Re: [PATCH v1 2/2] migration/multifd: Warn user when zerocopy not working

[Daniel P . Berrangé]

On Mon, Jun 27, 2022 at 10:09:09PM -0300, Leonardo Bras wrote:
> Some errors, like the lack of Scatter-Gather support by the network
> interface(NETIF_F_SG) may cause sendmsg(...,MSG_ZEROCOPY) to fail on using
> zero-copy, which causes it to fall back to the default copying mechanism.

How common is this lack of SG support ? What NICs did you have that
were affected ?

> After each full dirty-bitmap scan there should be a zero-copy flush
> happening, which checks for errors each of the previous calls to
> sendmsg(...,MSG_ZEROCOPY). If all of them failed to use zero-copy, then
> warn the user about it.
> 
> Since it happens once each full dirty-bitmap scan, even in worst case
> scenario it should not print a lot of warnings, and will allow tracking
> how many dirty-bitmap iterations were not able to use zero-copy send.

For long running migrations which are not converging, or converging
very slowly there could be 100's of passes.



> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  migration/multifd.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 684c014c86..9c62aec84e 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -624,6 +624,9 @@ int multifd_send_sync_main(QEMUFile *f)
>              if (ret < 0) {
>                  error_report_err(err);
>                  return -1;
> +            } else if (ret == 1) {
> +                warn_report("The network device is not able to use "
> +                            "zero-copy-send: copying is being used");
>              }
>          }
>      }
> -- 
> 2.36.1
> 

With regards,
Daniel
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