> 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. > >> -- >> Thanks, >> >> David / dhildenb > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
