+0
−5
Loading
net: Do not enable tx-nocache-copy by default
There are many cases where this feature does not improve performance or even reduces it. For example, here are the results from tests that I've run using 3.12.6 on one Intel Xeon W3565 and one i7 920 connected by ixgbe adapters. The results are from the Xeon, but they're similar on the i7. All numbers report the mean±stddev over 10 runs of 10s. 1) latency tests similar to what is described in "c6e1a0d1 net: Allow no-cache copy from user on transmit" There is no statistically significant difference between tx-nocache-copy on/off. nic irqs spread out (one queue per cpu) 200x netperf -r 1400,1 tx-nocache-copy off 692000±1000 tps 50/90/95/99% latency (us): 275±2/643.8±0.4/799±1/2474.4±0.3 tx-nocache-copy on 693000±1000 tps 50/90/95/99% latency (us): 274±1/644.1±0.7/800±2/2474.5±0.7 200x netperf -r 14000,14000 tx-nocache-copy off 86450±80 tps 50/90/95/99% latency (us): 334.37±0.02/838±1/2100±20/3990±40 tx-nocache-copy on 86110±60 tps 50/90/95/99% latency (us): 334.28±0.01/837±2/2110±20/3990±20 2) single stream throughput tests tx-nocache-copy leads to higher service demand throughput cpu0 cpu1 demand (Gb/s) (Gcycle) (Gcycle) (cycle/B) nic irqs and netperf on cpu0 (1x netperf -T0,0 -t omni -- -d send) tx-nocache-copy off 9402±5 9.4±0.2 0.80±0.01 tx-nocache-copy on 9403±3 9.85±0.04 0.838±0.004 nic irqs on cpu0, netperf on cpu1 (1x netperf -T1,1 -t omni -- -d send) tx-nocache-copy off 9401±5 5.83±0.03 5.0±0.1 0.923±0.007 tx-nocache-copy on 9404±2 5.74±0.03 5.523±0.009 0.958±0.002 As a second example, here are some results from Eric Dumazet with latest net-next. tx-nocache-copy also leads to higher service demand (cpu is Intel(R) Xeon(R) CPU X5660 @ 2.80GHz) lpq83:~# ./ethtool -K eth0 tx-nocache-copy on lpq83:~# perf stat ./netperf -H lpq84 -c MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to lpq84.prod.google.com () port 0 AF_INET Recv Send Send Utilization Service Demand Socket Socket Message Elapsed Send Recv Send Recv Size Size Size Time Throughput local remote local remote bytes bytes bytes secs. 10^6bits/s % S % U us/KB us/KB 87380 16384 16384 10.00 9407.44 2.50 -1.00 0.522 -1.000 Performance counter stats for './netperf -H lpq84 -c': 4282.648396 task-clock # 0.423 CPUs utilized 9,348 context-switches # 0.002 M/sec 88 CPU-migrations # 0.021 K/sec 355 page-faults # 0.083 K/sec 11,812,797,651 cycles # 2.758 GHz [82.79%] 9,020,522,817 stalled-cycles-frontend # 76.36% frontend cycles idle [82.54%] 4,579,889,681 stalled-cycles-backend # 38.77% backend cycles idle [67.33%] 6,053,172,792 instructions # 0.51 insns per cycle # 1.49 stalled cycles per insn [83.64%] 597,275,583 branches # 139.464 M/sec [83.70%] 8,960,541 branch-misses # 1.50% of all branches [83.65%] 10.128990264 seconds time elapsed lpq83:~# ./ethtool -K eth0 tx-nocache-copy off lpq83:~# perf stat ./netperf -H lpq84 -c MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to lpq84.prod.google.com () port 0 AF_INET Recv Send Send Utilization Service Demand Socket Socket Message Elapsed Send Recv Send Recv Size Size Size Time Throughput local remote local remote bytes bytes bytes secs. 10^6bits/s % S % U us/KB us/KB 87380 16384 16384 10.00 9412.45 2.15 -1.00 0.449 -1.000 Performance counter stats for './netperf -H lpq84 -c': 2847.375441 task-clock # 0.281 CPUs utilized 11,632 context-switches # 0.004 M/sec 49 CPU-migrations # 0.017 K/sec 354 page-faults # 0.124 K/sec 7,646,889,749 cycles # 2.686 GHz [83.34%] 6,115,050,032 stalled-cycles-frontend # 79.97% frontend cycles idle [83.31%] 1,726,460,071 stalled-cycles-backend # 22.58% backend cycles idle [66.55%] 2,079,702,453 instructions # 0.27 insns per cycle # 2.94 stalled cycles per insn [83.22%] 363,773,213 branches # 127.757 M/sec [83.29%] 4,242,732 branch-misses # 1.17% of all branches [83.51%] 10.128449949 seconds time elapsed CC: Tom Herbert <therbert@google.com> Signed-off-by:Benjamin Poirier <bpoirier@suse.de> Signed-off-by: