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Date: Mon, 23 Apr 2012 14:56:48 -0700 From: Rick Jones <rick.jones2@...com> To: Eric Dumazet <eric.dumazet@...il.com> CC: David Miller <davem@...emloft.net>, netdev@...r.kernel.org, therbert@...gle.com, ncardwell@...gle.com, maze@...gle.com, ycheng@...gle.com, ilpo.jarvinen@...sinki.fi Subject: Re: [PATCH 2/2 net-next] tcp: sk_add_backlog() is too agressive for TCP On 04/23/2012 02:51 PM, Rick Jones wrote: > On 04/23/2012 02:30 PM, Eric Dumazet wrote: >> On Mon, 2012-04-23 at 13:57 -0700, Rick Jones wrote: >>> Probably better to call that something other than 16K buffers - the send >>> size was probably 16K, which reflected SO_SNDBUF at the time the data >>> socket was created, but clearly SO_SNDBUF grew in that timeframe. >>> >> >> >> Maybe I was not clear : Application does sendmsg() of 16KB buffers. > > I'd probably call that a 16K send test. The root of the issue being > there being "send buffers" and "send socket buffers" (and their receive > versions). > > My "canonical" test - at least one that appears in most of my > contemporary scripts uses a 64K send size for the bulk transfer tests. I > switch back-and-forth between tests which allow the socket buffer size > to be determined automagically, and those where I set both sides' socket > buffers to 1M via the test-specific -s and -S options. In "netperf > speak" those would probably be "x64K" and "1Mx64k" respectively. More > generally "<socket buffer size>x<send size>" (I rarely set/specify the > receive size in those tests, leaving it at whatever SO_RCVBUF is at the > start. > >> Yet, in the small time it takes to perform this operation, softirq can >> queue up to 300 packets coming from the other side. > > There is more to it than just queue-up 16 KB right? I should have added that 300 ACKs seems huge as a backlog. At ack-every-other that is 300 * 1448 * 2 or 868800 bytes worth of ACKs. That sounds like a great deal more than just one 16KB send's worth of being held-off. I mean at 10Gbe speeds (using your 54 usec for 64KB) that represents data which took something like three quarters of a millisecond to transmit on the wire. rick -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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