Re: [PATCH] Preemption Latency Measurement Tool

[george anzinger <george@mvista.com>]

Robert Love wrote:
> 
> On Sat, 2001-09-22 at 19:40, safemode wrote:
> > ok. The preemption patch helps realtime applications in linux be a little
> > more close to realtime.  I understand that.  But your mp3 player shouldn't
> > need root permission or renicing or realtime priority flags to play mp3s.
> 
> It doesn't, it needs them to play with a dbench 32 running in the
> background.  This isn't nessecarily acceptable, either, but it is a
> difference.
> 
> Note one thing the preemption patch does is really make `realtime' apps
> accel.  Without it, regardless of the priority of the application, the
> app can be starved due to something in kernel mode.  Now it can't, and
> since said application is high priority, it will get the CPU when it
> wants it.
> 
> This is not to say the preemption patch is no good if you don't run
> stuff at realtime --  I don't (who uses nice, anyhow? :>), and I notice
> a difference.
> 
> > To
> > test how well the latency patches are working you should be running things
> > all at the same priority.  The main issue people are having with skipping
> > mp3s is not in the decoding of the mp3 or in the retrieving of the file, it's
> > in the playing in the soundcard.  That's being affected by dbench flooding
> > the system with irq requests.  I'm inclined to believe it's irq requests
> > because the _only_ time i have problems with mp3s (and i dont change priority
> > levels) is when A. i do a cdparanoia -Z -B "1-"    or dbench 32.   I bet if
> > someone did these tests on scsi hardware with the latency patch, they'd find
> > much better results than us users of ide devices.
> 
> The skips are really big to be irq requests, although perhaps you are
> right in that the handling of the irq (we disable preemption during
> irq_off, of course, but also during bottom half execution) is the
> problem.
> 
> However, I am more inclined to believe it is something else.  All these
> long held locks can indeed be the problem.
> 
> I am on an all UW2 SCSI system, and I have no major blips playing during
> a `dbench 16' (never ran 32).  However, many other users (Dieter, I
> believe) are on a SCSI system too.

Dieter, could you post your .config file?  It might have a clue or two.

George

> 
> > even i dont get any skips when i run the player at nice -n -20.   That
> > doesn't tell you much about the preemption patch though.  And it doesn't tell
> > you about performance when you dont give linux the chance to do what it does,
> >  multitask.  That's where the latency patch is directed at improving, i
> > think.
> 
> Agreed.
> 
> --
> Robert M. Love
> rml at ufl.edu
> rml at tech9.net