Re: [PATCH 3/5] tap: Don't use EPOLLET on Qemu sockets

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Jul 26, 2024 at 10:00:56AM +0200, Stefano Brivio wrote:
> On Fri, 26 Jul 2024 17:20:29 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > Currently we set EPOLLET (edge trigger) on the epoll flags for the
> > connected Qemu Unix socket.  It's not clear that there's a reason for
> > doing this: for TCP sockets we need to use EPOLLET, because we leave data
> > in the socket buffers for our flow control handling.  That consideration
> > doesn't apply to the way we handle the qemu socket however.
> 
> It significantly decreases epoll_wait() overhead on sustained data
> transfers, because we can read multiple TAP_BUF_SIZE buffers at a time
> instead of just one.

That's a reason to keep the loop, but not EPOLLET itself, AFAICT.  I'd
be happy enough to put the loop back in as an optimization (although,
I'd prefer to avoid the goto).

> I can check that now again with current QEMU and kernel versions, plus
> several fundamental changes in buffer handling, but I don't see a real
> reason why this shouldn't have changed meanwhile.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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