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

[Peter Xu <peterx@redhat.com>]

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?

> 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