On Mon, Feb 11, 2019 at 09:48:11AM -0800, Dave Hansen wrote: > 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. 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. > 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. -- MST
