On Tue, Jun 28, 2022 at 10:52 AM Dr. David Alan Gilbert <dgilb...@redhat.com> wrote: > > * Daniel P. Berrangé (berra...@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 <berra...@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 > > -- > > |: 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 / dgilb...@redhat.com / Manchester, UK >
