On Thu, Mar 26, 2020 at 08:54:04AM +0100, David Hildenbrand wrote: > > > > Am 26.03.2020 um 08:21 schrieb Michael S. Tsirkin <mst@xxxxxxxxxx>: > > > > On Thu, Mar 12, 2020 at 09:51:25AM +0100, David Hildenbrand wrote: > >>> On 12.03.20 09:47, Michael S. Tsirkin wrote: > >>> On Thu, Mar 12, 2020 at 09:37:32AM +0100, David Hildenbrand wrote: > >>>> 2. You are essentially stealing THPs in the guest. So the fastest > >>>> mapping (THP in guest and host) is gone. The guest won't be able to make > >>>> use of THP where it previously was able to. I can imagine this implies a > >>>> performance degradation for some workloads. This needs a proper > >>>> performance evaluation. > >>> > >>> I think the problem is more with the alloc_pages API. > >>> That gives you exactly the given order, and if there's > >>> a larger chunk available, it will split it up. > >>> > >>> But for balloon - I suspect lots of other users, > >>> we do not want to stress the system but if a large > >>> chunk is available anyway, then we could handle > >>> that more optimally by getting it all in one go. > >>> > >>> > >>> So if we want to address this, IMHO this calls for a new API. > >>> Along the lines of > >>> > >>> struct page *alloc_page_range(gfp_t gfp, unsigned int min_order, > >>> unsigned int max_order, unsigned int *order) > >>> > >>> the idea would then be to return at a number of pages in the given > >>> range. > >>> > >>> What do you think? Want to try implementing that? > >> > >> You can just start with the highest order and decrement the order until > >> your allocation succeeds using alloc_pages(), which would be enough for > >> a first version. At least I don't see the immediate need for a new > >> kernel API. > > > > OK I remember now. The problem is with reclaim. Unless reclaim is > > completely disabled, any of these calls can sleep. After it wakes up, > > we would like to get the larger order that has become available > > meanwhile. > > > > Yes, but that‘s a pure optimization IMHO. > So I think we should do a trivial implementation first and then see what we gain from a new allocator API. Then we might also be able to justify it using real numbers. > Well how do you propose implement the necessary semantics? I think we are both agreed that alloc_page_range is more or less what's necessary anyway - so how would you approximate it on top of existing APIs? > > > >> -- > >> Thanks, > >> > >> David / dhildenb > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
