux swap / Solaris

If not, you either need to [[UbuntuHelp:[addswap|create a swapfile]]] or create a swap partition. To create a swap partition you can

  • boot from your Ubuntu install CD, create a swap partition out of the free space on your hard disk and then interrupt your installation.
  • use Cfdisk.

Enabling a swap partition

In case you do have a swap partition, there are several ways of enabling it.

  • Use the following command
cat /etc/fstab
  • Ensure that there is a line link below. This enables swap on boot.
/dev/hda8       none            swap    sw              0       0
  • Then disable all swap, recreate it, then re-enable it with the following commands.
sudo swapoff -a
sudo /sbin/mkswap /dev/hda8
sudo swapon -a

Empty Swap

Even if you have lots of RAM and even if you have a low swappiness value, it is possible that your computer swaps. This can hurt the multitasking performance of your desktop system. You can use the following script to get the swap manually back into RAM:

  • place the script e.g. /usr/sbin:
sudo gedit /usr/sbin/swap2ram.sh
  • copy&paste the script into the file:
#!/bin/sh
err="not enough RAM to write swap back, nothing done"
mem=`free|grep Mem:|awk '{print $4}'`
swap=`free|grep Swap:|awk '{print $3}'`
test $mem -lt $swap && echo -e $err && exit 1
swapoff -a && swapon -a &&
exit 0
  • save and close gedit
  • make the script executable:
sudo chmod +x swap2ram.sh
  • execute:
sudo swap2ram.sh