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Date: Thu, 10 Mar 2011 00:24:58 -0500 From: Bill Fink <billfink@...dspring.com> To: Lucas Nussbaum <lucas.nussbaum@...ia.fr> Cc: Injong Rhee <rhee@...u.edu>, Stephen Hemminger <shemminger@...tta.com>, David Miller <davem@...emloft.net>, xiyou.wangcong@...il.com, netdev@...r.kernel.org, sangtae.ha@...il.com Subject: Re: [PATCH] Make CUBIC Hystart more robust to RTT variations On Wed, 9 Mar 2011, Lucas Nussbaum wrote: > On 08/03/11 at 20:30 -0500, Injong Rhee wrote: > > Now, both tools can be wrong. But that is not catastrophic since > > congestion avoidance can kick in to save the day. In a pipe where no > > other flows are competing, then exiting slow start too early can > > slow things down as the window can be still too small. But that is > > in fact when delays are most reliable. So those tests that say bad > > performance with hystart are in fact, where hystart is supposed to > > perform well. > > Hi, > > In my setup, there is no congestion at all (except the buffer bloat). > Without Hystart, transferring 8 Gb of data takes 9s, with CUBIC exiting > slow start at ~2000 packets. > With Hystart, transferring 8 Gb of data takes 19s, with CUBIC exiting > slow start at ~20 packets. > I don't think that this is "hystart performing well". We could just as > well remove slow start completely, and only do congestion avoidance, > then. > > While I see the value in Hystart, it's clear that there are some flaws > in the current implementation. It probably makes sense to disable > hystart by default until those problems are fixed. Here are some tests I performed across real networks, where congestion is generally not an issue, with a 2.6.35 kernel on the transmit side. 8 GB transfer across an 18 ms RTT path with autotuning and hystart: i7test7% nuttcp -n8g -i1 192.168.1.23 517.9375 MB / 1.00 sec = 4344.6096 Mbps 0 retrans 688.4375 MB / 1.00 sec = 5775.1998 Mbps 0 retrans 692.9375 MB / 1.00 sec = 5812.7462 Mbps 0 retrans 698.0625 MB / 1.00 sec = 5855.8078 Mbps 0 retrans 699.8750 MB / 1.00 sec = 5871.0123 Mbps 0 retrans 710.5625 MB / 1.00 sec = 5960.5707 Mbps 0 retrans 728.8125 MB / 1.00 sec = 6113.7652 Mbps 0 retrans 751.3750 MB / 1.00 sec = 6302.9210 Mbps 0 retrans 783.8750 MB / 1.00 sec = 6575.6201 Mbps 0 retrans 825.1875 MB / 1.00 sec = 6921.8145 Mbps 0 retrans 875.4375 MB / 1.00 sec = 7343.9811 Mbps 0 retrans 8192.0000 MB / 11.26 sec = 6102.4718 Mbps 11 %TX 28 %RX 0 retrans 18.92 msRTT Ramps up quickly to a little under 6 Gbps, then increases more slowly to 7+ Gbps, with no TCP retransmissions. 8 GB transfer across an 18 ms RTT path with 40 MB socket buffer and hystart: i7test7% nuttcp -n8g -w40m -i1 192.168.1.23 970.0625 MB / 1.00 sec = 8136.8475 Mbps 0 retrans 1181.1875 MB / 1.00 sec = 9909.0045 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9908.6369 Mbps 0 retrans 1181.3125 MB / 1.00 sec = 9909.8747 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9909.0531 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9908.8153 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9909.0729 Mbps 0 retrans 8192.0000 MB / 7.13 sec = 9633.5814 Mbps 17 %TX 42 %RX 0 retrans 18.91 msRTT Quickly ramps up to full 10-GigE line rate, with no TCP retrans. 