Zram vs zswap. Why choose one over the other

[cactux]

@mbod

E.g. My system has 64 GB of RAM with 12 GB of zram swap. I do the stress test with:

stress-ng --vm 2 --vm-bytes 32G --mmap 2 --mmap-bytes 32G --page-in

This swaps up to 12 GB to zram swap and the computer is still responsive. On the other hand, swapping out 12 GB to a file or a partition takes a few seconds during which the PC may not be responding. You can try this by yourself and feel the difference.

It would be interesting to see the result of a similar test in term of system responsiveness for a setup with 8GB RAM and 4/6/8GB Zram.

Also, I don’t think that this test reflects the majority of “real life” use cases where usually almost a negligible amount of pages are swapped out.

@mbod

Swapping to a file or a partition is really slow. Regardless if you swap to a fast SSD or NVME: The computer will become really slow when it swaps. May be even to a point where it is unresponsive.

I have switched to zswap plus swapfile. I just use my system as I normally would and I am monitoring the swap usage. So far I haven’t noticed any slowdown in the way of system’s operation let alone being unresponsive.

For now I cannot take your categorical statement at face value until I have used this setup for enough long period.

Current parameters for zswap:

 grep -r . /sys/module/zswap/parameters/
/sys/module/zswap/parameters/enabled:Y
/sys/module/zswap/parameters/shrinker_enabled:Y
/sys/module/zswap/parameters/max_pool_percent:20
/sys/module/zswap/parameters/compressor:lzo
/sys/module/zswap/parameters/zpool:zsmalloc
/sys/module/zswap/parameters/accept_threshold_percent:90

Current RAM and swap usage:

 free -h
               total        used        free      shared  buff/cache   available
Mem:            14Gi       4.5Gi       4.4Gi        47Mi       6.4Gi        10Gi
Swap:           15Gi          0B        15Gi

Current zswap usage: