Comment 0 for bug 1931725

Turns out that loading is always the slow part in loading initramfs into memory, decompression is always the final 10-20% or so of the task. It therefore makes sense to use the a good compressor that shrinks the initramfs as much as possible with little decompression overhead.

Benchmarking zstd vs lz4 shows that while zstd can be ~5x slower in decompression the image size is much smaller with zstd than lz4, and since ~80-90% of the boot time is loading the image it makes sense to use zstd.

Attached is a libreoffice spread sheet showing typical load and decompression times for a fairly standard 3.4 GHZ intel box with data for load times for a 5400 RPM, 7200 RPM and SATA SSD drives.

The conclusions from the test results (attached) show:

1. Loading time is always significantly slower than decompression time.
2. ZSTD is 5x slower than LZ4 in decompression speed but produces far
better compressed images
3. Given that loading time is the major factor in loading +
decompression, ZSTD is best for kernel and initramfs boot timings.

(Also refer to https://kernel.ubuntu.com/~cking/boot-speed-eoan-5.3/kernel-compression-method.txt for some raw data on drive load speeds for the same UEFI box I did a couple of years ago).