8 GB transfer across an 18 ms RTT path with autotuning and no hystart: i7test7% nuttcp -n8g -i1 192.168.1.23 845.4375 MB / 1.00 sec = 7091.5828 Mbps 0 retrans 1181.3125 MB / 1.00 sec = 9910.0134 Mbps 0 retrans 1181.0625 MB / 1.00 sec = 9907.1830 Mbps 0 retrans 1181.4375 MB / 1.00 sec = 9910.8936 Mbps 0 retrans 1181.1875 MB / 1.00 sec = 9908.1721 Mbps 0 retrans 1181.3125 MB / 1.00 sec = 9909.5774 Mbps 0 retrans 1181.1875 MB / 1.00 sec = 9908.6874 Mbps 0 retrans 8192.0000 MB / 7.25 sec = 9484.4524 Mbps 18 %TX 41 %RX 0 retrans 18.92 msRTT Also quickly ramps up to full 10-GigE line rate, with no TCP retrans. 8 GB transfer across an 18 ms RTT path with 40 MB socket buffer and no hystart: i7test7% nuttcp -n8g -w40m -i1 192.168.1.23 969.8750 MB / 1.00 sec = 8135.6571 Mbps 0 retrans 1181.3125 MB / 1.00 sec = 9909.3990 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9908.9342 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9909.4098 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9908.8252 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9909.0630 Mbps 0 retrans 1181.2500 MB / 1.00 sec = 9909.3504 Mbps 0 retrans 8192.0000 MB / 7.15 sec = 9611.8053 Mbps 18 %TX 42 %RX 0 retrans 18.95 msRTT Basically the same as the case with 40 MB socket buffer and hystart enabled. Now trying the same type of tests across an 80 ms RTT path. 8 GB transfer across an 80 ms RTT path with autotuning and hystart: i7test7% nuttcp -n8g -i1 192.168.1.18 11.3125 MB / 1.00 sec = 94.8954 Mbps 0 retrans 441.5625 MB / 1.00 sec = 3704.1021 Mbps 0 retrans 687.3750 MB / 1.00 sec = 5765.8657 Mbps 0 retrans 715.5625 MB / 1.00 sec = 6002.6273 Mbps 0 retrans 709.9375 MB / 1.00 sec = 5955.5958 Mbps 0 retrans 691.3125 MB / 1.00 sec = 5799.0626 Mbps 0 retrans 718.6250 MB / 1.00 sec = 6028.3538 Mbps 0 retrans 718.0000 MB / 1.00 sec = 6023.0205 Mbps 0 retrans 704.0000 MB / 1.00 sec = 5905.5387 Mbps 0 retrans 733.3125 MB / 1.00 sec = 6151.4096 Mbps 0 retrans 738.8750 MB / 1.00 sec = 6198.2381 Mbps 0 retrans 731.8750 MB / 1.00 sec = 6139.3695 Mbps 0 retrans 8192.0000 MB / 12.85 sec = 5348.9677 Mbps 10 %TX 23 %RX 0 retrans 80.81 msRTT Similar to the 20 ms RTT path, but achieving somewhat lower performance levels, presumably due to the larger RTT. Ramps up fairly quickly to a little under 6 Gbps, then increases more slowly to 6+ Gbps, with no TCP retransmissions. 8 GB transfer across an 80 ms RTT path with 100 MB socket buffer and hystart: i7test7% nuttcp -n8g -w100m -i1 192.168.1.18 103.9375 MB / 1.00 sec = 871.8378 Mbps 0 retrans 1086.5625 MB / 1.00 sec = 9114.6102 Mbps 0 retrans 1106.6875 MB / 1.00 sec = 9283.5583 Mbps 0 retrans 1109.3125 MB / 1.00 sec = 9305.5226 Mbps 0 retrans 1111.1875 MB / 1.00 sec = 9321.9596 Mbps 0 retrans 1112.8125 MB / 1.00 sec = 9334.8452 Mbps 0 retrans 1113.6875 MB / 1.00 sec = 9341.6620 Mbps 0 retrans 1120.2500 MB / 1.00 sec = 9398.0054 Mbps 0 retrans 8192.0000 MB / 8.37 sec = 8207.2049 Mbps 16 %TX 38 %RX 0 retrans 80.81 msRTT Quickly ramps up to 9+ Gbps and then slowly increases further, with no TCP retrans. 8 GB transfer across an 80 ms RTT path with autotuning and no hystart: i7test7% nuttcp -n8g -i1 192.168.1.18 11.2500 MB / 1.00 sec = 94.3703 Mbps 0 retrans 519.0625 MB / 1.00 sec = 4354.1596 Mbps 0 retrans 861.2500 MB / 1.00 sec = 7224.7970 Mbps 0 retrans 871.0000 MB / 1.00 sec = 7306.4191 Mbps 0 retrans 860.7500 MB / 1.00 sec = 7220.4438 Mbps 0 retrans 869.0625 MB / 1.00 sec = 7290.3340 Mbps 0 retrans 863.4375 MB / 1.00 sec = 7242.7707 Mbps 0 retrans 860.4375 MB / 1.00 sec = 7218.0606 Mbps 0 retrans 875.5000 MB / 1.00 sec = 7344.3071 Mbps 0 retrans 863.1875 MB / 1.00 sec = 7240.8257 Mbps 0 retrans 8192.0000 MB / 10.98 sec = 6259.4379 Mbps 12 %TX 27 %RX 0 retrans 80.81 msRTT Ramps up quickly to 7+ Gbps, then appears to stabilize at that level, with no TCP retransmissions. Performance is somewhat better than with autotuning enabled, but less than using a manually set 100 MB socket buffer. 