Re: [PATCH net v4 1/2] net: sched: fix packet stuck problem for lockless qdisc
From: Yunsheng Lin
Date: Wed Apr 21 2021 - 04:22:01 EST
On 2021/4/21 13:31, Michal Kubecek wrote:
> On Wed, Apr 21, 2021 at 09:52:40AM +0800, Yunsheng Lin wrote:
>> On 2021/4/21 4:34, Michal Kubecek wrote:
>>> However, I noticed something disturbing in the results of a simple
>>> 1-thread TCP_STREAM test (client sends data through a TCP connection to
>>> server using long writes, we measure the amount of data received by the
>>> server):
>>>
>>> server: 172.17.1.1, port 12543
>>> iterations: 20, threads: 1, test length: 30
>>> test: TCP_STREAM, message size: 1048576
>>>
>>> 1 927403548.4 B/s, avg 927403548.4 B/s, mdev 0.0 B/s ( 0.0%)
>>> 2 1176317172.1 B/s, avg 1051860360.2 B/s, mdev 124456811.8 B/s ( 11.8%), confid. +/- 1581348251.3 B/s (150.3%)
>>> 3 927335837.8 B/s, avg 1010352186.1 B/s, mdev 117354970.3 B/s ( 11.6%), confid. +/- 357073677.2 B/s ( 35.3%)
>>> 4 1176728045.1 B/s, avg 1051946150.8 B/s, mdev 124576544.7 B/s ( 11.8%), confid. +/- 228863127.8 B/s ( 21.8%)
>>> 5 1176788216.3 B/s, avg 1076914563.9 B/s, mdev 122102985.3 B/s ( 11.3%), confid. +/- 169478943.5 B/s ( 15.7%)
>>> 6 1158167055.1 B/s, avg 1090456645.8 B/s, mdev 115504209.5 B/s ( 10.6%), confid. +/- 132805140.8 B/s ( 12.2%)
>>> 7 1176243474.4 B/s, avg 1102711907.0 B/s, mdev 111069717.1 B/s ( 10.1%), confid. +/- 110956822.2 B/s ( 10.1%)
>>> 8 1176771142.8 B/s, avg 1111969311.5 B/s, mdev 106744173.5 B/s ( 9.6%), confid. +/- 95417120.0 B/s ( 8.6%)
>>> 9 1176206364.6 B/s, avg 1119106761.8 B/s, mdev 102644185.2 B/s ( 9.2%), confid. +/- 83685200.5 B/s ( 7.5%)
>>> 10 1175888409.4 B/s, avg 1124784926.6 B/s, mdev 98855550.5 B/s ( 8.8%), confid. +/- 74537085.1 B/s ( 6.6%)
>>> 11 1176541407.6 B/s, avg 1129490061.2 B/s, mdev 95422224.8 B/s ( 8.4%), confid. +/- 67230249.7 B/s ( 6.0%)
>>> 12 934185352.8 B/s, avg 1113214668.9 B/s, mdev 106114984.5 B/s ( 9.5%), confid. +/- 70420712.5 B/s ( 6.3%)
>>> 13 1176550558.1 B/s, avg 1118086660.3 B/s, mdev 103339448.9 B/s ( 9.2%), confid. +/- 65002902.4 B/s ( 5.8%)
>>> 14 1176521808.8 B/s, avg 1122260599.5 B/s, mdev 100711151.3 B/s ( 9.0%), confid. +/- 60333655.0 B/s ( 5.4%)
>>> 15 1176744840.8 B/s, avg 1125892882.3 B/s, mdev 98240838.2 B/s ( 8.7%), confid. +/- 56319052.3 B/s ( 5.0%)
>>> 16 1176593778.5 B/s, avg 1129061688.3 B/s, mdev 95909740.8 B/s ( 8.5%), confid. +/- 52771633.5 B/s ( 4.7%)
>>> 17 1176583967.4 B/s, avg 1131857116.5 B/s, mdev 93715582.2 B/s ( 8.3%), confid. +/- 49669258.6 B/s ( 4.4%)
>>> 18 1176853301.8 B/s, avg 1134356904.5 B/s, mdev 91656530.2 B/s ( 8.1%), confid. +/- 46905244.8 B/s ( 4.1%)
>>> 19 1176592845.7 B/s, avg 1136579848.8 B/s, mdev 89709043.8 B/s ( 7.9%), confid. +/- 44424855.9 B/s ( 3.9%)
>>> 20 1176608117.3 B/s, avg 1138581262.2 B/s, mdev 87871692.6 B/s ( 7.7%), confid. +/- 42193098.5 B/s ( 3.7%)
>>> all avg 1138581262.2 B/s, mdev 87871692.6 B/s ( 7.7%), confid. +/- 42193098.5 B/s ( 3.7%)
>>>
>>> Each line shows result of one 30 second long test and average, mean
>>> deviation and 99% confidence interval half width through the iterations
>>> so far. While 17 iteration results are essentially the wire speed minus
>>> TCP overhead, iterations 1, 3 and 12 are more than 20% lower. As results
>>> of the same test on unpatched 5.12-rc7 are much more consistent (the
>>> lowest iteration result through the whole test was 1175939718.3 and the
>>> mean deviation only 276889.1 B/s), it doesn't seeem to be just a random
>>> fluctuation.
