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

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 26 Jul 2024 10:00:56 +0200
Stefano Brivio <sbrivio@redhat.com> 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.
> 
> 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.

Surprisingly, this doesn't affect throughput at all on my setup, no
matter the packet size.

I didn't check with perf(1), though, and we probably should give all the
recent substantial changes a pass with it (it's been a while since I
last checked where overhead typically is...).

-- 
Stefano