8 GB transfer across an 80 ms RTT path with 100 MB socket buffer and no hystart: i7test7% nuttcp -n8g -w100m -i1 192.168.1.18 102.8750 MB / 1.00 sec = 862.9487 Mbps 0 retrans 522.8750 MB / 1.00 sec = 4386.2811 Mbps 414 retrans 881.5625 MB / 1.00 sec = 7394.6534 Mbps 0 retrans 1164.3125 MB / 1.00 sec = 9766.6682 Mbps 0 retrans 1170.5625 MB / 1.00 sec = 9819.7042 Mbps 0 retrans 1166.8125 MB / 1.00 sec = 9788.2067 Mbps 0 retrans 1159.8750 MB / 1.00 sec = 9729.1530 Mbps 0 retrans 811.1250 MB / 1.00 sec = 6804.8017 Mbps 21 retrans 73.2500 MB / 1.00 sec = 614.4674 Mbps 0 retrans 884.6250 MB / 1.00 sec = 7420.2900 Mbps 0 retrans 8192.0000 MB / 10.34 sec = 6647.9394 Mbps 13 %TX 31 %RX 435 retrans 80.81 msRTT Disabling hystart on a large RTT path does not seem to play nice with a manually specified socket buffer, resulting in TCP retransmissions that limit the effective network performance. This is a repeatable but extremely variable phenomenon. i7test7% nuttcp -n8g -w100m -i1 192.168.1.18 103.7500 MB / 1.00 sec = 870.3015 Mbps 0 retrans 1146.3750 MB / 1.00 sec = 9616.4520 Mbps 0 retrans 1175.9375 MB / 1.00 sec = 9864.6070 Mbps 0 retrans 615.6875 MB / 1.00 sec = 5164.7353 Mbps 21 retrans 139.2500 MB / 1.00 sec = 1168.1253 Mbps 0 retrans 1090.0625 MB / 1.00 sec = 9143.8053 Mbps 0 retrans 1170.4375 MB / 1.00 sec = 9818.6654 Mbps 0 retrans 1174.5625 MB / 1.00 sec = 9852.8754 Mbps 0 retrans 1174.8750 MB / 1.00 sec = 9855.6052 Mbps 0 retrans 8192.0000 MB / 9.42 sec = 7292.9879 Mbps 14 %TX 34 %RX 21 retrans 80.81 msRTT And: i7test7% nuttcp -n8g -w100m -i1 192.168.1.18 102.8125 MB / 1.00 sec = 862.4227 Mbps 0 retrans 1148.4375 MB / 1.00 sec = 9633.6860 Mbps 0 retrans 1177.4375 MB / 1.00 sec = 9877.3086 Mbps 0 retrans 1168.1250 MB / 1.00 sec = 9798.9133 Mbps 11 retrans 133.1250 MB / 1.00 sec = 1116.7457 Mbps 0 retrans 479.8750 MB / 1.00 sec = 4025.4631 Mbps 0 retrans 1150.6875 MB / 1.00 sec = 9652.4830 Mbps 0 retrans 1177.3125 MB / 1.00 sec = 9876.0624 Mbps 0 retrans 1177.3750 MB / 1.00 sec = 9876.0139 Mbps 0 retrans 320.2500 MB / 1.00 sec = 2686.6452 Mbps 19 retrans 64.9375 MB / 1.00 sec = 544.7363 Mbps 0 retrans 73.6250 MB / 1.00 sec = 617.6113 Mbps 0 retrans 8192.0000 MB / 12.39 sec = 5545.7570 Mbps 12 %TX 26 %RX 30 retrans 80.80 msRTT Re-enabling hystart immediately gives a clean test with no TCP retrans. i7test7% nuttcp -n8g -w100m -i1 192.168.1.18 103.8750 MB / 1.00 sec = 871.3353 Mbps 0 retrans 1086.7500 MB / 1.00 sec = 9116.4474 Mbps 0 retrans 1105.8125 MB / 1.00 sec = 9276.2276 Mbps 0 retrans 1109.4375 MB / 1.00 sec = 9306.5339 Mbps 0 retrans 1111.3125 MB / 1.00 sec = 9322.5327 Mbps 0 retrans 1111.3750 MB / 1.00 sec = 9322.8053 Mbps 0 retrans 1113.7500 MB / 1.00 sec = 9342.8962 Mbps 0 retrans 1120.3125 MB / 1.00 sec = 9397.5711 Mbps 0 retrans 8192.0000 MB / 8.38 sec = 8204.8394 Mbps 16 %TX 39 %RX 0 retrans 80.80 msRTT -Bill -- 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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