From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1Jt9PV-0000tL-Nf
	for qemu-devel@nongnu.org; Mon, 05 May 2008 18:43:53 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1Jt9PT-0000sS-FV
	for qemu-devel@nongnu.org; Mon, 05 May 2008 18:43:52 -0400
Received: from [199.232.76.173] (port=54425 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1Jt9PT-0000sN-8H
	for qemu-devel@nongnu.org; Mon, 05 May 2008 18:43:51 -0400
Received: from py-out-1112.google.com ([64.233.166.181])
	by monty-python.gnu.org with esmtp (Exim 4.60)
	(envelope-from <anthony@codemonkey.ws>) id 1Jt9PS-0006uk-Vd
	for qemu-devel@nongnu.org; Mon, 05 May 2008 18:43:51 -0400
Received: by py-out-1112.google.com with SMTP id u52so1672341pyb.10
	for <qemu-devel@nongnu.org>; Mon, 05 May 2008 15:43:50 -0700 (PDT)
Message-ID: <481F8DA1.60605@codemonkey.ws>
Date: Mon, 05 May 2008 17:43:45 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [4341] qemu: generate signals on tap I/O
References: <E1Jt8Cq-0007Qs-U7@cvs.savannah.gnu.org>
In-Reply-To: <E1Jt8Cq-0007Qs-U7@cvs.savannah.gnu.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Reply-To: qemu-devel@nongnu.org
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: qemu-devel@nongnu.org

Aurelien Jarno wrote:
> Revision: 4341
>           http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=4341
> Author:   aurel32
> Date:     2008-05-05 21:26:43 +0000 (Mon, 05 May 2008)
>
> Log Message:
> -----------
> qemu: generate signals on tap I/O
>
> Currently tap does not generate signals on I/O; this causes
> network latency to be dependent on the timer tick (1ms without
> dyntick, guest dependent with dyntick).  By generating a signal
> on I/O, we can inform the guest immediately that a packet has
> arrived.
>
> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
>
> Modified Paths:
> --------------
>     trunk/vl.c
>
> Modified: trunk/vl.c
> ===================================================================
> --- trunk/vl.c	2008-05-05 21:26:31 UTC (rev 4340)
> +++ trunk/vl.c	2008-05-05 21:26:43 UTC (rev 4341)
> @@ -4030,6 +4030,7 @@
>      if (!s)
>          return NULL;
>      s->fd = fd;
> +    enable_sigio_timer(fd);
>      s->vc = qemu_new_vlan_client(vlan, tap_receive, NULL, s);
>      qemu_set_fd_handler(s->fd, tap_send, NULL, s);
>      snprintf(s->vc->info_str, sizeof(s->vc->info_str), "tap: fd=%d", fd);
>   

I still haven't seen anyone explain why this results in a performance 
improvement.  The SIGIO handler is tied to the host_alarm_handler which 
will not dispatch IO.  It surprises me that it has any affect at all.

FWIW, we're getting rid of SIGIO in KVM.  It doesn't improve performance 
verses a properly implemented select lop.

Regards,

Anthony Liguori

>
>
>