On Sat, Jun 6, 2020 at 10:04 AM Richard W.M. Jones <rjones(a)redhat.com> wrote:
On Fri, Jun 05, 2020 at 04:07:44PM -0600, Chris Murphy wrote:
> This laptop with 8GiB RAM is running two VMs at the same time: Windows
> 10 and Fedora Workstation 32. The host is Fedora Workstation 32.
So this suggests another interesting question: If I have Fedora
virtual machines running on a Fedora host, and both are using zram,
could there be some negative interaction? ie. With the pages not
being compressible twice. I suppose (without looking at the source)
that zram will skip compressing a page if the compressed page is not
I've not seen a negative interaction. But I am not a scientific sample :D
However, it's surely true that compressed pages in the guest VM
"memory" would not compress again in the host if those same pages are
subject to eviction. Thus, two consequences:
- the host 'zramctl' statistics will show lower compression ratio, as
this type of eviction happens.
- it might be an example workload where the host is better off with
zswap, which is intended to work with swap-on-drive. (not part of this
But also the zram devices are still pretty small so the negative
effects are limited. Of course this gets tricky anytime there's
significant overcommit of memory no matter the swap scheme.