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, and just have a trace point in this location instead. 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 :|
