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. - Alex
