Re: [PATCH v34 0/4] Virtio-balloon: support free page reporting

[Michael S. Tsirkin]

On Fri, Jun 29, 2018 at 03:52:40PM +0000, Wang, Wei W wrote:
> On Friday, June 29, 2018 10:46 PM, Michael S. Tsirkin wrote:
> > To: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: Wang, Wei W <wei.w.wang@xxxxxxxxx>; virtio-dev@xxxxxxxxxxxxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx;
> > kvm@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; mhocko@xxxxxxxxxx;
> > akpm@xxxxxxxxxxxxxxxxxxxx; torvalds@xxxxxxxxxxxxxxxxxxxx;
> > pbonzini@xxxxxxxxxx; liliang.opensource@xxxxxxxxx;
> > yang.zhang.wz@xxxxxxxxx; quan.xu0@xxxxxxxxx; nilal@xxxxxxxxxx;
> > riel@xxxxxxxxxx; peterx@xxxxxxxxxx
> > Subject: Re: [PATCH v34 0/4] Virtio-balloon: support free page reporting
> > 
> > On Wed, Jun 27, 2018 at 01:06:32PM +0200, David Hildenbrand wrote:
> > > On 25.06.2018 14:05, Wei Wang wrote:
> > > > This patch series is separated from the previous "Virtio-balloon
> > > > Enhancement" series. The new feature,
> > > > VIRTIO_BALLOON_F_FREE_PAGE_HINT, implemented by this series
> > enables
> > > > the virtio-balloon driver to report hints of guest free pages to the
> > > > host. It can be used to accelerate live migration of VMs. Here is an
> > introduction of this usage:
> > > >
> > > > Live migration needs to transfer the VM's memory from the source
> > > > machine to the destination round by round. For the 1st round, all
> > > > the VM's memory is transferred. From the 2nd round, only the pieces
> > > > of memory that were written by the guest (after the 1st round) are
> > > > transferred. One method that is popularly used by the hypervisor to
> > > > track which part of memory is written is to write-protect all the guest
> > memory.
> > > >
> > > > This feature enables the optimization by skipping the transfer of
> > > > guest free pages during VM live migration. It is not concerned that
> > > > the memory pages are used after they are given to the hypervisor as
> > > > a hint of the free pages, because they will be tracked by the
> > > > hypervisor and transferred in the subsequent round if they are used and
> > written.
> > > >
> > > > * Tests
> > > > - Test Environment
> > > >     Host: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
> > > >     Guest: 8G RAM, 4 vCPU
> > > >     Migration setup: migrate_set_speed 100G, migrate_set_downtime 2
> > > > second
> > > >
> > > > - Test Results
> > > >     - Idle Guest Live Migration Time (results are averaged over 10 runs):
> > > >         - Optimization v.s. Legacy = 284ms vs 1757ms --> ~84% reduction
> > > >     - Guest with Linux Compilation Workload (make bzImage -j4):
> > > >         - Live Migration Time (average)
> > > >           Optimization v.s. Legacy = 1402ms v.s. 2528ms --> ~44% reduction
> > > >         - Linux Compilation Time
> > > >           Optimization v.s. Legacy = 5min6s v.s. 5min12s
> > > >           --> no obvious difference
> > > >
> > >
> > > Being in version 34 already, this whole thing still looks and feels
> > > like a big hack to me. It might just be me, but especially if I read
> > > about assumptions like "QEMU will not hotplug memory during
> > > migration". This does not feel like a clean solution.
> > >
> > > I am still not sure if we really need this interface, especially as
> > > real free page hinting might be on its way.
> > >
> > > a) we perform free page hinting by setting all free pages
> > > (arch_free_page()) to zero. Migration will detect zero pages and
> > > minimize #pages to migrate. I don't think this is a good idea but
> > > Michel suggested to do a performance evaluation and Nitesh is looking
> > > into that right now.
> > 
> > Yes this test is needed I think. If we can get most of the benefit without PV
> > interfaces, that's nice.
> > 
> > Wei, I think you need this as part of your performance comparison
> > too: set page poisoning value to 0 and enable KSM, compare with your
> > patches.
> 
> Do you mean live migration with zero pages?
> I can first share the amount of memory transferred during live migration I saw,
> Legacy is around 380MB,
> Optimization is around 340MB.
> This proves that most pages have already been 0 and skipped during the legacy live migration. But the legacy time is still much larger because zero page checking is costly. 
> (It's late night here, I can get you that with my server probably tomorrow)
> 
> Best,
> Wei

Sure thing.

Also we might want to look at optimizing the RLE compressor for
the common case of pages full of zeroes.

Here are some ideas:
https://rusty.ozlabs.org/?p=560

Note Epiphany #2 as well as comments Paolo Bonzini and by Victor Kaplansky.

-- 
MST