[linux-audio-dev] Audio/Midi system - RT prios..

[Florian Schmidt]

Hi,

i was wondering:

With the new shiny -rt kernels and realtime scheduling available to non
root users via the usual mechanisms, there's the possibility of really
finetuning an audio/midi system.

The main issue i am interested in is the interplay between midi and
audio in such a system. How to tune the audio side to get a very
reliable system is pretty easy these days, thanks to the great jack
audio connection kit, alsa and the new -rt kernels.

But now i wonder how midi software fits into this. I'm here interested
in the special case of a software sequencer (like i.e. Rosegarden)
driving a softsynth (like i.e. om-synth or supercollider3) or whatever.

Ok, on a normal audio tuned -rt equipped linux system the SCHED_FIFO
priorities which are used for the different components look something
like this:

99 - system timer
98 - RTC

81 - soundcard IRQ handler
80 - jack watchdog
70 - jack main loop
69 - jack clients' process loops

50 - the other IRQ handlers

Now, i wonder how midi threads would fit in best into this scheme. Let's
assume our midi sequencer uses either sleep() or RTC to get woken up at
regular intervals, and let's further assume that it properly deals with
these timing sources to get relatively jitter free midi output given
that it get's woken up often enough by the scheduler. I further assume
that the alsa seq event system is used and midi events are not queued
for future delivery but always delivered immediately.

All this implies that for midi delivery timing to not be influenced by
audio processing on the system (which gets a problem especially at large
buffer size, where quite a bit of work is done at a time), all the stuff
that handles midi should run with realtime priorities above the jack
stuff (i.e. around 90). I wonder whether it also needs to have a higher
priority than the soundcard irq handler, too. Does the jackd main loop
"inherit" the priority of the soundcard irq handler?

Anyways, one more thing to note is for this to work nicely, the
softsynth needs to have an extra midi handling thread that is also
running with a priority in the 90s range, so it can timestamp the event
properly when it arrives.

So i wonder now: Assuming our system is setup as described above and all
midi handling is done from threads with sufficiently high pririties not
to get disturbed by audio stuff, will the alsa event system play nice?

I ask this, because i have setup a system as above with a simple midi
generator (see code below) and some different softsynths (one of which i
have written which does have its midi thread at an appropriate priority.
you can get a tarball here.

http://affenbande.org/~tapas/ughsynth-0.0.3.tgz

Beware it eats unbelievable amounts of cpu and is in no way considered
being finished. it just lay around handy for this test ;)). But i still
get some regular jitter in my sound.

Here's recorded example output (running jackd at a periodsize of 1024
and the test notes are produced at a frequency of 8hz). First with
ughsynth then with jack-dssi-host hexter.so. The effect is less
prominent with hexter, i suppose because the jack load with it is only
at 2 or 3% as opposed to ughsynth that uses 50% here on my athlon 1.2
ghz box. In case you don't hear what i mean: The timing of every ca. 7th
or 8th note is a little bit off.

http://affenbande.org/~tapas/midi_timing.ogg

So i wonder: what's going wrong? Is the priorities setup described above
not correct? Is alsa seq handling somehow not done with RT priority?
What else could be wrong? Please enlighten me :)

And yeah, i do _not_ want to hear about jack midi. It's a good thing,
and i'm all for it as it will make at least some scenarios work great
(sequencer and softsynth both being jack midi clients), but not all.

Thanks in advance, 
Flo

midi_timer.cc:

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>
#include <cstdlib>
#include <iomanip>

#include <pthread.h>
#include <linux/rtc.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <poll.h>
#include <signal.h>
#include <time.h>

#include <alsa/asoundlib.h>

#define RTC_FREQ  2048.0
#define NOTE_FREQ    8.0
#define RT_PRIO     85

int main() 
{
	int fd;

	fd = open("/dev/rtc", O_RDONLY);

	if (fd ==  -1) {
		perror("/dev/rtc");
		exit(errno);
	}

	int retval = ioctl(fd, RTC_IRQP_SET, (int)RTC_FREQ);

	if (retval == -1) {
		perror("ioctl");
		exit(errno);
	}

	std::cout << "locking memory" << std::endl;
	mlockall(MCL_CURRENT);

	// std::cout << "sleeping 1 sec" << std::endl;
	// sleep(1);

	snd_seq_t *seq_handle;

	int err, port_no;

	err = snd_seq_open(&seq_handle, "default", SND_SEQ_OPEN_OUTPUT, 0);

	if (err < 0) {
		std::cout << "error" << std::endl;
		exit(0);
	}

	std::string port_name = "midi_timer";
	// set the name to something reasonable..

	err = snd_seq_set_client_name(seq_handle, port_name.c_str());

	if (err < 0) {
		std::cout << "error" << std::endl;
		exit(0);
	}

	// this is the port others can connect to. we don't do autoconnect ourself
	err = snd_seq_create_simple_port(seq_handle, "midi_timer:output", SND_SEQ_PORT_CAP_READ|SND_SEQ_PORT_CAP_SUBS_READ, SND_SEQ_PORT_TYPE_MIDI_GENERIC);

	if (err < 0) {
		std::cout << "error" << std::endl;
		exit(0);
	}

	// on success we know our port no
	port_no = err;

	struct sched_param param;
	int policy;

	pthread_getschedparam(pthread_self(), &policy, &param);
	param.sched_priority = RT_PRIO;
	policy = SCHED_FIFO;
	pthread_setschedparam(pthread_self(), policy, &param);
	

	std::cout << "turning irq on" << std::endl;
	retval = ioctl(fd, RTC_PIE_ON, 0);

	if (retval == -1) {
		perror("ioctl");
		exit(errno);
	}

	snd_seq_event_t ev;

	unsigned long data;
	
	int ticks_passed = 0;

	while(1) {
		// then we read it
		retval = read(fd, &data, sizeof(unsigned long));
		if (retval == -1) {
			perror("read");
			exit(errno);
		}
		if ((float)ticks_passed >= (RTC_FREQ/NOTE_FREQ)) {
			// std::cout << "play note" << std::endl;
			ticks_passed -= (long int)(RTC_FREQ/NOTE_FREQ);

			// play note
			snd_seq_ev_clear(&ev);
			snd_seq_ev_set_direct(&ev);
			snd_seq_ev_set_subs(&ev);
			snd_seq_ev_set_source(&ev, port_no);
			ev.type = SND_SEQ_EVENT_NOTEON;
			ev.data.note.note = 53;
			ev.data.note.velocity = 100;
			
			snd_seq_event_output_direct(seq_handle, &ev);

			snd_seq_drain_output(seq_handle);
		}
		data = (data >> 8);
		// std::cout << data << std::endl;
		ticks_passed += data;
	}
	return 0;
}


-- 
Palimm Palimm! 
http://tapas.affenbande.org