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2015-11-27 2:19 GMT+08:00 Daniel P. Berrange <berrange at redhat.com>:

> On Thu, Nov 26, 2015 at 05:39:04PM +0000, Daniel P. Berrange wrote:
> > On Thu, Nov 26, 2015 at 11:55:31PM +0800, 少合冯 wrote:
> > > 3.  dynamically choose when to activate xbzrle compress for live
> migration.
> > >      This is the best.
> > >      xbzrle really wants to be used if the network is not able to keep
> up
> > > with the dirtying rate of the guest RAM.
> > >      But how do I check the coming migration fit this situation？
> >
> > FWIW, if we decide we want compression support in Nova, I think that
> > having the Nova libvirt driver dynamically decide when to use it is
> > the only viable approach. Unfortunately the way the QEMU support
> > is implemented makes it very hard to use, as QEMU forces you to decide
> > to use it upfront, at a time when you don't have any useful information
> > on which to make the decision :-(  To be useful IMHO, we really need
> > the ability to turn on compression on the fly for an existing active
> > migration process. ie, we'd start migration off and let it run and
> > only enable compression if we encounter problems with completion.
> > Sadly we can't do this with QEMU as it stands today :-(
> >
>
[Shaohe Feng]
Add more guys working on kernel/hypervisor in our loop.
Wonder whether there will be any good solutions to improve it in QEMU in
future.


> > Oh and of course we still need to address the issue of RAM usage and
> > communicating that need with the scheduler in order to avoid OOM
> > scenarios due to large compression cache.
> >
> > I tend to feel that the QEMU compression code is currently broken by
> > design and needs rework in QEMU before it can be pratically used in
> > an autonomous fashion :-(
>
> Actually thinking about it, there's not really any significant
> difference between Option 1 and Option 3. In both cases we want
> a nova.conf setting live_migration_compression=on|off to control
> whether we want to *permit* use  of compression.
>
> The only real difference between 1 & 3 is whether migration has
> compression enabled always, or whether we turn it on part way
> though migration.
>
> So although option 3 is our desired approach (which we can't
> actually implement due to QEMU limitations), option 1 could
> be made fairly similar if we start off with a very small
> compression cache size which would have the effect of more or
> less disabling compression initially.
>
> We already have logic in the code for dynamically increasing
> the max downtime value, which we could mirror here
>
> eg something like
>
>  live_migration_compression=on|off
>
>   - Whether to enable use of compression
>
>  live_migration_compression_cache_ratio=0.8
>
>   - The maximum size of the compression cache relative to
>     the guest RAM size. Must be less than 1.0
>
>  live_migration_compression_cache_steps=10
>
>   - The number of steps to take to get from initial cache
>     size to the maximum cache size
>
>  live_migration_compression_cache_delay=75
>
>   - The time delay in seconds between increases in cache
>     size
>
>
> In the same way that we do with migration downtime, instead of
> increasing cache size linearly, we'd increase it in ever larger
> steps until we hit the maximum. So we'd start off fairly small
> a few MB, and monitoring the cache hit rates, we'd increase it
> periodically.  If the number of steps configured and time delay
> between steps are reasonably large, that would have the effect
> that most migrations would have a fairly small cache and would
> complete without needing much compression overhead.
>
> Doing this though, we still need a solution to the host OOM scenario
> problem. We can't simply check free RAM at start of migration and
> see if there's enough to spare for compression cache, as the schedular
> can spawn a new guest on the compute host at any time, pushing us into
> OOM. We really need some way to indicate that there is a (potentially
> very large) extra RAM overhead for the guest during migration.
>
> ie if live_migration_compression_cache_ratio is 0.8 and we have a
> 4 GB guest, we need to make sure the schedular knows that we are
> potentially going to be using 7.2 GB of memory during migration
>
>
[Shaohe Feng]
These suggestions sounds good.
Thank you, Daneil.

Do we need to consider this factor:
  Seems, XBZRLE compress is executed after bulk stage. During the bulk
stage,
  calculate an transfer rate. If the transfer rate bellow a certain
  threshold value, we can set a bigger cache size.




> Regards,
> Daniel
> --
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>

BR
Shaohe Feng
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