On Tue, Jun 20, 2017 at 01:29:00PM -0400, Rik van Riel wrote: > On Tue, 2017-06-20 at 18:49 +0200, David Hildenbrand wrote: > > On 20.06.2017 18:44, Rik van Riel wrote: > > > > Nitesh Lal (on the CC list) is working on a way > > > to efficiently batch recently freed pages for > > > free page hinting to the hypervisor. > > > > > > If that is done efficiently enough (eg. with > > > MADV_FREE on the hypervisor side for lazy freeing, > > > and lazy later re-use of the pages), do we still > > > need the harder to use batch interface from this > > > patch? > > > > > > > David's opinion incoming: > > > > No, I think proper free page hinting would be the optimum solution, > > if > > done right. This would avoid the batch interface and even turn > > virtio-balloon in some sense useless. > > I agree with that. Let me go into some more detail of > what Nitesh is implementing: > > 1) In arch_free_page, the being-freed page is added > to a per-cpu set of freed pages. > 2) Once that set is full, arch_free_pages goes into a > slow path, which: > 2a) Iterates over the set of freed pages, and > 2b) Checks whether they are still free, and > 2c) Adds the still free pages to a list that is > to be passed to the hypervisor, to be MADV_FREEd. > 2d) Makes that hypercall. > > Meanwhile all arch_alloc_pages has to do is make sure it > does not allocate a page while it is currently being > MADV_FREEd on the hypervisor side. > > The code Wei is working on looks like it could be > suitable for steps (2c) and (2d) above. Nitesh already > has code for steps 1 through 2b. > > -- > All rights reversed So my question is this: Wei posted these numbers for balloon inflation times: inflating 7GB of an 8GB idle guest: 1) allocating pages (6.5%) 2) sending PFNs to host (68.3%) 3) address translation (6.1%) 4) madvise (19%) It takes about 4126ms for the inflating process to complete. It seems that this is an excessive amount of time to stay under a lock. What are your estimates for Nitesh's work? -- MST
