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

[Michael S. Tsirkin]

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.


> b) we perform free page hinting using something that Nitesh proposed. We
> get in QEMU blocks of free pages that we can MADV_FREE. In addition we
> could e.g. clear the dirty bit of these pages in the dirty bitmap, to
> hinder them from getting migrated. Right now the hinting mechanism is
> synchronous (called from arch_free_page()) but we might be able to
> convert it into something asynchronous.
> 
> So we might be able to completely get rid of this interface.

The way I see it, hinting during alloc/free will always add
overhead which might be unacceptable for some people.  So even with
Nitesh's patches there's value in enabling / disabling hinting
dynamically. And Wei's patches would then be useful to set
the stage where we know the initial page state.


> And looking at all the discussions and problems that already happened
> during the development of this series, I think we should rather look
> into how clean free page hinting might solve the same problem.

I'm not sure I follow the logic. We found that neat tricks
especially re-using the max order free page for reporting.

> If it can't be solved using free page hinting, fair enough.

I suspect Nitesh will need to find a way not to have mm code
call out to random drivers or subsystems before that code
is acceptable.


-- 
MST