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

[Leonardo Bras Soares Passos]

On Tue, Jun 28, 2022 at 10:52 AM Dr. David Alan Gilbert
<dgilbert@redhat.com> wrote:
>
> * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > On Tue, Jun 28, 2022 at 09:32:04AM -0300, Leonardo Bras Soares Passos wrote:
> > > 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.
> >
> > Well I'm mostly wondering what we're expecting the user todo with this
> > information.


My rationale on that:
- zero-copy-send is a feature that is supposed to improve send
throughput by reducing cpu usage.
- there is a chance the sendmsg(MSG_ZEROCOPY) fails to use zero-copy
- if this happens, there will be a potential throughput decrease on sendmsg()
- the user (or management app) need to know when zero-copy-send is
degrading throughput, so it can be disabled
- this is also important for performance testing, given it can be
confusing having zero-copy-send improving throughput in some cases,
and degrading in others, without any apparent reason why.

> > Generally a log file containing warnings ends up turning
> > into a bug report. If we think it is important for users and/or mgmt
> > apps to be aware of this info, then it might be better to actually
> > put a field in the query-migrate stats to report if zero-copy is
> > being honoured or not,
>
> Yeh just a counter would work there I think.

The warning idea was totally due to my inexperience on this mgmt app
interface, since I had no other idea on how to deal with that.

I think having it in query-migrate is a much better idea than a
warning, since it should be much easier to parse and disable
zero-copy-send if desired.
Even in my current qemu test script, it's much better having it in
query-migrate.

>
> > and just have a trace point in this location
> > instead.
>
> Yeh.
>

Yeap, the counter idea seems great!
Will it be always printed there, or only when zero-copy-send is enabled?

Best regards,
Leo

> Dave
>
> > With regards,
> > Daniel
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> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>