Re: [PATCH v5 09/16] multifd: Add COLO support

[Fabiano Rosas <farosas@suse.de>]

Lukas Straub <lukasstraub2@web.de> writes:

> 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, this is already worth it for the precopy phase as it helps with
> converging. Moreover, 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
>
> Evaluation
> While multifd has slightly higher latency when the guest idles, it is
> 10ms faster under load and more importantly it's worst case latency is
> less than 1/2 of plain under load as can be seen in the 99. Percentile.
>
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> Signed-off-by: Lukas Straub <lukasstraub2@web.de>

Reviewed-by: Fabiano Rosas <farosas@suse.de>