On Sat, Jun 06, 2020 at 03:10:37PM -0700, Samuel Sieb wrote:
On 6/6/20 9:04 AM, Richard W.M. Jones 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 considered this as well. But that would only be a concern if the
live memory of the VM is getting swapped out. Is that something
that could happen?
That's a good point. The answer is yes (VMs are just regular
processes). However usually if that does happen then the end result
isn't great, with potentially huge guest pauses when a guest tries to
run and its memory needs to be swapped back in. Serious
virtualization hosts will try to ensure that memory is not
overcommitted like this.
KSM (kernel samepage merging) might also be affected. I wonder if the
same original page content always compresses to exactly the same zram
page? I'm going to guess not, in which case KSM would be defeated by
zram. Although personally I've never found KSM does anything.
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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