On 10/2/19 3:13 AM, David Hildenbrand wrote: > 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. +1 > >> 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. Sure. I am always open to suggestions of different benchmarks/setup where I can run my patch-set. > > 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. Exactly, first, we need to be sure that the overhead caused by bitmap scanning is not significant. If we are fine with the approach, I will certainly look into this as this would be an excellent enhancement. -- Thanks Nitesh
