Re: Low Latency vs. Real Time Kernel - actual latencies ?

[Brian Monroe <briancmonroe@xxxxxxxxx>]

I didn't have time tonight to get tests on all the kernels under load, but here are the results, relatively load free. 

I need to write a short python program to put the systems under duress. Hopefully I'll have time tommorrow to work on it and I can have the rest of the results in. 

PS:I'm attaching some results from an RT kernel on a debian system below that they used while the box was recieving flood ping from an external source and repeated loops of hackbench 25 and ls -Ral /. 

# cyclictest -a -t -n -p99 -i100 -d50
560.44 586.11 606.12 211/1160 3727
T: 0 (18617) P:99 I:100 C:1011846111 Min:  2 Act:  4 Avg:  5 Max: 39
T: 1 (18618) P:98 I:150 C: 708641019 Min:  2 Act:  5 Avg: 11 Max: 57

On Thu, Feb 16, 2012 at 10:37 AM, Fernando Lopez-Lezcano <nando@xxxxxxxxxxxxxxxxxx> wrote:

On 02/16/2012 04:33 AM, David Timms wrote:

On 15/02/12 16:49, Brian Monroe wrote:

On Tue, Feb 14, 2012 at 12:21 PM, Brendan Jones
<brendan.jones.it@xxxxxxxxx>wrote:

Latency times are really relative things based on many factors. Have you
been getting better latency times than ta

yeah.

I would be good to see real numbers associated with claims like the
above; also how it is measured..

Cyclictest is widely used for measuring latencies:
 https://rt.wiki.kernel.org/articles/c/y/c/Cyclictest.html

Also, indicate those various kernel patches and or options they use or
have tested.

The rt patches for the latest kernels (3.2 at this point) are here:
 http://www.kernel.org/pub/linux/kernel/projects/rt/3.2/

This is pretty much all I add to the rt patched kernels for Fedora (with the proper configuration options, of course, I'm attaching the extra options I'm currently using). To get the best performance in the Jack world you need to tune the irq kernel thread priorities, that is usually done using rtirq (http://www.rncbc.org/jack/#rtirq), and of course jackd should run with the proper rt priority as well.

-- Fernando

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