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Re: [Qemu-devel] [PATCH 00/11] kvm/migration: support KVM_CLEAR_DIRTY_LO
Re: [Qemu-devel] [PATCH 00/11] kvm/migration: support KVM_CLEAR_DIRTY_LOG
Thu, 16 May 2019 17:26:03 +0800
On Thu, May 09, 2019 at 10:33:19AM +0800, Peter Xu wrote:
> On Wed, May 08, 2019 at 01:55:07PM +0200, Paolo Bonzini wrote:
> > On 08/05/19 06:39, Peter Xu wrote:
> > >> The disadvantage of this is that you won't clear in the kernel those
> > >> dirty bits that come from other sources (e.g. vhost or
> > >> address_space_map). This can lead to double-copying of pages.
> > >>
> > >> Migration already makes a local copy in rb->bmap, and
> > >> memory_region_snapshot_and_clear_dirty can also do the clear. Would it
> > >> be possible to invoke the clear using rb->bmap instead of the KVMSlot's
> > >> new bitmap?
> > >
> > > Actually that's what I did in the first version before I post the work
> > > but I noticed that there seems to have a race condition with the
> > > design. The problem is we have multiple copies of the same dirty
> > > bitmap from KVM and the race can happen with those multiple users
> > > (bitmaps of the users can be a merged version containing KVM and other
> > > sources like vhost, address_space_map, etc. but let's just make it
> > > simpler to not have them yet).
> > I see now. And in fact the same double-copying inefficiency happens
> > already without this series, so you are improving the situation anyway.
> > Have you done any kind of benchmarking already?
> Not yet. I posted the series for some initial reviews first before
> moving on with performance tests.
> My plan of the test scenario could be:
> - find a guest with relatively large memory (I would guess it is
> better to have memory like 64G or even more to make some big
> - run random dirty memory workload upon most of the mem, with dirty
> rate X Bps.
> - setup the migration bandwidth to Y Bps (Y should be bigger than X
> but not that big. One could be X=800M and Y=1G to emulate 10G nic
> with a workload that we can still converge with precopy only) and
> start precopy migration.
> - measure total migration time with CLEAR_LOG on & off. We should
> expect the guest to have these with CLEAR_LOG: (1) not hang during
> log_sync, and (2) migration should complete faster.
Some updates on performance numbers.
Summary: the ideal case below shows ~40% or even more time reduced to
migrate the same VM with the same workload. In other words, it could
be seen as ~40% faster than before.
Test environment: 13G guest, 10G test mem (so I leave 3G untouched),
dirty rate 900MB/s, BW 10Gbps to emulate ixgbe, downtime 100ms.
IO pattern: I pre-fault all the 10G mem then I do random writes (with
command "mig_mon mm_dirty 10240 900 random" ) upon the test memory
with a constant dirty rate (900MB/s, as mentioned). Then I migrate
during the IOs.
Here's the total migration time of such VM (for each scenario, I run
the migration 5 times then I get an average migration total time
| scenario | migration times (s) | average (s) |
| no CLEAR_LOG | 55, 54, 56, 74, 54 | 58 |
| 1G chunk | 40, 39, 41, 39, 40 | 40 |
| 128M chunk | 38, 40, 37, 40, 38 | 38 |
| 16M chunk | 42, 40, 38, 41, 38 | 39 |
| 1M chunk | 37, 40, 36, 40, 39 | 38 |
The first "no CLEAR_LOG" means the master branch which still uses the
GET_DIRTY only. The latter four scenarios are all with the new
CLEAR_LOG interface, aka, this series. The test result shows that
128M chunk size seems to be a good default value instead of 1G (which
this series used). I'll adjust that accordingly when I post the next
- [Qemu-devel] [PATCH 06/11] memory: Introduce memory listener hook log_clear(), (continued)
- [Qemu-devel] [PATCH 06/11] memory: Introduce memory listener hook log_clear(), Peter Xu, 2019/05/08
- [Qemu-devel] [PATCH 07/11] kvm: Update comments for sync_dirty_bitmap, Peter Xu, 2019/05/08
- [Qemu-devel] [PATCH 08/11] kvm: Persistent per kvmslot dirty bitmap, Peter Xu, 2019/05/08
- [Qemu-devel] [PATCH 09/11] kvm: Introduce slots lock for memory listener, Peter Xu, 2019/05/08
- [Qemu-devel] [PATCH 10/11] kvm: Support KVM_CLEAR_DIRTY_LOG, Peter Xu, 2019/05/08
- [Qemu-devel] [PATCH 11/11] migration: Split log_clear() into smaller chunks, Peter Xu, 2019/05/08
- Re: [Qemu-devel] [PATCH 00/11] kvm/migration: support KVM_CLEAR_DIRTY_LOG, Paolo Bonzini, 2019/05/08