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

[Leonardo Bras Soares Passos]

On Tue, Jun 28, 2022 at 4:53 AM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> 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 ?

I am not aware of any NIC without SG available for testing, nor have
any idea on how common they are.
But since we can detect sendmsg() falling back to copying we should
warn the user if this ever happens.

There is also a case in IPv6 related to fragmentation that may cause
MSG_ZEROCOPY to fall back to the copying mechanism, so it's also
covered.

>
> > 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.
>

I could change it so it only warns once, if that is too much output.

Best regards,
Leo

>
>
> > 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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