[Cake] Cake3 - source code and some questions

[Adrian Popescu]

Hello Toke,

Thanks to your experiment and your statement regarding CPU load on
your box during testing, I was able to fix the problem.

It looks like this problem was being caused by power saving. Something
changed between the older kernels and the newer ones. Changing the
power saving settings in the BIOS brings back latency below 0.5
milliseconds.

This might have an impact some benchmarks which don't load up all CPU
cores or which don't need a lot of CPU power. This is certainly
something to keep an eye on when doing any kind of testing involving
really low latencies or network schedulers.


On Thu, Apr 23, 2015 at 12:01 PM, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
> Adrian Popescu <adriannnpopescu at gmail.com> writes:
>
>> Seeing worse latency under load (20-100 milliseconds) isn't uncommon.
>> I believe this to be a regression in the kernel or in the network
>> drivers.
>
> I don't see this behaviour at all:
>
> $ ls -l /sys/class/net/enp0s25/device/driver                                                                                                                                                                                   :(
> 0 lrwxrwxrwx 1 root root 0 Apr 21 16:05 /sys/class/net/enp0s25/device/driver -> ../../../bus/pci/drivers/e1000e/
>
> $ ping 130.243.26.1 -c 100 # this is my default gateway
> ..snip...
> --- 130.243.26.1 ping statistics ---
> 100 packets transmitted, 100 received, 0% packet loss, time 99000ms
> rtt min/avg/max/mdev = 0.341/0.801/28.260/2.769 ms
>
> $ cat /proc/loadavg
> 9.29 8.43 5.30 14/508 6665
>
> (yes, this is while running a cpu-hungry data processing application in
> the background on all eight cores)
>
> $ uname -a
> Linux alrua-kau 3.19.3-3-ARCH #1 SMP PREEMPT Wed Apr 8 14:10:00 CEST 2015 x86_64 GNU/Linux
>
> $  tc qdisc show dev enp0s25
> qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
>
> (hmm, why am I running pfifo_fast?)
>
> Repeating with sch_fq:
>
> $ ping 130.243.26.1 -c 100
> ...snip...
> --- 130.243.26.1 ping statistics ---
> 100 packets transmitted, 100 received, 0% packet loss, time 98998ms
> rtt min/avg/max/mdev = 0.358/0.468/1.278/0.151 ms
>
> -Toke