On 02.10.19 02:55, Alexander Duyck wrote: > On Tue, Oct 1, 2019 at 12:16 PM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: >> >> >> 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 > > So I tried testing with the spin_lock calls replaced with spin_lock > _irq to resolve the IRQ issue. I also had shuffle enabled in order to > increase the number of pages being dirtied. > > With that setup the bitmap approach is running significantly worse > then my approach, even with the boundary removed. Since I had to It would make sense to share the setup+benchmark+performance indication that you measured. You don't have to share the actual numbers. > modify the code to even getting working I am not comfortable posting > numbers. My suggestion would be to look at reworking the patch set and > post numbers for my patch set versus the bitmap approach and we can > look at them then. I would prefer not to spend my time fixing and > tuning a patch set that I am still not convinced is viable. I agree, I think Nitesh should work on his patch set and try to reproduce what you are seeing. Also, I think to make a precise statement of "which overhead comes with external tracking", Nitesh should switch to an approach (motivated by Michal) like 1. Sense lockless if a page is still free 2. start_isolate_page_range() -> Failed? Skip 3. test_pages_isolated() -> No? undo_isolate_page_range(), skip 4. Repeat for multiple pages + report 5. undo_isolate_page_range() That is the bare minimum any external tracking will need = some overhead for the tracking data. As a nice side effect, it get's rid of taking the zone lock manually AFAIKS. But that's unrelated to your series, only to quantify "how much" does external tracking actually cost. -- Thanks, David / dhildenb
