Re: [PATCH v1 2/3] io: Add zerocopy and errqueue

[Peter Xu <peterx@redhat.com>]

On Wed, Sep 08, 2021 at 09:30:58AM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > On Tue, Sep 07, 2021 at 12:06:15PM +0100, Dr. David Alan Gilbert wrote:
> > > > > What if we do the 'flush()' before we start post-copy, instead of after each
> > > > > iteration? would that be enough?
> > > > 
> > > > Possibly, yes. This really need David G's input since he understands
> > > > the code in way more detail than me.
> > > 
> > > Hmm I'm not entirely sure why we have the sync after each iteration;
> > 
> > We don't have that yet, do we?
> 
> I think multifd does; I think it calls multifd_send_sync_main sometimes,
> which I *think* corresponds to iterations.

Oh.. Then we may need to:

  (1) Make that sync work for zero-copy too; say, semaphores may not be enough
      with it - we need to make sure all async buffers are consumed by checking
      the error queue of the sockets,

  (2) When we make zero-copy work without multi-fd, we may want some similar
      sync on the main stream too

Thanks,

> 
> Dave
> 
> > > the case I can think of is if we're doing async sending, we could have
> > > two versions of the same page in flight (one from each iteration) -
> > > you'd want those to get there in the right order.
> > 
> > From MSG_ZEROCOPY document:
> > 
> >         For protocols that acknowledge data in-order, like TCP, each
> >         notification can be squashed into the previous one, so that no more
> >         than one notification is outstanding at any one point.
> > 
> >         Ordered delivery is the common case, but not guaranteed. Notifications
> >         may arrive out of order on retransmission and socket teardown.
> > 
> > So no matter whether we have a flush() per iteration before, it seems we may
> > want one when zero copy enabled?
> > 
> > Thanks,
> > 
> > -- 
> > Peter Xu
> > 
> -- 
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 

-- 
Peter Xu