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