From: "Michael S. Tsirkin" <mst@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: Wei Wang <wei.w.wang@intel.com>,
virtio-dev@lists.oasis-open.org, linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
linux-mm@kvack.org, mhocko@kernel.org, akpm@linux-foundation.org,
torvalds@linux-foundation.org, pbonzini@redhat.com,
liliang.opensource@gmail.com, yang.zhang.wz@gmail.com,
quan.xu0@gmail.com, nilal@redhat.com, riel@redhat.com,
peterx@redhat.com
Subject: Re: [PATCH v34 0/4] Virtio-balloon: support free page reporting
Date: Fri, 29 Jun 2018 17:45:48 +0300 [thread overview]
Message-ID: <20180629172216-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <c4dd0a13-91fb-c0f5-b41f-54421fdacca9@redhat.com>
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
next prev parent reply other threads:[~2018-06-29 14:45 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-25 12:05 [PATCH v34 0/4] Virtio-balloon: support free page reporting Wei Wang
2018-06-25 12:05 ` [PATCH v34 1/4] mm: support to get hints of free page blocks Wei Wang
2018-06-25 12:05 ` [PATCH v34 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT Wei Wang
2018-06-26 1:37 ` Michael S. Tsirkin
2018-06-26 3:46 ` Wei Wang
2018-06-26 3:56 ` Michael S. Tsirkin
2018-06-26 12:27 ` Wei Wang
2018-06-26 13:34 ` Michael S. Tsirkin
2018-06-27 1:24 ` Wei Wang
2018-06-27 2:41 ` Michael S. Tsirkin
2018-06-27 3:00 ` Wei Wang
2018-06-27 3:58 ` Michael S. Tsirkin
2018-06-27 5:27 ` Wei Wang
2018-06-27 16:53 ` [virtio-dev] " Michael S. Tsirkin
2018-06-25 12:05 ` [PATCH v34 3/4] mm/page_poison: expose page_poisoning_enabled to kernel modules Wei Wang
2018-06-25 12:05 ` [PATCH v34 4/4] virtio-balloon: VIRTIO_BALLOON_F_PAGE_POISON Wei Wang
2018-06-27 11:06 ` [PATCH v34 0/4] Virtio-balloon: support free page reporting David Hildenbrand
2018-06-29 3:51 ` Wei Wang
2018-06-29 7:46 ` David Hildenbrand
2018-06-29 11:31 ` Wei Wang
2018-06-29 11:53 ` David Hildenbrand
2018-06-29 15:55 ` Wang, Wei W
2018-06-29 16:03 ` David Hildenbrand
2018-06-29 14:45 ` Michael S. Tsirkin [this message]
2018-06-29 15:28 ` David Hildenbrand
2018-06-29 15:52 ` Wang, Wei W
2018-06-29 16:32 ` Michael S. Tsirkin
2018-06-30 4:31 ` Wei Wang
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