Re: [PATCH v4 08/19] migration: Add postcopy-preempt capability

[Daniel P . Berrangé]

On Thu, Mar 31, 2022 at 11:08:46AM -0400, Peter Xu wrote:
> Firstly, postcopy already preempts precopy due to the fact that we do
> unqueue_page() first before looking into dirty bits.
> 
> However that's not enough, e.g., when there're host huge page enabled, when
> sending a precopy huge page, a postcopy request needs to wait until the whole
> huge page that is sending to finish.  That could introduce quite some delay,
> the bigger the huge page is the larger delay it'll bring.
> 
> This patch adds a new capability to allow postcopy requests to preempt 
> existing
> precopy page during sending a huge page, so that postcopy requests can be
> serviced even faster.
> 
> Meanwhile to send it even faster, bypass the precopy stream by providing a
> standalone postcopy socket for sending requested pages.
> 
> Since the new behavior will not be compatible with the old behavior, this will
> not be the default, it's enabled only when the new capability is set on both
> src/dst QEMUs.
> 
> This patch only adds the capability itself, the logic will be added in follow
> up patches.
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/migration.c | 23 +++++++++++++++++++++++
>  migration/migration.h |  1 +
>  qapi/migration.json   |  8 +++++++-
>  3 files changed, 31 insertions(+), 1 deletion(-)


> diff --git a/qapi/migration.json b/qapi/migration.json
> index 18e2610e88..3523f23386 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -463,6 +463,12 @@
>  #                       procedure starts. The VM RAM is saved with running 
> VM.
>  #                       (since 6.0)
>  #
> +# @postcopy-preempt: If enabled, the migration process will allow postcopy
> +#                    requests to preempt precopy stream, so postcopy requests
> +#                    will be handled faster.  This is a performance feature 
> and
> +#                    should not affect the correctness of postcopy migration.
> +#                    (since 7.0)

Now 7.1

  Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


With regards,
Daniel
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