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Memory compression on hypervisor vs host (lemmy.ca)

submitted 2 days ago* (last edited 2 days ago) by rbos@lemmy.ca to c/sysadmin@lemmy.world

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I've been wondering whether it's better for memory pages to be compressed at the hypervisor level, or on the VM level.

I'm leaning toward the VM level, because

1: VMs have better knowledge of memory pressure by the application, and can better decide when to swap pages out to zram. The VM has access to information about memory pages that the hypervisor doesn't have.

2: if pages are compressed on the hypervisor level, the VM doesn't "see" any increased memory available. The host box gains free memory, but the application never sees it to make use of it, it'll just see the same 8GB as it always has, so it never really benefits. This maybe lets you host more VMs on one box, but at the cost of the applications not being as efficient.

Is this a reasonable position? I'm wondering if I'm missing something obvious.

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[–] brucethemoose@lemmy.world 5 points 2 days ago* (last edited 2 days ago)

It depends on the application.

Do you have some apps that are inactive for long periods of time, and “wake up?” Better to do it at the highest level. That gives the OS power and o essentially shunt whole VMs away and give the active ones full power.

Are they all pretty active all the time? Are memory performance requirements not too high? Is latency a priority? Best to do it inside, I suppose.

EDIT: For what it’s worth, I found that no zram is best in some scenarios. Sometimes applications just barely, rarely scrape the memory limit, and if I enable a big chunk of zram they scrape it more frequently, then don’t give it up and keep active pages in zram. Rare swapping to an ssd ended up much, much faster.