zram: speed up old computers

Modern devices have GBs of RAM and swapping is not very common without running lots of applications together. But what if you have a older device with 512MB or less RAM? You’ll see a lot of data being swapped in and out of your hard disk swap partition. Hard disks are much slower than RAM and thus swapping enables you to use virtual memory at the cost of performance.

zram is a Linux kernel feature (enabled from version 3.2) to use a compressed virtual memory in-RAM. It mimics a LZ4 compressed block device in the RAM for swapping data. As compression is used, more data can be stored in RAM than in actual capacity. The technique uses more processing to compress and decompress data but the lag is negligible when compared to swapping data in and out of hard disk.

To enable zram on Ubuntu, run:

$ sudo apt-get install zram-config

The feature is initialized as soon as you install it. By default it uses half your RAM as compressed swap. To view the device nodes and amount of RAM used by zram, run:

$ cat /proc/swaps
Filename       Type        Size     Used   Priority
/dev/zram0     partition   492980   0      5
/dev/zram1     partition   492980   0      5
/dev/zram2     partition   492980   0      5
/dev/zram3     partition   492980   0      5

When used, the Used field changes.

To tweak the amount of RAM used by zram, open /etc/init/zram-config.conf and find the following line:

mem=$(((totalmem / 2 / ${NRDEVICES}) * 1024))

As you can see, mem is calculated as half the (total RAM available / number of CPU cores) in KBs. The zram devices are created one per CPU core and the memory is distributed equally (refer the output of /proc/swaps above).