Re: [PATCH v2] flow, repair: Wait for a short while for passt-repair to connect

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Mar 12, 2025 at 09:39:10PM +0100, Stefano Brivio wrote:
> On Wed, 12 Mar 2025 12:29:11 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Tue, Mar 11, 2025 at 10:55:32PM +0100, Stefano Brivio wrote:
> > > On Tue, 11 Mar 2025 12:13:46 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
[snip]
> > > > Now, as it happens, the default downtime limit is 300ms, so an
> > > > additional 10ms is probably fine (though 100ms really wasn't).
> > > > Nonetheless the reasoning above isn't valid.  
> > > 
> > > ~50 ms is actually quite easy to get with a few (8) gigabytes of
> > > memory,  
> > 
> > 50ms as measured above?  That's a bit surprising, because there's no
> > particular reason for it to depend on memory size.  AFAICT
> > SET_DEVICE_STATE_FD is called close to immediately before actually
> > reading/writing the stream from the backend.
> 
> Oops, right, this figure I had in mind actually came from a rather
> different measurement, that is, checking when the guest appeared to
> resume from traffic captures with iperf3 running.

Ok.  That is a reasonable measure of the downtime, at least as long as
the guest is continuously trying to send, which it will with iperf3.

Which means by adding a 100ms delay, you'd triple the downtime, which
isn't really ok.  With more RAM and/or smaller migration bandwidth
this would increase up to the 300ms limit.  In that case 100ms would
still be a 33% (unaccounted for) increase, which still isn't really
ok.

> I definitely can't see this difference if I repeat the same measurement
> as above.
> 
> > The memory size will of course affect the total migration time, and
> > maybe the downtime.  As soon as qemu thinks it can transfer all
> > remaining RAM within its downtime limit, qemu will go to the stopped
> > phase.  With a fast local to local connection, it's possible qemu
> > could enter that stopped phase almost immediately.
> > 
> > > that's why 100 ms also looked fine to me, but sure, 10 ms
> > > sounds more reasonable.
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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