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

[Paul Davis <paul@linuxaudiosystems.com>]

(sorry for the slow turnaround. i am also going offline for the next 6
hours, so this is my last reply till later ...)

>> the "max delay" measurement isn't a reflection of the runtime activity
>> of jackd. its simply a measurement of the delay between when jackd
>> expected to be next woken from poll and when it actually was.
>> 
>> as you noted, jackd generally goes back to sleep in poll typically a
>> long time before the next interrupt is expected. hence any delay in
>> the wakeup is between the interrupt handler and the scheduler getting
>> jackd's main thread back on the processor. i think.
>
>this brings up the next question: does the jackd measurement also
>timestamp the time when it calls poll() - hence detecting in-jackd
>processing delays? If yes, which value is this from Rui's stats? If not
>then it might make sense to add it.

i'm not quite sure what you mean. the timestamps used to measure
max_delay look like this:
	  

                ... set up poll fd struct  ....

		poll_enter = jack_get_microseconds ();

		if (poll (driver->pfd, nfds, driver->poll_timeout) < 0) {

			if (errno == EINTR) {
				printf ("poll interrupt\n");
				// this happens mostly when run
				// under gdb, or when exiting due to a signal
				if (under_gdb) {
					goto again;
				}
				*status = -2;
				return 0;
			}
			
			jack_error ("ALSA: poll call failed (%s)",
				    strerror (errno));
			*status = -3;
			return 0;
			
		}

		poll_ret = jack_get_microseconds ();

		if (extra_fd < 0) {
			if (driver->poll_next && poll_ret > driver->poll_next) {
				*delayed_usecs = poll_ret - driver->poll_next;
			} 
			driver->poll_last = poll_ret;
			driver->poll_next = poll_ret + driver->period_usecs;
			driver->engine->transport_cycle_start (driver->engine, 
							       poll_ret);
		}

you can view this here in context:

   http://jackit.sourceforge.net/cgi-bin/lxr/http/source/drivers/alsa/alsa_driver.c
  
(its at line 1195)

the max_delay value is taken from "delayed_usecs" above. each backend
(the one shown is the ALSA driver) sets this every time its "wait"
function is called; jackd collects the figures and these days manages
them as a set of statistics.

as you can see, poll_enter is available, but we don't use it for much
at all (mostly just some information in the very unusual event that
poll times out).

jack_get_microseconds() uses the cycle timer to compute elapsed time,
there is no system call involved.

--p