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

From: "Daniel P. Berrange" <berrange@redhat.com>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH 00/16] Multifd v4
Date: Tue, 14 Mar 2017 11:45:04 +0000	[thread overview]
Message-ID: <20170314114504.GI2652@redhat.com> (raw)
In-Reply-To: <20170314114019.GG2445@work-vm>

On Tue, Mar 14, 2017 at 11:40:20AM +0000, Dr. David Alan Gilbert wrote:
> * 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.
> > 
> > multi-fd will certainly benefit encrypted migration, since we'll be able
> > to burn multiple CPUs for AES instead of bottlenecking on one CPU, and
> > thus able to take greater advantage of networks with > 1-GigE bandwidth.
> 
> Yes, that's one I really want to see.  It might be odd using lots of fd's
> just to do that, but probably the easiest way.

In theory you could have multiple threads doing encryption, all writing to
a single FD, but the AFAICT the GNUTLS library doesn't make this possible
as it encapsulates both encryption + i/o behind a single API call. Even if
we called the same API call from multiple threads, I'm pretty sure it will
serialize the encryption with internal locking, so you would not gain
anything. So using multiple distinct TLS connections is the only viable
option.

Regards,
Daniel
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