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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Leonardo Brás (leobras@redhat.com) wrote:
> On Tue, 2022-06-28 at 17:56 +0100, Dr. David Alan Gilbert wrote:
> > * 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
> 
> Thanks for this feedback!
> 
> I have everything already working, but I am struggling with a good property
> name. 
> 
> I am currently using zero_copy_copied (or zero-copy-copied in json), but it does
> not look like a good Migration stat name. 
> 
> Do you have any suggestion?

Shrug; I'm not going to fuss over the name too much as long as it's
reasonable. 'zero-copied' might be OK.

Dave

> Best regards,
> Leo
> 
> 
> > 
> > > 
> > > Best regards,
> > > Leo
> > > 
> > > > Dave
> > > > 
> > > > > With regards,
> > > > > Daniel
> > > > > --
> > > > > > : https://berrange.com      -o-   
> > > > > > https://www.flickr.com/photos/dberrange :|
> > > > > > : https://libvirt.org         -o-           
> > > > > > https://fstop138.berrange.com :|
> > > > > > : https://entangle-photo.org    -o-   
> > > > > > https://www.instagram.com/dberrange :|
> > > > > 
> > > > --
> > > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > > > 
> > > 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK