From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=38015 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PkIeA-00034P-LQ for qemu-devel@nongnu.org; Tue, 01 Feb 2011 11:00:06 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PkIe9-00036z-6g for qemu-devel@nongnu.org; Tue, 01 Feb 2011 11:00:02 -0500 Received: from mail-vw0-f45.google.com ([209.85.212.45]:58277) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PkIe8-00036u-Rf for qemu-devel@nongnu.org; Tue, 01 Feb 2011 11:00:01 -0500 Received: by vws12 with SMTP id 12so2321551vws.4 for ; Tue, 01 Feb 2011 08:00:00 -0800 (PST) Message-ID: <4D482DF5.7030708@codemonkey.ws> Date: Tue, 01 Feb 2011 09:59:49 -0600 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH 0.14] tap: safe sndbuf default References: <20110201122540.GA19108@redhat.com> In-Reply-To: <20110201122540.GA19108@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: markmc@redhat.com, qemu-devel@nongnu.org, herbert@gondor.hengli.com.au On 02/01/2011 06:25 AM, Michael S. Tsirkin wrote: > With current sndbuf default value, a blocked > target guest can prevent another guest from > transmitting any packets. While current > sndbuf value (1M) is reported to help some > UDP based workloads, the default should > be safe (0). > Can you be more specific about the workload this helps and by how much? Regards, Anthony Liguori > Signed-off-by: Michael S. Tsirkin > --- > > I think this should go into 0.14. > Comments? > > net/tap-linux.c | 13 +++++++++---- > qemu-options.hx | 2 +- > 2 files changed, 10 insertions(+), 5 deletions(-) > > diff --git a/net/tap-linux.c b/net/tap-linux.c > index f7aa904..ff8cad0 100644 > --- a/net/tap-linux.c > +++ b/net/tap-linux.c > @@ -82,12 +82,17 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required > return fd; > } > > -/* sndbuf should be set to a value lower than the tx queue > - * capacity of any destination network interface. > +/* sndbuf implements a kind of flow control for tap. > + * Unfortunately when it's enabled, and packets are sent > + * to other guests on the same host, the receiver > + * can lock up the transmitter indefinitely. > + * > + * To avoid packet loss, sndbuf should be set to a value lower than the tx > + * queue capacity of any destination network interface. > * Ethernet NICs generally have txqueuelen=1000, so 1Mb is > - * a good default, given a 1500 byte MTU. > + * a good value, given a 1500 byte MTU. > */ > -#define TAP_DEFAULT_SNDBUF 1024*1024 > +#define TAP_DEFAULT_SNDBUF 0 > > int tap_set_sndbuf(int fd, QemuOpts *opts) > { > diff --git a/qemu-options.hx b/qemu-options.hx > index 11c93a2..ca4d5c9 100644 > --- a/qemu-options.hx > +++ b/qemu-options.hx > @@ -1057,7 +1057,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net, > " use '[down]script=no' to disable script execution\n" > " use 'fd=h' to connect to an already opened TAP interface\n" > " use 'sndbuf=nbytes' to limit the size of the send buffer (the\n" > - " default of 'sndbuf=1048576' can be disabled using 'sndbuf=0')\n" > + " default is disabled 'sndbuf=0' to enable flow control set 'sndbuf=1048576')\n" > " use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n" > " use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n" > " use vhost=on to enable experimental in kernel accelerator\n" >