Re: 1 of 23 peformance problems at 30 to 40 percent system load

[linuxball <linuxball@gmail.com>]

Hi Joël,

see remarks in the context below.

Best regards,

Wolfgang

On 17.07.2015 22:05, Joël Krähemann wrote:
> Hi all,
>
> My name is Joël Krähemann, I'm developing:
>
> http://gsequencer.org
>
> and I'm using:
>
> Linux debian 4.0.5 #1 SMP PREEMPT Sat Jul 11 16:32:49 CEST 2015 x86_64 GNU/Linux
 From the output it seems that the kernel you are using is NOT a RT 
kernel (otherwise "uname -v" should say "#1 SMP PREEMPT RT ..."). If you 
want to use a RT kernel you should build the kernel with PREEMPT_RT_FULL 
defined.
> For now I encounter on my system performance problems at a load of 30
> to 40 % system load, all 8 virtual cpu's have same average load.
What do you mean when you talk about "performance problems"? How do they 
manifest?

> * `chrt` to higher priority doesn't give wished throughput.
As you certainly know in general a RT kernel has worse throughput (but 
better response times / lower latencies for time critical threads) than 
a generic kernel.
> * `taskset` has ff as default.
> * `cpufreq-set -g performance` brings a little improvement for first seconds
>
> This is my CPU:
>
> Architecture:          x86_64
> CPU op-mode(s):        32-bit, 64-bit
> Byte Order:            Little Endian
> CPU(s):                8
> On-line CPU(s) list:   0-7
> Thread(s) per core:    2
> Core(s) per socket:    4
> Socket(s):             1
> NUMA node(s):          1
> Vendor ID:             GenuineIntel
> CPU family:            6
> Model:                 58
> Model name:            Intel(R) Core(TM) i7-3740QM CPU @ 2.70GHz
> Stepping:              9
> CPU MHz:               3275.648
> CPU max MHz:           3700.0000
> CPU min MHz:           1200.0000
> BogoMIPS:              5387.58
> Virtualization:        VT-x
> L1d cache:             32K
> L1i cache:             32K
> L2 cache:              256K
> L3 cache:              6144K
> NUMA node0 CPU(s):     0-7
>
>
> GSequencer uses many threads but doesn't stay in realtime. What am I
> doing wrong? Or how to gain more Performance out of the system?
How do you define performance? If you want more throughput then try a 
generic kernel. If you want shorter response times (lower latencies) for 
selected RT threads than use a RT kernel and tweak the thread priorities 
of the respective user and kernel threads in the processing chain. Make 
sure that you don't get priority inversion by using improper RT prio 
assignment, e.g. participating IRQ threads from kernel should have 
higher or same RT prio than threads processing data delivered by those 
IRQ threads. A good example for audio applications is 
http://subversion.ffado.org/wiki/IrqPriorities which shows priority 
settings for JACK audio applications.
> I could imagine on my code side that vary frequency segmentation would
> bring better throughput. By modeifing AGS_THREAD_DEFAULT_JIFFIE,
> AGS_THREAD_MAX_PRECISION and related.
>
> But for now I search for documentation about linux kernel performance counters.
>
> Best regards,
> Joël Krähemann
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