Boost Linux Performance with zSwap, zRAM, and Zstandard Compression

Why Memory Compression Matters

Compressing memory reduces space usage; I/O operations are far slower than RAM access, so frequent I/O not only shortens flash lifespan but also degrades system performance. Memory compression smooths I/O transitions, mitigating performance loss caused by memory pressure.

Introducing zSwap

zSwap is a Linux kernel feature that provides a compressed write‑back cache for swap pages. Instead of moving pages directly to a swap device, zSwap compresses them and stores them in a dynamically allocated RAM pool, postponing or even avoiding writes to the swap device and thus reducing Linux I/O at the cost of extra CPU cycles for compression.

By using zSwap, Linux can utilize RAM more efficiently, effectively increasing available memory without a proportional increase in CPU usage. The feature exists in the kernel but is disabled by default; it must be enabled via configuration.

Main Memory‑Compression Technologies

zSwap

zRAM

zCache

How zSwap Works

zSwap acts as a cache layer between memory and flash. When a page needs to be swapped out to disk, it is first compressed and placed in zSwap, which grows on demand. Once a certain threshold is reached, pages are evicted in LRU order (provided the memory allocator supports LRU), decompressed, and written to the swap device.

Drawbacks include the requirement for an LRU‑capable allocator and similarity to zRAM when the backing store is fully swapped.

zRAM

zRAM creates a compressed RAM block device. It never writes to an actual block device; compressed pages remain in RAM, offering much faster access than I/O and making it popular on mobile devices. Issues include configuring the size, increased CPU usage by the kswapd process in low‑memory scenarios, and potential fragmentation from large zRAM allocations.

zCache

Proposed by Oracle, zCache compresses file pages and sits between memory and block devices, similar to zSwap but targeting file‑page compression. In contrast, zSwap and zRAM compress anonymous pages.

zstd Compression Algorithm

Zstandard (zstd) is a real‑time compression algorithm offering high compression ratios with a wide range of speed/ratio trade‑offs. It includes a fast decoder and supports dictionary compression for small data sets. The library is released under the BSD license.

Enabling zSwap

# edit grub as root
sudo vim /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash zswap.enabled=1 zswap.compressor=zstd zswap.zpool=z3fold"

# update grub
sudo update-grub

# install zstd and Z3fold
sudo apt install checkinstall build-essential
sudo apt install zstd  # not the latest version

# edit the modules file
sudo vim /etc/initramfs-tools/modules
# add the following lines
zstd
zstd_compress
z3fold

# update initramfs
sudo update-initramfs -u -k all

# restart and verify
cat /sys/module/zswap/parameters/enabled   # should output Y
sudo dmesg | grep -i zswap:
[    1.059997] zswap: loaded using pool zstd/z3fold

References

Improving Elementary OS Performance using ZSWAP, zstd and z3fold

Linux 内存压缩浅析之原理