Zram vs zswap. Why choose one over the other

[Archie1]

@wolfn

+1
In the Arch-wiki they linked an article: link
There are so many aspects, I think you will never hit the bulls eye…
And one aspect I am missing totally here: afaik all Arch-based (or systend?) systems use half of the available ram for a ramdisk with /tmp & some more inside to speedup the system, and adjust it at actual demand.

Here, I use my swap-partitions for hibernating, too. So it is no question, how large…
And I think, using a modern nvme (a part of) for a old fashioned swap, it should be fast enough, especially, if you recall the linked article.
Sure, a minimum of real ram, adequate for your working needs cannot be replaced by any swap (compare it to a car…)

@wolfn you are right, almost all of the distros these days mount /run and other partitions in RAM. They typically reserve 10% or 2GB of RAM for this. In addition if /tmp is also mounted as tmpfs then it will also consume RAM. So reduce SWAP pressure and disk writes, we need RAM. But RAM is already provisioned for many things.
What surprised me was when @mbod clarified that Fedora was using ZRAM equal to physical RAM and that too Out-of-the-box. I might be one of the very few distros that do this.

@mbod

You can try this out with the stress-ng command and monitoring swap usage with your system monitor of choice.

See these instructions from Red Hat:
https://docs.redhat.com/en/documentation/red_hat_enterprise_linux_for_real_time/8/html/optimizing_rhel_8_for_real_time_for_low_latency_operation/assembly_stress-testing-real-time-systems-with-stress-ng_optimizing-rhel8-for-real-time-for-low-latency-operation#proc_generating-a-virtual-memory-pressure_assembly_stress-testing-real-time-systems-with-stress-ng

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

stress-ng is part of which package? It is not installed in my EOS system.
Also is there any other package apart from stress-ng that ca be used?

@mbod

@Schlaefer

True, but in the end we put these in-memory solutions (zswap, zram) in front of the disk-swap to avoid that situation.

zram is not “in front of the disk-swap”. zram does not use any disk swap. zswap does use disk swap after compressing swapable pages in memory. zswap is a cache layer for disk swap.

I was under the impression that zswap compresses and hold the swap-able pages in RAM and not in disk/Swap partition/Swap file. It does not write the disk at all, unless the memory page is not cant be compressed. Is that not the case?
Disk in this case would be NVMe or SATA disk or Flash memory.

@dalto

@Archie1

I am thinking more in terms of laptop slowness with time, due to battery degradation. With time all the lithium ion batteries degrade. Thus they are unable to deliver the required level of voltage/power to the CPU to make it function at its best. When that happens, running zram and possibly zswap would also result in issues.

I don’t think this is likely to cause a material issue. The amount of CPU overhead by this compression is pretty low. If your CPU performance degrades to the point where this would be an issue, you are going to have bigger issues.

Thanks for clarifying that. I am a big fan of Dell Laptops and after 8+ years Dell throttles all of its laptops to 1.5GHz or below. Irrespective whether the laptop is connected to power or is running on battery. I was coming from that angle.

@cactux, @mbod, @dalto, @wolfn, @MyNameIsRichard and @Schlaefer thanks for your input and comments. Please do add something if you think it would be helpful.