On Tue, 2022-06-28 at 17:56 +0100, Dr. David Alan Gilbert wrote: > * Leonardo Bras Soares Passos (leob...@redhat.com) wrote: > > 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. > > 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? 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 / dgilb...@redhat.com / Manchester, UK > > > > >