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From: Stefano Brivio <sbrivio@redhat.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: passt-dev@passt.top
Subject: Re: [PATCH 3/5] tap: Don't use EPOLLET on Qemu sockets
Date: Fri, 26 Jul 2024 12:44:11 +0200	[thread overview]
Message-ID: <20240726124411.5dbe9cfa@elisabeth> (raw)
In-Reply-To: <20240726100056.235b65de@elisabeth>

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


  reply	other threads:[~2024-07-26 10:44 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-07-26  7:20 [PATCH 0/5] Fix assorted errors in the Qemu socket tap receive handler David Gibson
2024-07-26  7:20 ` [PATCH 1/5] tap: Better report errors receiving from QEMU socket David Gibson
2024-07-26 11:25   ` Stefano Brivio
2024-07-26 11:50     ` Stefano Brivio
2024-07-26 12:02     ` David Gibson
2024-07-26  7:20 ` [PATCH 2/5] tap: Don't attempt to carry on if we get a bad frame length from qemu David Gibson
2024-07-26 11:26   ` Stefano Brivio
2024-07-26  7:20 ` [PATCH 3/5] tap: Don't use EPOLLET on Qemu sockets David Gibson
2024-07-26  8:00   ` Stefano Brivio
2024-07-26 10:44     ` Stefano Brivio [this message]
2024-07-26 12:12     ` David Gibson
2024-07-26 13:25       ` Stefano Brivio
2024-07-29  1:15         ` David Gibson
2024-07-26  7:20 ` [PATCH 4/5] tap: Correctly handle frames of odd length David Gibson
2024-07-26  7:20 ` [PATCH 5/5] tap: Improve handling of partially received frames on qemu socket David Gibson
2024-07-26 11:39   ` Stefano Brivio
2024-07-26 12:33     ` David Gibson
2024-07-26 13:19 ` [PATCH 0/5] Fix assorted errors in the Qemu socket tap receive handler Stefano Brivio

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