Re: [PATCH v2 4/8] multifd: Add COLO support

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jan 20, 2026 at 12:13:58PM -0500, Peter Xu wrote:
> On Sat, Jan 17, 2026 at 03:09:11PM +0100, Lukas Straub wrote:
> > Like in the normal ram_load() path, put the received pages into the
> > colo cache and mark the pages in the bitmap so that they will be
> > flushed to the guest later.
> > 
> > Multifd with COLO is useful to reduce the VM pause time during checkpointing
> > for latency sensitive workloads. In such workloads the worst-case latency
> > is especially important.
> > 
> > Also, multifd migration is the preferred way to do migration nowadays and this
> > allows to use multifd compression with COLO.
> > 
> > Benchmark:
> > Cluster nodes
> >  - Intel Xenon E5-2630 v3
> >  - 48Gb RAM
> >  - 10G Ethernet
> > Guest
> >  - Windows Server 2016
> >  - 6Gb RAM
> >  - 4 cores
> > Workload
> >  - Upload a file to the guest with SMB to simulate moderate
> >    memory dirtying
> >  - Measure the memory transfer time portion of each checkpoint
> >  - 600ms COLO checkpoint interval
> > 
> > Results
> > Plain
> >  idle mean: 4.50ms 99per: 10.33ms
> >  load mean: 24.30ms 99per: 78.05ms
> > Multifd-4
> >  idle mean: 6.48ms 99per: 10.41ms
> >  load mean: 14.12ms 99per: 31.27ms
> 
> Thanks for the numbers.  They're persuasive at least from 1st look.
> 
> Said that, one major question is, multifd should only help with throughput
> when cpu is a bottleneck sending, in your case it's 10Gbps NIC.  Normally
> any decent cpu should be able to push closer to 10Gbps even without
> multifd.

That assumes the CPUs used by migration are otherwise idle though. If the
host is busy running guest workloads, only small timeslices may be available
for use by migration threads. Using multifd would better utilize what's
available if multiple host CPUs have partial availability.


With regards,
Daniel
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