>>
>> I think I need to relearn the statistial math to understand the above
>> "99% confidence interval half width ":)
>
> An easy way to understand it is that if the last column shows 42 MB/s,
> it means that with 99% confidence (probability), the measured average
> is within 42 MB/s off the actual one.
>
>> But the problem do not seems related too much with "99% confidence
>> interval half width ", but with "mean deviation"?
>
> Mean deviation is a symptom here. What worries me is that most results
> show the same value (corresponding to fully saturated line) with very
> little variation, in some iterations (1, 3 and 12 here) we can suddenly
> see much lower value (by ~2.5 GB/s, i.e. 20-25%). And as each iteration
> runs the connection for 30 seconds, it cannot be just some short glitch.
>
> I managed to get tcpdump captures yesterday but they are huge even with
> "-s 128" (client ~5.6 GB, server ~9.0 GB) so that working with them is
> rather slow so I did not find anything interesting yet.
>
>> I tried using netperf, which seems only show throughput of 9415.06
>> (10^6bits/sec) using 10G netdev. which tool did you used to show the
>> above number?
>
> 9415.06 * 10^6 b/s is 1176.9 * 10^6 B/s so it's about the same as the
> numbers above (the good ones, that is). As this was part of a longer
> test trying different thread counts from 1 to 128, I was using another
> utility I started writing recently:
I tried using below shell to simulate your testcase:
#!/bin/sh
for((i=0; i<20; i++))
do
taskset -c 0-31 netperf -t TCP_STREAM -H 192.168.100.2 -l 30 -- -m 1048576
done
And I got a quite stable result: 9413~9416 (10^6bits/sec) for 10G netdev.
>
> https://github.com/mkubecek/nperf
>
> It is still raw and a lot of features are missing but it can be already
> used for multithreaded TCP_STREAM and TCP_RR tests. In particular, the
> output above was with
>
> nperf -H 172.17.1.1 -l 30 -i 20 --exact -t TCP_STREAM -M 1
I tried your nperf too, unfortunately it does not seem to work on my
system(arm64), which exits very quickly and output the blow result:
root@(none):/home# nperf -H 192.168.100.2 -l 30 -i 20 --exact -t TCP_STREAM -M 1
server: 192.168.100.2, port 12543
iterations: 20, threads: 1, test length: 30
test: TCP_STREAM, message size: 1048576
1 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%)
2 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
3 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- nan B/s ( nan%)
4 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
5 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
6 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
7 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
8 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
9 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
10 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
11 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
12 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
13 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
14 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
15 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
16 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
17 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
18 4.0 B/s, avg 4.0 B/s, mdev 0.0 B/s ( 0.0%), confid. +/- 0.0 B/s ( 0.0%)
19 4.0 B/s, avg 4.0 B/s, mdev nan B/s ( nan%), confid. +/- nan B/s ( nan%)
20 4.0 B/s, avg 4.0 B/s, mdev nan B/s ( nan%), confid. +/- nan B/s ( nan%)
all avg 4.0 B/s, mdev nan B/s ( nan%), confid. +/- nan B/s ( nan%)
>
> The results are with 1 thread so that they should be also reproducible
> with netperf too. But it needs to be repeated enough times, when
> I wanted to get the packet captures, I did 40 iterations and only two of
> them showed lower result.
>
> Michal
>
> .
>