On 10/1/19 12:21 PM, Alexander Duyck wrote: > On Tue, 2019-10-01 at 17:35 +0200, David Hildenbrand wrote: >> On 01.10.19 17:29, Alexander Duyck wrote: >>> This series provides an asynchronous means of reporting to a hypervisor >>> that a guest page is no longer in use and can have the data associated >>> with it dropped. To do this I have implemented functionality that allows >>> for what I am referring to as unused page reporting. The advantage of >>> unused page reporting is that we can support a significant amount of >>> memory over-commit with improved performance as we can avoid having to >>> write/read memory from swap as the VM will instead actively participate >>> in freeing unused memory so it doesn't have to be written. >>> >>> The functionality for this is fairly simple. When enabled it will allocate >>> statistics to track the number of reported pages in a given free area. >>> When the number of free pages exceeds this value plus a high water value, >>> currently 32, it will begin performing page reporting which consists of >>> pulling non-reported pages off of the free lists of a given zone and >>> placing them into a scatterlist. The scatterlist is then given to the page >>> reporting device and it will perform the required action to make the pages >>> "reported", in the case of virtio-balloon this results in the pages being >>> madvised as MADV_DONTNEED. After this they are placed back on their >>> original free list. If they are not merged in freeing an additional bit is >>> set indicating that they are a "reported" buddy page instead of a standard >>> buddy page. The cycle then repeats with additional non-reported pages >>> being pulled until the free areas all consist of reported pages. >>> >>> In order to try and keep the time needed to find a non-reported page to >>> a minimum we maintain a "reported_boundary" pointer. This pointer is used >>> by the get_unreported_pages iterator to determine at what point it should >>> resume searching for non-reported pages. In order to guarantee pages do >>> not get past the scan I have modified add_to_free_list_tail so that it >>> will not insert pages behind the reported_boundary. Doing this allows us >>> to keep the overhead to a minimum as re-walking the list without the >>> boundary will result in as much as 18% additional overhead on a 32G VM. >>> >>> > <snip> > >>> As far as possible regressions I have focused on cases where performing >>> the hinting would be non-optimal, such as cases where the code isn't >>> needed as memory is not over-committed, or the functionality is not in >>> use. I have been using the will-it-scale/page_fault1 test running with 16 >>> vcpus and have modified it to use Transparent Huge Pages. With this I see >>> almost no difference with the patches applied and the feature disabled. >>> Likewise I see almost no difference with the feature enabled, but the >>> madvise disabled in the hypervisor due to a device being assigned. With >>> the feature fully enabled in both guest and hypervisor I see a regression >>> between -1.86% and -8.84% versus the baseline. I found that most of the >>> overhead was due to the page faulting/zeroing that comes as a result of >>> the pages having been evicted from the guest. >> I think Michal asked for a performance comparison against Nitesh's >> approach, to evaluate if keeping the reported state + tracking inside >> the buddy is really worth it. Do you have any such numbers already? (or >> did my tired eyes miss them in this cover letter? :/) >> > I thought what Michal was asking for was what was the benefit of using the > boundary pointer. I added a bit up above and to the description for patch > 3 as on a 32G VM it adds up to about a 18% difference without factoring in > the page faulting and zeroing logic that occurs when we actually do the > madvise. > > Do we have a working patch set for Nitesh's code? The last time I tried > running his patch set I ran into issues with kernel panics. If we have a > known working/stable patch set I can give it a try. Did you try the v12 patch-set [1]? I remember that you reported the CPU stall issue, which I fixed in the v12. [1] https://lkml.org/lkml/2019/8/12/593 > > - Alex > -- Thanks Nitesh
