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

[Dr. David Alan Gilbert]

* 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.

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