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

[Dave Hansen]

On 2/9/19 4:49 PM, Michael S. Tsirkin wrote:
> On Mon, Feb 04, 2019 at 10:15:52AM -0800, Alexander Duyck wrote:
>> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>
>> Add guest support for providing free memory hints to the KVM hypervisor for
>> freed pages huge TLB size or larger. I am restricting the size to
>> huge TLB order and larger because the hypercalls are too expensive to be
>> performing one per 4K page.
> Even 2M pages start to get expensive with a TB guest.

Yeah, but we don't allocate and free TB's of memory at a high frequency.

> 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.

A virtio ring seems like an overblown solution to a non-existent problem.