Linux swap partition (detailed explanation)

linux

1. What is SWAP

$ swapon -s
Filename Type Size Used Priority
/swap.img file 2097148 0 -2

Functionally, the swap partition is mainly used to swap part of the data in the memory to the swap space when the memory is insufficient, so that the system will not cause OOM or more fatal situations due to insufficient memory. Therefore, when there is pressure on memory usage and memory recycling begins to be triggered, swap space may be used.

2. What does swappiness adjust?

The file /proc/sys/vm/swappiness can be used to adjust swap-related parameters. The default value of this file is 60, and the possible value range is 0-100

$ cat /proc/sys/vm/swappiness
60
$ sysctl -q vm.swappiness
vm.swappiness = 60

$ sysctl vm.swappiness=10
$ sysctl -q vm.swappiness
vm.swappiness = 10

Persistent Operations

$ vim /etc/sysctl.conf
vm.swappiness=10 #To the last line, restart is required to take effect

Defines how aggressively the kernel uses swap:

• The higher the value, the more actively the kernel will use swap;
• The lower the value, the less motivated to use swap.
• If this value is 0, the total amount of memory used by free and file-backed pages is less than the high water mark.
• mark), no exchange will occur. Adjusting it to 0 means trying to reclaim memory by clearing the cache.
• A value of 100 means that when memory is reclaimed, reclaiming memory from the cache has the same priority as swapping. That is to say, if 100M of memory is currently required, there is a high probability that 50M of memory will be cleared from the cache, and then 50M of anonymous pages will be swapped out, and the recovered memory will be given to the application for use. But this also depends on whether there is space in the cache and whether swap can exchange 50m.

file-backed: This is the size of the file mapping page mentioned above.

3. When will the swap operation be performed?

There are two memory recycling mechanisms: kswapd cycle checking and direct memory recycling. When the requested memory is greater than the remaining memory, direct recycling will be triggered. So what are the conditions that trigger recycling during the kswapd process periodic check? From a design perspective, the kswapd process needs to periodically check the memory and start memory recycling when a certain threshold is reached. This so-called threshold can be understood as the current memory usage pressure. That is, although we still have remaining memory, when the remaining memory is relatively small, that is, when the memory pressure is high, we should start trying to reclaim some memory. This can ensure that the system has enough memory as much as possible for sudden memory requests.

Kswapd decides whether to start reclaiming memory based on the memory water level mark. If the mark reaches low, it starts reclaiming until the remaining memory reaches high.

View the current system memory watermark
$ cat /proc/zoneinfo

4. Priority of swap partition

You can use the -p parameter to specify the priority of the swap space. The larger the value, the higher the priority. The range of numbers that can be specified is from -1 to 32767.

$ swapoff /dev/sdc1; swapon -p 0 /dev/sdc1
$ swapon -s
Filename Type Size Used Priority
/dev/sdc1 file 2097148 0 0

$ cat /proc/swaps
Filename Type Size Used Priority
/dev/sdc1 file 2097148 0 0

Put an entry in /etc/ fstab to make it take effect on every Linux reboot:

/dev/sdc1 swap swap pri=0 0 0

5. Start and stop swap

$ swapoff -a stop $ swapon -a start

6. Create swap space

Create swap file dd if=/dev/sda3 of=./swapfile bs=1M count=1G
mkswap ./swapfile

Enable swap file $ swapon swapfile

$ swapon -s
Filename Type Size Used Priority
/swap.img file 2097148 3340 0
/mnt/swapfile file 6388156 0 -2

Turn off swap space $ swapoff swapfile
$ swapon -s
Filename Type Size Used Priority
/swap.img file 2097148 3156 0

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