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 "c6e1a0d 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: David S. Miller <davem@davemloft.net>
cdb3f4a31b