Running out of RAM in Ubuntu? Enable ZRAM

Whenever I ran out of RAM on a Linux system, I used to enable swap memory using the storage device to provide an extra bit of memory.  The main advantage is that it’s does not require extra hardware, but come at the cost of much slower access, and potential issues or wear and tear, unless you only use it temporary.

This week-end, I compiled Arm Compute Library on ODROID-XU4Q board, and the first time it crashed because the system ran out of memory, so I enable swap on the eMMC flash module to restart and complete the build successfully. However, I was told it would have been better to enable ZRAM instead.

So what is ZRAM? Wikipedia explains:

zram, formerly called compcache, is a Linux kernel module for creating a compressed block device in RAM, i.e. a RAM disk, but with on-the-fly “disk” compression.

So it’s similar to swap, expect it operates in RAM and compresses memory. It’s also possible to combine it with swap, but if you have to go that route, it may be worth considering upgrading the memory, or changing to a more powerful hardware platform. ZRAM support has been considered stable since Linux 3.14 released in early 2014.

Before showing how to use ZRAM, let’s check the memory in my board.

That’s about 2GB RAM, and swap is disabled. In theory, enabling ZRAM in Ubuntu or Debian should be one simple step:

Installation went just fine, but it did not enable ZRAM. The first thing to check is whether ZRAM is enabled in the Linux kernel by checking out /proc/config or /proc/config.gz:

ZRAM is built as a module. I tried different things to enable it and check, but I was not sure where to go at this stage at it was almost time to go to bed. What’s usually the best course of action in this case? Stay awake and work overnight to fix the issue? Nope! Rookie mistake. Years of experience have taught me, you just turn off your equipment, and have a good night sleep.

Morning time, breakfast, walk to office, turn on computer and board, et voilà:

Success! So I would just have had to reboot the board to make it work the previous day… So we have 1GB ZRAM swap enabled across 8 block devices. I assume those are used as needed to avoid eating RAM necessary.

So let’s try to use that swap by making it a bit more challenging that just building the Arm Compute Library, by running a few programs like Chromium and Thunderbird, and monitoring RAM usage at the same time with htop.

At the final stage we can see 506 MB “swap” is used, with 1.11GB memory, probably since the rest of the RAM is used for ZRAM. I was however wrong in myinitial assumption that ZRAM block devices would be used one by one as needed, as all eight devices were show to hold about the same amount of data:

That’s the output of free for reference:

The good news is that the build could complete just fine with ZRAM, even with Chromium and Firefox running in the background.

ZRAM requires compressing and decompressing data constantly, and reduces the amount of uncompressed RAM your system can access, so it may actually decrease system performance. However, if you run out of RAM frequently or for a specific application it may be worth enabling it. I just needed ZRAM for a single build, so I could disable it now by removing it:

However, you may also consider tweaking it by using zramctl utility:

Running zramctl without parameters allows use to see which how much actual compressed / uncompressed data is used per block:

zramswap service is handled by systemd in Ubuntu 18.04  and you could check out /etc/systemd/system/ to understand how it is setup and control it at boot time too. For example, we can see one block device is set per processor. Exynos 5422 is an eight core processor, and that’s why we have 8 block devices here. Any other tips are appreciated in comments.

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