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. > > If another process needs to perform a massive manipulation of the free > list, such as compaction, it can either reset a given individual boundary > which will push the boundary back to the list_head, or it can clear the > bit indicating the zone is actively processing which will result in the > reporting process resetting all of the boundaries for a given zone. > > I am leaving a number of things hard-coded such as limiting the lowest > order processed to pageblock_order, and have left it up to the guest to > determine what the limit is on how many pages it wants to allocate to > process the hints. The upper limit for this is based on the size of the > queue used to store the scatterlist. > > I wanted to avoid gaming the performance testing for this. As far as > possible gain a significant performance improvement should be visible in > cases where guests are forced to write/read from swap. As such, testing > it would be more of a benchmark of copying a page from swap versus just > allocating a zero page. I have been verifying that the memory is being > freed using memhog to allocate all the memory on the guest, and then > watching /proc/meminfo to verify the host sees the memory returned after > the test completes. > > 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? :/) -- Thanks, David / dhildenb
