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Date: Fri, 23 Dec 2011 08:13:06 -0500 From: "John A. Sullivan III" <jsullivan@...nsourcedevel.com> To: Eric Dumazet <eric.dumazet@...il.com> Cc: netdev@...r.kernel.org Subject: Re: SFQ on HFSC leaf does not seem to work On Fri, 2011-12-23 at 09:10 +0100, Eric Dumazet wrote: > Le vendredi 23 décembre 2011 à 01:00 -0500, John A. Sullivan III a > écrit : > > > Where might this 3000 ms delay be coming from? > > > > Certainly not from SFQ > > You could use tcpdump to check if delay is at egress or ingress side. > > > That's perplexing as well. Tracing on the eth1 interface of the test devices, we see the packets going out with near perfect regularity at one second intervals and replies returning immediately. They are also completely in sequence so it is not as if we are matching packet 4 with packet 7 and seeing an immediate reply that is really offset by three seconds. Thus, the delay seems to be registered only in the ICMP application itself. The CPU is virtually idle - very impressed with Debian's handling of interrupts on this old system would have been spending almost 60% on hardware interrupts. Another weird characteristic is that the delay is introduced gradually over several seconds: 64 bytes from 192.168.223.84: icmp_req=4 ttl=64 time=58.0 ms 64 bytes from 192.168.223.84: icmp_req=5 ttl=64 time=52.4 ms 64 bytes from 192.168.223.84: icmp_req=6 ttl=64 time=48.7 ms 64 bytes from 192.168.223.84: icmp_req=7 ttl=64 time=118 ms 64 bytes from 192.168.223.84: icmp_req=8 ttl=64 time=834 ms 64 bytes from 192.168.223.84: icmp_req=9 ttl=64 time=896 ms 64 bytes from 192.168.223.84: icmp_req=10 ttl=64 time=897 ms 64 bytes from 192.168.223.84: icmp_req=11 ttl=64 time=1081 ms 64 bytes from 192.168.223.84: icmp_req=12 ttl=64 time=1257 ms 64 bytes from 192.168.223.84: icmp_req=13 ttl=64 time=1744 ms 64 bytes from 192.168.223.84: icmp_req=14 ttl=64 time=2107 ms 64 bytes from 192.168.223.84: icmp_req=15 ttl=64 time=2532 ms 64 bytes from 192.168.223.84: icmp_req=16 ttl=64 time=2948 ms 64 bytes from 192.168.223.84: icmp_req=17 ttl=64 time=3191 ms 64 bytes from 192.168.223.84: icmp_req=18 ttl=64 time=3163 ms While the delay builds, we can see the replied noticeably delayed however, once we hit 3000ms, the display updates one packet per second. I would expect one packet per three seconds unless we were interleaving packets but we are not according to the packet trace. So I am guessing an inbound queue but where? Ah, netem. I pulled out netem and I seem vastly different results. I get a very occasional lost packet but no impact to latency: 64 bytes from 192.168.223.84: icmp_req=48 ttl=64 time=0.802 ms 64 bytes from 192.168.223.84: icmp_req=49 ttl=64 time=0.843 ms 64 bytes from 192.168.223.84: icmp_req=50 ttl=64 time=0.739 ms 64 bytes from 192.168.223.84: icmp_req=51 ttl=64 time=0.769 ms 64 bytes from 192.168.223.84: icmp_req=52 ttl=64 time=0.833 ms 64 bytes from 192.168.223.84: icmp_req=53 ttl=64 time=0.872 ms 64 bytes from 192.168.223.84: icmp_req=54 ttl=64 time=0.786 ms 64 bytes from 192.168.223.84: icmp_req=55 ttl=64 time=0.766 ms 64 bytes from 192.168.223.84: icmp_req=56 ttl=64 time=0.715 ms 64 bytes from 192.168.223.84: icmp_req=57 ttl=64 time=0.710 ms 64 bytes from 192.168.223.84: icmp_req=58 ttl=64 time=0.784 ms 64 bytes from 192.168.223.84: icmp_req=59 ttl=64 time=0.766 ms 64 bytes from 192.168.223.84: icmp_req=60 ttl=64 time=0.748 ms So netem has bufferbloat ;) Thanks - John -- 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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