All about swap area

Linux beginners always get confused what is swap area? or why do we need swap area? Well I was confused about it too whenever I was running a fresh install of Linux on my machine so finally I read all about it and felt like sharing the resource with everyone. I read all of this information from Ubuntu forums here, its explained well in simple language and if you want to read everything you should visit that page. An excerpt from the page is given below for a quick reference.

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 LibreOffice, 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?

For less then 1GB of physical memory (RAM), it’s highly recommended that the swap space should, as a base minimum, be equal to the amount of RAM. Also, it’s recommended that the swap space is maximum twice the amount of RAM depending upon the amount of hard disk space available for the system because of diminishing returns.

For more modern systems (>1GB), your swap space should be at a minimum minimum be equal to your physical memory (RAM) size “if you use hibernation”, otherwise you need a minimum of round(sqrt(RAM)) and a maximum of twice the amount of RAM. The only downside to having more swap space than you will actually use, is the disk space you will be reserving for it.

The “diminishing returns” means that if you need more swap space than twice your RAM size, you’d better add more RAM as Hard Disk Drive (HDD) access is about 10³ slower then RAM access, so something that would take 1 second, suddenly takes more then 15 minutes! And still more then a minute on a fast Solid State Drive (SSD)…

Example Scenarios

(last 3 columns denote swap space)

        RAM(MB) No hibernation  With Hibernation  Maximum
         256     256              512               512
         512     512             1024              1024
        1024    1024             2048              2048

        RAM(GB) No hibernation  With Hibernation  Maximum
          1      1                2                   2
          2      1                3                   4
          3      2                5                   6
          4      2                6                   8
          5      2                7                  10
          6      2                8                  12
          8      3               11                  16
         12      3               15                  24
         16      4               20                  32
         24      5               29                  48
         32      6               38                  64
         64      8               72                 128
        128     11              139                 256

Read how to add more swap? and more details.


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