Re: [Fwd: Re: [patch] Real-Time Preemption, -RT-2.6.9-mm1-V0.4]

[Paul Davis <paul@linuxaudiosystems.com>]

  [ I trimmed the CC: line because several people there are on
    jackit-devel. ]

>> compiles and boots fine. no observable change in xrun behaviour though. 
>
>ok, so there's something else going on as well - or i missed an ioctl. 

i really don't think the ioctl's are relevant. 

consider what will happen if jackd does make a system call that causes
a major delay (say, because of the BKL). we will get an xrun, yes, but
this will cause jackd to stop the audio interface and
restart. max_delay is not affected by this behaviour.
 
as far as i can tell, the number reported by max_delay entirely (or
almost entirely) represents problems in kernel scheduling, specifically
with a combination of:

     a) handling the audio interface interrupt in time.
     b) marking the relevant jackd thread runnable
     c) context switching back to the relevant jackd thread

things that jackd does once its running do not, it appear to me, have
any impact on max_delay, which is based on the simple observation: 

   "i was just woken, i expect to be awakened again in N usecs or
   less.

--p