From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1MLZFF-0007AR-L7 for qemu-devel@nongnu.org; Tue, 30 Jun 2009 05:03:17 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1MLZFA-00077r-Pc for qemu-devel@nongnu.org; Tue, 30 Jun 2009 05:03:16 -0400 Received: from [199.232.76.173] (port=40977 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MLZFA-00077c-Jk for qemu-devel@nongnu.org; Tue, 30 Jun 2009 05:03:12 -0400 Received: from mx2.redhat.com ([66.187.237.31]:44031) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1MLZF9-0005sD-Pw for qemu-devel@nongnu.org; Tue, 30 Jun 2009 05:03:12 -0400 Received: from int-mx2.corp.redhat.com (int-mx2.corp.redhat.com [172.16.27.26]) by mx2.redhat.com (8.13.8/8.13.8) with ESMTP id n5U93Apj031847 for ; Tue, 30 Jun 2009 05:03:10 -0400 From: Mark McLoughlin Content-Type: text/plain Date: Tue, 30 Jun 2009 10:02:57 +0100 Message-Id: <1246352577.3749.32.camel@blaa> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] [PATCH] net: set a default value for sndbuf= Reply-To: Mark McLoughlin List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel Cc: Herbert Xu On reflection, perhaps it does make sense to set a default value for the sndbuf= tap parameter. For best effect, sndbuf= should be set to just below the capacity of the physical NIC. Setting it higher will cause packets to be dropped before the limit is hit. Setting it much lower will not cause any problems unless you set it low enough such that the guest cannot queue up new packets before the NIC has emptied its queue. In Linux, txqueuelen=1000 by default for ethernet NICs. Given a 1500 byte MTU, 1Mb is a good choice for sndbuf. If it turns out that txqueuelen is actually much lower than this, then sndbuf is essentially disabled. In the event that txqueuelen is much higher, it's unlikely that the NIC will be able to empty a 1Mb queue. Thanks to Herbert Xu for this logic. Signed-off-by: Mark McLoughlin Cc: Herbert Xu --- net.c | 34 +++++++++++++++++++++++++++++----- qemu-options.hx | 3 ++- 2 files changed, 31 insertions(+), 6 deletions(-) diff --git a/net.c b/net.c index 6d82d59..21842d0 100644 --- a/net.c +++ b/net.c @@ -1396,17 +1396,39 @@ static void tap_send(void *opaque) } while (size > 0); } -static void tap_set_sndbuf(TAPState *s, int sndbuf, Monitor *mon) -{ #ifdef TUNSETSNDBUF +/* 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. + */ +#define TAP_DEFAULT_SNDBUF 1024*1024 + +static void tap_set_sndbuf(TAPState *s, const char *sndbuf_str, Monitor *mon) +{ + int sndbuf = TAP_DEFAULT_SNDBUF; + + if (sndbuf_str) { + sndbuf = atoi(sndbuf_str); + } + + if (!sndbuf) { + sndbuf = INT_MAX; + } + if (ioctl(s->fd, TUNSETSNDBUF, &sndbuf) == -1) { config_error(mon, "TUNSETSNDBUF ioctl failed: %s\n", strerror(errno)); } +} #else - config_error(mon, "No '-net tap,sndbuf=' support available\n"); -#endif +static void tap_set_sndbuf(TAPState *s, const char *sndbuf_str, Monitor *mon) +{ + if (sndbuf_str) { + config_error(mon, "No '-net tap,sndbuf=' support available\n"); + } } +#endif /* TUNSETSNDBUF */ static void tap_cleanup(VLANClientState *vc) { @@ -2653,9 +2675,11 @@ int net_client_init(Monitor *mon, const char *device, const char *p) s = net_tap_init(vlan, device, name, ifname, setup_script, down_script); } if (s != NULL) { + const char *sndbuf_str = NULL; if (get_param_value(buf, sizeof(buf), "sndbuf", p)) { - tap_set_sndbuf(s, atoi(buf), mon); + sndbuf_str = buf; } + tap_set_sndbuf(s, sndbuf_str, mon); ret = 0; } else { ret = -1; diff --git a/qemu-options.hx b/qemu-options.hx index a94f9d3..8947d05 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -778,7 +778,8 @@ 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" #ifdef TUNSETSNDBUF - " use 'sndbuf=nbytes' to limit the size of the send buffer\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" #endif #endif "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n" -- 1.6.2.2