Re: [Qemu-devel] [PATCH 00/16] Multifd v4

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrange (berrange@redhat.com) wrote:
> On Tue, Mar 14, 2017 at 10:21:43AM +0000, Dr. David Alan Gilbert wrote:
> > * Juan Quintela (quintela@redhat.com) wrote:
> > > Hi
> > > 
> > > This is the 4th version of multifd. Changes:
> > > - XBZRLE don't need to be checked for
> > > - Documentation and defaults are consistent
> > > - split socketArgs
> > > - use iovec instead of creating something similar.
> > > - We use now the exported size of target page (another HACK removal)
> > > - created qio_chanel_{wirtev,readv}_all functions.  the _full() name
> > >   was already taken.
> > >   What they do is the same that the without _all() function, but if it
> > >   returns due to blocking it redo the call.
> > > - it is checkpatch.pl clean now.
> > > 
> > > Please comment, Juan.
> > 
> > High level things,
> >   a) I think you probably need to do some bandwidth measurements to show
> >     that multifd is managing to have some benefit - it would be good
> >     for the cover letter.
> 
> Presumably this would be a building block to solving the latency problems
> with post-copy, by reserving one channel for use transferring out of band
> pages required by target host page faults.

Right, it's on my list to look at;  there's some interesting questions about
the way in which the main fd carrying the headers interacts, and also what
happens to pages immediately after the requested page; for example, lets
say we're currently streaming at address 'S' and a postcopy request (P) comes in;
so what we currently have on one FD is:

    S,S+1....S+n,P,P+1,P+2,P+n

Note that when a request comes in we flip location so we start sending background
pages from P+1 on the assumption that they'll be wanted soon.

with 3 FDs this would go initially as:
    S    S+3 P+1 P+4
    S+1  S+4 P+2 ..
    S+2  P   P+3 ..

now if we had a spare FD for postcopy we'd do:
    S    S+3 P+1 P+4
    S+1  S+4 P+2 ..
    S+2  S+5 P+3 ..
    -    P   -   -

So 'P' got there quickly - but P+1 is stuck behind the S's; is that what we want?
An interesting alternative would be to switch which fd we keep free:
    S    S+3 -   -   -
    S+1  S+4 P+2 P+4
    S+2  S+5 P+3 P+5
    -    P   P+1 P+6
  
So depending on your buffering P+1 might also now be pretty fast; but that's
starting to get into heuristics about guessing how much you should put on
your previously low-queue'd fd.

Dave

> Regards,
> Daniel
> -- 
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK