Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints

From: Dave Hansen
Date: Mon Feb 11 2019 - 13:19:17 EST

On 2/11/19 9:58 AM, Michael S. Tsirkin wrote:
>>> Really it seems we want a virtio ring so we can pass a batch of these.
>>> E.g. 256 entries, 2M each - that's more like it.
>> That only makes sense for a system that's doing high-frequency,
>> discontiguous frees of 2M pages. Right now, a 2M free/realloc cycle
>> (THP or hugetlb) is *not* super-high frequency just because of the
>> latency for zeroing the page.
> Heh but with a ton of free memory, and a thread zeroing some of
> it out in the background, will this still be the case?
> It could be that we'll be able to find clean pages
> at all times.

In a systems where we have some asynchrounous zeroing of memory where
freed, non-zeroed memory is sequestered out of the allocator, yeah, that
could make sense.

But, that's not what we have today.

>> A virtio ring seems like an overblown solution to a non-existent problem.
> It would be nice to see some traces to help us decide one way or the other.

Yeah, agreed. Sounds like we need some more testing to see if these
approaches hit bottlenecks anywhere.