Re: [PATCH] Preemption Latency Measurement Tool

[george anzinger <george@mvista.com>]

Andre Pang wrote:
> 
> On Sat, Sep 22, 2001 at 02:10:18AM -0400, Robert Love wrote:
> 
> > > i did a test of it on linux-2.4.10-pre13 with Benno Senoner's
> > > lowlatency program, which i hacked up a bit to output
> > > /proc/latencytimes after each of the graphs.  test results are at
> > >
> > >     http://www.algorithm.com.au/hacking/linux-lowlatency/2.4.10-pre13-pes/
> > >
> > > and since i stared at the results in disbelief, i won't even try
> > > to guess what's going on :).  maybe you can make some sense of
> > > it?
> >
> > Well, its not hard to decipher...and really, its actually fairly good.
> > the latency test program is giving you a max latency of around 12ms in
> > each test, which is OK.
> 
> arrgh!  i just realised my script buggered up and was producing the same
> graph for all the results.  please have a look at the page again, sorry.
> 
> apart from that, i'm still confused.  compared to other graphs produced
> by the latencytest program, my system seems to have huge latencies.
> unless i'm reading it wrongly, the graph is saying that i'm getting
> latencies of up to 30ms, and a lot of overruns.  compare this to
> 
>     http://www.gardena.net/benno/linux/audio/2.4.0-test2/3x256.html
> 
> which shows latencytest on 2.4.0-test2, and
> 
>     http://www.gardena.net/benno/linux/audio/2.2.10-p133-3x128/3x128.html
> 
> which are the results for latencytest on 2.2.10.  admittedly these
> kernels are much older, but i'm consistently getting far more latency
> than those kernels.  that's the bit i'm confused about :)  i've tried
> Andrew Morton's low-latency patches as well, to no avail.  i've made
> sure i've tuned my hard disks correctly, and i don't have any other
> realtime processes running.
> 
> am i concerned with a different issue than the one you're addressing?
> 
> > the preemption-test patch is showing _MAX_ latencies of 0.8ms through
> > 12ms.  this is fine, too.
> 
> yep, i agree with that ... so why is latencytest showing scheduling
> latencies of > 30ms?  i get the feeling i'm confusing two different
> issues here.  from what i understand, /proc/latencytimes shows the
> how long it takes for various functions in the kernel to finish, and
> the latencytest result shows how long it takes for it to be
> re-scheduled (represented by the white line on the graph).

The one thing the latancytimes patch doesn't monitor is interrupt off
time.  Maybe it should...


George