Re: [RFC PATCH 0/4] kvm: Report unused guest pages to host

Alexander Duyck
2019-02-05T18:43:23Z

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