On Fri, 2020-11-20 at 09:08 -0500, Steven Rostedt wrote: > On Fri, 20 Nov 2020 14:43:21 +0100 > Dario Faggioli <dfaggioli@xxxxxxxx> wrote: > > > > So, you often say that "the accuracy of the synchronization > > protocol is > > XX ms". Now, I guess that means that an event in the guest and the > > Note, we are usually microsecond (us) apart, not millisecond (ms) ;-) > Ah, yes, sure... And sorry about that! I know it us, I'm not sure how I ended up writing ms. That would be quite terrible indeed! :-D > > corresponding event in the host (or vice versa) are XX ms apart. > > And > > that's even after the synchronization of the two traces, is that > > right? > > At plumbers we talked with Thomas Gleixner and he suggested ideas of > how to > get to the actual shifts used in the hardware that should give us > exact > timestamp offsets. We are currently working on that. > Yes, I remember that, I attended the BoF. > But in the mean time, > the P2P is giving us somewhere between 5 and 10 us accuracy. And > that's > simply because the jitter of the vsock connection (which is used for > the > synchronization at start and end of the traces) has a 5 to 10 us > jitter, > and it's not possible to get a more accurate than the medium that is > being > used. > Yes, with a student that I was helping with his thesis, we applied one debug patch to trace-cmd that you have on this list, and we tried the different synchronization strategies, frequency, etc. > > Question is, how do you measure that? Sure, I can look manually for > > an > > occurrence of the pattern that I described above: i.e., an event in > > the > > guest, then the corresponding one in the host and compute the > > difference between the timestamps. > > You mean, how we measure the accuracy? It's usually done by seeing > when we > have events from the guest showing up when we should be in the host > (it's > like seeing events from userspace when you are in the kernel). > Ok, makes sense. I need to try it first hand to make sure I've properly understood it, though. I'll collect some more tracing and looks for situations like these. Thanks! > > But do you have a way to do so automatically, or with a > > script/program, > > etc? > > We have talked about having something scan to find cases where the > guest > event happens in kernel and do some post processing shifting, but > haven't > gotten there yet. > Yep, as said, I was thinking at it as a way to measure how accurately the traces are synched, but indeed once one has it, it can even use it to actually synch them better. But I understand how it's rather tricky. > If the hardware settings can work, then there will be no > need to do so. > Indeed. Well, perhaps it could still be useful, as a test/check whether things are working? :-) Regards -- Dario Faggioli, Ph.D http://about.me/dario.faggioli Virtualization Software Engineer SUSE Labs, SUSE https://www.suse.com/ ------------------------------------------------------------------- <<This happens because _I_ choose it to happen!>> (Raistlin Majere)
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