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

[Dr. David Alan Gilbert]

* Leonardo Bras Soares Passos (leobras@redhat.com) wrote:
> 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.

Yeh it's not too silly an idea!
The way some of these warning or stats get to us can be a bit random,
but sometimes can confuse things.

> 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?

You could make it either if it's enabled or if it's none zero.
(I guess you want it to reset to 0 at the start of a new migration).

Dave

> 
> Best regards,
> Leo
> 
> > Dave
> >
> > > With regards,
> > > Daniel
> > > --
> > > |: https://berrange.com      -o-    
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> > > |: https://libvirt.org         -o-            
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> > >
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK