Re: [LSF/MM/BPF TOPIC] VM Memory Overcommit

[David Hildenbrand <david@redhat.com>]

On 28.02.23 23:38, SeongJae Park wrote:
> On Tue, 28 Feb 2023 10:20:57 +0100 David Hildenbrand <david@redhat.com> 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 :)

> Hope to attend this session and discuss about that together.

Absolutely!

-- 
Thanks,

David / dhildenb