On Tue, 2019-02-05 at 12:25 -0500, Nitesh Narayan Lal wrote: > On 2/4/19 1:15 PM, Alexander Duyck wrote: > > This patch set provides a mechanism by which guests can notify the host of > > pages that are not currently in use. Using this data a KVM host can more > > easily balance memory workloads between guests and improve overall system > > performance by avoiding unnecessary writing of unused pages to swap. > > > > In order to support this I have added a new hypercall to provided unused > > page hints and made use of mechanisms currently used by PowerPC and s390 > > architectures to provide those hints. To reduce the overhead of this call > > I am only using it per huge page instead of of doing a notification per 4K > > page. By doing this we can avoid the expense of fragmenting higher order > > pages, and reduce overall cost for the hypercall as it will only be > > performed once per huge page. > > > > Because we are limiting this to huge pages it was necessary to add a > > secondary location where we make the call as the buddy allocator can merge > > smaller pages into a higher order huge page. > > > > This approach is not usable in all cases. Specifically, when KVM direct > > device assignment is used, the memory for a guest is permanently assigned > > to physical pages in order to support DMA from the assigned device. In > > this case we cannot give the pages back, so the hypercall is disabled by > > the host. > > > > Another situation that can lead to issues is if the page were accessed > > immediately after free. For example, if page poisoning is enabled the > > guest will populate the page *after* freeing it. In this case it does not > > make sense to provide a hint about the page being freed so we do not > > perform the hypercalls from the guest if this functionality is enabled. > > > > My testing up till now has consisted of setting up 4 8GB VMs on a system > > with 32GB of memory and 4GB of swap. To stress the memory on the system I > > would run "memhog 8G" sequentially on each of the guests and observe how > > long it took to complete the run. The observed behavior is that on the > > systems with these patches applied in both the guest and on the host I was > > able to complete the test with a time of 5 to 7 seconds per guest. On a > > system without these patches the time ranged from 7 to 49 seconds per > > guest. I am assuming the variability is due to time being spent writing > > pages out to disk in order to free up space for the guest. > > Hi Alexander, > > Can you share the host memory usage before and after your run. (In both > the cases with your patch-set and without your patch-set) Here are some snippets from the /proc/meminfo for the system both before and after the test. W/O patch -- Before -- MemTotal: 32881396 kB MemFree: 21363724 kB MemAvailable: 25891228 kB Buffers: 2276 kB Cached: 4760280 kB SwapCached: 0 kB Active: 7166952 kB Inactive: 1474980 kB Active(anon): 3893308 kB Inactive(anon): 8776 kB Active(file): 3273644 kB Inactive(file): 1466204 kB Unevictable: 16756 kB Mlocked: 16756 kB SwapTotal: 4194300 kB SwapFree: 4194300 kB Dirty: 29812 kB Writeback: 0 kB AnonPages: 3896540 kB Mapped: 75568 kB Shmem: 10044 kB -- After -- MemTotal: 32881396 kB MemFree: 194668 kB MemAvailable: 51356 kB Buffers: 24 kB Cached: 129036 kB SwapCached: 224396 kB Active: 27223304 kB Inactive: 2589736 kB Active(anon): 27220360 kB Inactive(anon): 2481592 kB Active(file): 2944 kB Inactive(file): 108144 kB Unevictable: 16756 kB Mlocked: 16756 kB SwapTotal: 4194300 kB SwapFree: 35616 kB Dirty: 0 kB Writeback: 0 kB AnonPages: 29476628 kB Mapped: 22820 kB Shmem: 5516 kB W/ patch -- Before -- MemTotal: 32881396 kB MemFree: 26618880 kB MemAvailable: 27056004 kB Buffers: 2276 kB Cached: 781496 kB SwapCached: 0 kB Active: 3309056 kB Inactive: 393796 kB Active(anon): 2932728 kB Inactive(anon): 8776 kB Active(file): 376328 kB Inactive(file): 385020 kB Unevictable: 16756 kB Mlocked: 16756 kB SwapTotal: 4194300 kB SwapFree: 4194300 kB Dirty: 96 kB Writeback: 0 kB AnonPages: 2935964 kB Mapped: 75428 kB Shmem: 10048 kB -- After -- MemTotal: 32881396 kB MemFree: 22677904 kB MemAvailable: 26543092 kB Buffers: 2276 kB Cached: 4205908 kB SwapCached: 0 kB Active: 3863016 kB Inactive: 3768596 kB Active(anon): 3437368 kB Inactive(anon): 8772 kB Active(file): 425648 kB Inactive(file): 3759824 kB Unevictable: 16756 kB Mlocked: 16756 kB SwapTotal: 4194300 kB SwapFree: 4194300 kB Dirty: 1336180 kB Writeback: 0 kB AnonPages: 3440528 kB Mapped: 74992 kB Shmem: 10044 kB
