On 02.03.23 04:26, David Rientjes wrote:
On Tue, 28 Feb 2023, David Hildenbrand wrote:
On 28.02.23 23:38, SeongJae Park wrote:
On Tue, 28 Feb 2023 10:20:57 +0100 David Hildenbrand <david@xxxxxxxxxx>
wrote:
On 23.02.23 00:59, T.J. Alumbaugh wrote:
Hi,
This topic proposal would be to present and discuss multiple MM
features to improve host memory overcommit while running VMs. There
are two general cases:
1. The host and its guests operate independently,
2. The host and its guests cooperate by techniques like ballooning.
In the first case, we would discuss some new techniques, e.g., fast
access bit harvesting in the KVM MMU, and some difficulties, e.g.,
double zswapping.
In the second case, we would like to discuss a novel working set size
(WSS) notifier framework and some improvements to the ballooning
policy. The WSS notifier, when available, can report WSS to its
listeners. VM Memory Overcommit is one of its use cases: the
virtio-balloon driver can register for WSS notifications and relay WSS
to the host. The host can leverage the WSS notifications and improve
the ballooning policy.
This topic would be of interest to a wide range of audience, e.g.,
phones, laptops and servers.
Co-presented with Yuanchu Xie.
In general, having the WSS available to the hypervisor might be
beneficial. I recall, that there was an idea to leverage MGLRU and to
communicate MGLRU statistics to the hypervisor, such that the hypervisor
can make decisions using these statistics.
But note that I don't think that the future will be traditional memory
balloon inflation/deflation. I think it might be useful in related
context, though.
What we actually might want is a way to tell the OS ruining inside the
VM to "please try not using more than XXX MiB of physical memory" but
treat it as a soft limit. So in case we mess up, or there is a sudden
peak in memory consumption due to a workload, we won't harm the guest
OS/workload, and don't have to act immediately to avoid trouble. One can
think of it like an evolution of memory ballooning: instead of creating
artificial memory pressure by inflating the balloon that is fairly event
driven and requires explicit memory deflation, we teach the OS to do it
natively and pair it with free page reporting.
All free physical memory inside the VM can be reported using free page
reporting to the hypervisor, and the OS will try sticking to the
requested "logical" VM size, unless there is real demand for more memory.
I think use of DAMON_RECLAIM[1] inside VM together with free pages reporting
could be an option. Some users tried that in a manual way and reported some
positive results. I'm trying to find a good way to provide some control of
the
in-VM DAMON_RECLAIM utilization to hypervisor.
I think we might want to go one step further and not only reclaim
(pro)actively, but also limit e.g., the growth of caches, such as the
pagecache, to make them also aware of a soft-limit. Having that said, I still
have to learn more about DAMON reclaim :)
I'm curious, is this limitation possible to impose with memcg today or are
specifically looking to provide a cap on page cache, dentries, inodes,
etc, without specifically requiring memcg?
Good question, I remember the last time that topic was raised, the
common understanding was that existing mechanisms (i.e., memcg) were not
sufficient. But I am no expert on this, so this sure sounds like a good
topic to discuss in a bigger group, with hopefully some memcg experts
around :)
--
Thanks,
David / dhildenb
