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Date: Fri, 13 Dec 2013 20:59:41 -0500 From: Neal Cardwell <ncardwell@...gle.com> To: Eric Dumazet <eric.dumazet@...il.com> Cc: David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>, Yuchung Cheng <ycheng@...gle.com>, Nandita Dukkipati <nanditad@...gle.com>, Van Jacobson <vanj@...gle.com> Subject: Re: [PATCH v3 net-next] tcp: refine TSO splits On Fri, Dec 13, 2013 at 4:51 PM, Eric Dumazet <eric.dumazet@...il.com> wrote: > From: Eric Dumazet <edumazet@...gle.com> > > While investigating performance problems on small RPC workloads, > I noticed linux TCP stack was always splitting the last TSO skb > into two parts (skbs). One being a multiple of MSS, and a small one > with the Push flag. This split is done even if TCP_NODELAY is set, > or if no small packet is in flight. > > Example with request/response of 4K/4K > > IP A > B: . ack 68432 win 2783 <nop,nop,timestamp 6524593 6525001> > IP A > B: . 65537:68433(2896) ack 69632 win 2783 <nop,nop,timestamp 6524593 6525001> > IP A > B: P 68433:69633(1200) ack 69632 win 2783 <nop,nop,timestamp 6524593 6525001> > IP B > A: . ack 68433 win 2768 <nop,nop,timestamp 6525001 6524593> > IP B > A: . 69632:72528(2896) ack 69633 win 2768 <nop,nop,timestamp 6525001 6524593> > IP B > A: P 72528:73728(1200) ack 69633 win 2768 <nop,nop,timestamp 6525001 6524593> > IP A > B: . ack 72528 win 2783 <nop,nop,timestamp 6524593 6525001> > IP A > B: . 69633:72529(2896) ack 73728 win 2783 <nop,nop,timestamp 6524593 6525001> > IP A > B: P 72529:73729(1200) ack 73728 win 2783 <nop,nop,timestamp 6524593 6525001> > > We can avoid this split by including the Nagle tests at the right place. > > Note : If some NIC had trouble sending TSO packets with a partial > last segment, we would have hit the problem in GRO/forwarding workload already. > > tcp_minshall_update() is moved to tcp_output.c and is updated as we might > feed a TSO packet with a partial last segment. > > This patch tremendously improves performance, as the traffic now looks > like : > > IP A > B: . ack 98304 win 2783 <nop,nop,timestamp 6834277 6834685> > IP A > B: P 94209:98305(4096) ack 98304 win 2783 <nop,nop,timestamp 6834277 6834685> > IP B > A: . ack 98305 win 2768 <nop,nop,timestamp 6834686 6834277> > IP B > A: P 98304:102400(4096) ack 98305 win 2768 <nop,nop,timestamp 6834686 6834277> > IP A > B: . ack 102400 win 2783 <nop,nop,timestamp 6834279 6834686> > IP A > B: P 98305:102401(4096) ack 102400 win 2783 <nop,nop,timestamp 6834279 6834686> > IP B > A: . ack 102401 win 2768 <nop,nop,timestamp 6834687 6834279> > IP B > A: P 102400:106496(4096) ack 102401 win 2768 <nop,nop,timestamp 6834687 6834279> > IP A > B: . ack 106496 win 2783 <nop,nop,timestamp 6834280 6834687> > IP A > B: P 102401:106497(4096) ack 106496 win 2783 <nop,nop,timestamp 6834280 6834687> > IP B > A: . ack 106497 win 2768 <nop,nop,timestamp 6834688 6834280> > IP B > A: P 106496:110592(4096) ack 106497 win 2768 <nop,nop,timestamp 6834688 6834280> > > > Before : > > lpq83:~# nstat >/dev/null;perf stat ./super_netperf 200 -t TCP_RR -H lpq84 -l 20 -- -r 4K,4K > 280774 > > Performance counter stats for './super_netperf 200 -t TCP_RR -H lpq84 -l 20 -- -r 4K,4K': > > 205719.049006 task-clock # 9.278 CPUs utilized > 8,449,968 context-switches # 0.041 M/sec > 1,935,997 CPU-migrations # 0.009 M/sec > 160,541 page-faults # 0.780 K/sec > 548,478,722,290 cycles # 2.666 GHz [83.20%] > 455,240,670,857 stalled-cycles-frontend # 83.00% frontend cycles idle [83.48%] > 272,881,454,275 stalled-cycles-backend # 49.75% backend cycles idle [66.73%] > 166,091,460,030 instructions # 0.30 insns per cycle > # 2.74 stalled cycles per insn [83.39%] > 29,150,229,399 branches # 141.699 M/sec [83.30%] > 1,943,814,026 branch-misses # 6.67% of all branches [83.32%] > > 22.173517844 seconds time elapsed > > lpq83:~# nstat | egrep "IpOutRequests|IpExtOutOctets" > IpOutRequests 16851063 0.0 > IpExtOutOctets 23878580777 0.0 > > After patch : > > lpq83:~# nstat >/dev/null;perf stat ./super_netperf 200 -t TCP_RR -H lpq84 -l 20 -- -r 4K,4K > 280877 > > Performance counter stats for './super_netperf 200 -t TCP_RR -H lpq84 -l 20 -- -r 4K,4K': > > 107496.071918 task-clock # 4.847 CPUs utilized > 5,635,458 context-switches # 0.052 M/sec > 1,374,707 CPU-migrations # 0.013 M/sec > 160,920 page-faults # 0.001 M/sec > 281,500,010,924 cycles # 2.619 GHz [83.28%] > 228,865,069,307 stalled-cycles-frontend # 81.30% frontend cycles idle [83.38%] > 142,462,742,658 stalled-cycles-backend # 50.61% backend cycles idle [66.81%] > 95,227,712,566 instructions # 0.34 insns per cycle > # 2.40 stalled cycles per insn [83.43%] > 16,209,868,171 branches # 150.795 M/sec [83.20%] > 874,252,952 branch-misses # 5.39% of all branches [83.37%] > > 22.175821286 seconds time elapsed > > lpq83:~# nstat | egrep "IpOutRequests|IpExtOutOctets" > IpOutRequests 11239428 0.0 > IpExtOutOctets 23595191035 0.0 > > Indeed, the occupancy of tx skbs (IpExtOutOctets/IpOutRequests) is higher : > 2099 instead of 1417, thus helping GRO to be more efficient when using FQ packet > scheduler. > > Many thanks to Neal for review and ideas. > > Signed-off-by: Eric Dumazet <edumazet@...gle.com> > Cc: Yuchung Cheng <ycheng@...gle.com> > Cc: Neal Cardwell <ncardwell@...gle.com> > Cc: Nandita Dukkipati <nanditad@...gle.com> > Cc: Van Jacobson <vanj@...gle.com> > --- > v3: Add the Nagle tests to make sure we can send the whole skb > according to Nagle rules. Factorize the code. > > v2: changed tcp_minshall_update() as Neal pointed out. > > include/net/tcp.h | 7 ---- > net/ipv4/tcp_output.c | 65 +++++++++++++++++++++++++--------------- > 2 files changed, 42 insertions(+), 30 deletions(-) I love it! Thanks, Eric. Acked-by: Neal Cardwell <ncardwell@...gle.com> Tested-by: Neal Cardwell <ncardwell@...gle.com> neal -- 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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