Re: [PATCH 1/2] docs/migration: document that postcopy can be used with multifd

[Fabiano Rosas <farosas@suse.de>]

Bin Guo <guobin@linux.alibaba.com> writes:

> Since commit e27418861288 ("migration: enable multifd and postcopy
> together") in the 10.1 release, the multifd and postcopy-ram capabilities
> are no longer mutually exclusive, but the postcopy documentation does not
> mention multifd at all, leaving users unaware that the two features can be
> combined.
>
> Add a short section describing how they interact: multifd carries the
> pages of the precopy phase, the channels are flushed and synced before the
> switchover (because loading a page in a multifd receive thread is not
> atomic with regard to a running vCPU), and the postcopy phase itself does
> not use the multifd channels.
>
> Signed-off-by: Bin Guo <guobin@linux.alibaba.com>
> ---
>  docs/devel/migration/postcopy.rst | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/docs/devel/migration/postcopy.rst b/docs/devel/migration/postcopy.rst
> index e319388d8f..570a3a1ce6 100644
> --- a/docs/devel/migration/postcopy.rst
> +++ b/docs/devel/migration/postcopy.rst
> @@ -294,6 +294,32 @@ the background migration channel.  Anyone who cares about latencies of page
>  faults during a postcopy migration should enable this feature.  By default,
>  it's not enabled.
>  
> +Postcopy with multifd
> +---------------------
> +
> +The ``multifd`` capability can be enabled together with ``postcopy-ram``
> +since the 10.1 QEMU release.  The two features apply to different phases of
> +the migration:
> +
> +  - During the precopy phase, guest pages are sent over the multifd channels
> +    as usual, so the initial RAM transfer can use all of them.
> +
> +  - Just before switching to postcopy, the source flushes and syncs the
> +    multifd channels.  This guarantees that all the pages already queued on
> +    those channels are loaded on the destination *before* the destination
> +    CPUs are started, which is required because loading a page in a multifd
> +    receive thread is not atomic with regard to a running vCPU.
> +
> +  - During the postcopy phase the multifd channels are no longer used for
> +    guest pages.  Both the background stream and the pages requested by the
> +    destination go through the background migration channel instead, or
> +    through the preempt channel for the requested pages when postcopy
> +    preemption is enabled.
> +
> +Consequently, multifd only speeds up the precopy phase of a postcopy
> +migration.  The bandwidth available once postcopy has started is the same as
> +without multifd.
> +
>  Postcopy blocktime statistics
>  -----------------------------

Looks good to me, I'll wait a bit to allow Peter to comment.

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