1. 18 Oct, 2014 10 commits
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  4. 15 Oct, 2014 11 commits
  5. 14 Oct, 2014 1 commit
    • Eric Dumazet's avatar
      tcp: TCP Small Queues and strange attractors · 9b462d02
      Eric Dumazet authored
      TCP Small queues tries to keep number of packets in qdisc
      as small as possible, and depends on a tasklet to feed following
      packets at TX completion time.
      Choice of tasklet was driven by latencies requirements.
      
      Then, TCP stack tries to avoid reorders, by locking flows with
      outstanding packets in qdisc in a given TX queue.
      
      What can happen is that many flows get attracted by a low performing
      TX queue, and cpu servicing TX completion has to feed packets for all of
      them, making this cpu 100% busy in softirq mode.
      
      This became particularly visible with latest skb->xmit_more support
      
      Strategy adopted in this patch is to detect when tcp_wfree() is called
      from ksoftirqd and let the outstanding queue for this flow being drained
      before feeding additional packets, so that skb->ooo_okay can be set
      to allow select_queue() to select the optimal queue :
      
      Incoming ACKS are normally handled by different cpus, so this patch
      gives more chance for these cpus to take over the burden of feeding
      qdisc with future packets.
      
      Tested:
      
      lpaa23:~# ./super_netperf 1400 --google-pacing-rate 3028000 -H lpaa24 -l 3600 &
      
      lpaa23:~# sar -n DEV 1 10 | grep eth1
      06:16:18 AM      eth1 595448.00 1190564.00  38381.09 1760253.12      0.00      0.00      1.00
      06:16:19 AM      eth1 594858.00 1189686e.00  38340.76 1758952.72      0.00      0.00      0.00
      06:16:20 AM      eth1 597017.00 1194019.00  38480.79 1765370.29      0.00      0.00      1.00
      06:16:21 AM      eth1 595450.00 1190936.00  38380.19 1760805.05      0.00      0.00      0.00
      06:16:22 AM      eth1 596385.00 1193096.00  38442.56 1763976.29      0.00      0.00      1.00
      06:16:23 AM      eth1 598155.00 1195978.00  38552.97 1768264.60      0.00      0.00      0.00
      06:16:24 AM      eth1 594405.00 1188643.00  38312.57 1757414.89      0.00      0.00      1.00
      06:16:25 AM      eth1 593366.00 1187154.00  38252.16 1755195.83      0.00      0.00      0.00
      06:16:26 AM      eth1 593188.00 1186118.00  38232.88 1753682.57      0.00      0.00      1.00
      06:16:27 AM      eth1 596301.00 1192241.00  38440.94 1762733.09      0.00      0.00      0.00
      Average:         eth1 595457.30 1190843.50  38381.69 1760664.84      0.00      0.00      0.50
      lpaa23:~# ./tc -s -d qd sh dev eth1 | grep backlog
       backlog 7606336b 2513p requeues 167982
       backlog 224072b 74p requeues 566
       backlog 581376b 192p requeues 5598
       backlog 181680b 60p requeues 1070
       backlog 5305056b 1753p requeues 110166    // Here, this TX queue is attracting flows
       backlog 157456b 52p requeues 1758
       backlog 672216b 222p requeues 3025
       backlog 60560b 20p requeues 24541
       backlog 448144b 148p requeues 21258
      
      lpaa23:~# echo 1 >/proc/sys/net/ipv4/tcp_tsq_enable_tcp_wfree_ksoftirqd_detect
      
      Immediate jump to full bandwidth, and traffic is properly
      shard on all tx queues.
      
      lpaa23:~# sar -n DEV 1 10 | grep eth1
      06:16:46 AM      eth1 1397632.00 2795397.00  90081.87 4133031.26      0.00      0.00      1.00
      06:16:47 AM      eth1 1396874.00 2793614.00  90032.99 4130385.46      0.00      0.00      0.00
      06:16:48 AM      eth1 1395842.00 2791600.00  89966.46 4127409.67      0.00      0.00      1.00
      06:16:49 AM      eth1 1395528.00 2791017.00  89946.17 4126551.24      0.00      0.00      0.00
      06:16:50 AM      eth1 1397891.00 2795716.00  90098.74 4133497.39      0.00      0.00      1.00
      06:16:51 AM      eth1 1394951.00 2789984.00  89908.96 4125022.51      0.00      0.00      0.00
      06:16:52 AM      eth1 1394608.00 2789190.00  89886.90 4123851.36      0.00      0.00      1.00
      06:16:53 AM      eth1 1395314.00 2790653.00  89934.33 4125983.09      0.00      0.00      0.00
      06:16:54 AM      eth1 1396115.00 2792276.00  89984.25 4128411.21      0.00      0.00      1.00
      06:16:55 AM      eth1 1396829.00 2793523.00  90030.19 4130250.28      0.00      0.00      0.00
      Average:         eth1 1396158.40 2792297.00  89987.09 4128439.35      0.00      0.00      0.50
      
      lpaa23:~# tc -s -d qd sh dev eth1 | grep backlog
       backlog 7900052b 2609p requeues 173287
       backlog 878120b 290p requeues 589
       backlog 1068884b 354p requeues 5621
       backlog 996212b 329p requeues 1088
       backlog 984100b 325p requeues 115316
       backlog 956848b 316p requeues 1781
       backlog 1080996b 357p requeues 3047
       backlog 975016b 322p requeues 24571
       backlog 990156b 327p requeues 21274
      
      (All 8 TX queues get a fair share of the traffic)
      Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      9b462d02