seq: maximum number of events w.r.t. ordering in queue

seq: maximum number of events w.r.t. ordering in queue

[Gerald Grabner]

Hi there,

for simultaneously playing many midi tracks each with a large number
of events, it would be convenient to output one track after the other
in a simple loop, without caring about the ordering of the events.

Of course, this requires an output buffer large enough to hold all
events, otherwise events of the later are postponed every time the
buffer is full. But one question remains, at least for me:
Is there a maximum number of events w.r.t. the ordering in a queue?

Below is a simple test program that pumps a number of events in
reverse tick order into the output buffer, i.e. starting with the
latest tick (high hotes) and finishing with tick=0 (low notes).
output_buffer_size, client_pool_output and client_pool_output_room are
increased to have the capacity for all events. But still, the (high)
notes with the lowest ticks are played only towards the end instead of
at the beginning - obviously because they were output last. The
ordering is failing.

Is there a way to ensure that all events are played in correct order,
regardless of when the are outputted?

Many thanks,
Gerald

BTW: alsa version = 1.0.11

**

#include <alsa/asoundlib.h>
#include <stdio.h>

int main()
{
  // open sequencer
  snd_seq_t* seq;
  snd_seq_open(&seq, "default", SND_SEQ_OPEN_DUPLEX, 0) ;
  int my_port = snd_seq_create_simple_port
    ( seq, "standard port",
      SND_SEQ_PORT_CAP_READ | SND_SEQ_PORT_CAP_SUBS_READ,
      SND_SEQ_PORT_TYPE_APPLICATION) ;

  // number of events
  unsigned int n = 600 ;

  // buffer size for all events
  unsigned int b = (n+1)*sizeof(snd_seq_event_t);

  // set output buffer and client pool size
  if ( snd_seq_set_output_buffer_size(seq,b) )
    printf("Error setting output_buffer_size\n");
  if ( snd_seq_set_client_pool_output(seq,b) )
    printf("Error setting client_pool_output\n");
  if ( snd_seq_set_client_pool_output_room(seq,b) )
    printf("Error setting client_pool_output_room\n");

  // connect to 21:0 (OPL3)
  snd_seq_connect_to(seq, my_port, 21, 0);

  // queue (default: 96 PPQ and 120 BPM)
  int queue = snd_seq_alloc_queue(seq);
  snd_seq_start_queue (seq, queue, NULL);
  snd_seq_drain_output(seq);

  // set and schedule events
  snd_seq_event_t ev ;
  snd_seq_ev_clear(&ev);
  snd_seq_ev_set_subs(&ev);
  snd_seq_ev_set_noteon(&ev,0,60,127);
  unsigned int i;
  for ( i=0 ; i<n ; i++ )
    {
      // note range: 12-107
      ev.data.note.note = (unsigned char) (12+(107-12)*i/(n-1));
      // tick in decreasing order, total length = 4 quarters
      snd_seq_ev_schedule_tick(&ev, queue, 0, (n-1-i)*96*4/n);
      // output event
      snd_seq_event_output(seq, &ev);
    }
  printf("scheduling done...\n");

  // send events and wait for end
  snd_seq_drain_output(seq);
  snd_seq_sync_output_queue(seq);
  sleep(1);

  // done
  snd_seq_close ( seq ) ;
  return 0 ;
}

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Re: seq: maximum number of events w.r.t. ordering in queue

[Clemens Ladisch]

Gerald Grabner wrote:
> for simultaneously playing many midi tracks each with a large number
> of events, it would be convenient to output one track after the other
> in a simple loop, without caring about the ordering of the events.
> 
> Of course, this requires an output buffer large enough to hold all
> events, otherwise events of the later are postponed every time the
> buffer is full. But one question remains, at least for me:
> Is there a maximum number of events w.r.t. the ordering in a queue?

Yes.  Your client buffer can have any size, but the events stored in the
kernel for a queue are sent from an interrupt handler and must therefore
be in locked kernel memory.  The kernel buffer is currently restricted
to 2000 events for one client.


Regards,
Clemens

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Re: seq: maximum number of events w.r.t. ordering in queue

[Gerald Grabner]

Clemens Ladisch wrote:
> Gerald Grabner wrote:
>> for simultaneously playing many midi tracks each with a large number
>> of events, it would be convenient to output one track after the other
>> in a simple loop, without caring about the ordering of the events.
>>
>> Of course, this requires an output buffer large enough to hold all
>> events, otherwise events of the later are postponed every time the
>> buffer is full. But one question remains, at least for me:
>> Is there a maximum number of events w.r.t. the ordering in a queue?
> 
> Yes.  Your client buffer can have any size, but the events stored in the
> kernel for a queue are sent from an interrupt handler and must therefore
> be in locked kernel memory.  The kernel buffer is currently restricted
> to 2000 events for one client.

Ok, I see, many thanks!

In my test application, the threshold is 500 events. How am I loosing
the remaining 1500 events?

Regards,
Gerald

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Re: seq: maximum number of events w.r.t. ordering in queue

[Clemens Ladisch]

Gerald Grabner wrote:
>   // buffer size for all events
>   unsigned int b = (n+1)*sizeof(snd_seq_event_t);
> 
>   // set output buffer and client pool size
>   if ( snd_seq_set_output_buffer_size(seq,b) )
>     printf("Error setting output_buffer_size\n");
>   if ( snd_seq_set_client_pool_output(seq,b) )
>     printf("Error setting client_pool_output\n");

What the documentation failed to tell you is that the parameter to
snd_seq_set_client_pool_output() is measured in events, not in bytes,
and that a value that is too big is silently ignored.


HTH
Clemens

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Re: seq: maximum number of events w.r.t. ordering in queue

[Gerald Grabner]

Clemens Ladisch wrote:
> Gerald Grabner wrote:
>>   // buffer size for all events
>>   unsigned int b = (n+1)*sizeof(snd_seq_event_t);
>>
>>   // set output buffer and client pool size
>>   if ( snd_seq_set_output_buffer_size(seq,b) )
>>     printf("Error setting output_buffer_size\n");
>>   if ( snd_seq_set_client_pool_output(seq,b) )
>>     printf("Error setting client_pool_output\n");
>
> What the documentation failed to tell you is that the parameter to
> snd_seq_set_client_pool_output() is measured in events, not in
> bytes, and that a value that is too big is silently ignored.

Indeed, that did the trick!


Besides the fact that ALSA is a great piece of software, the
documentation is a bit poor in some places - like this one. Since I am
not a developer of the ALSA lib, I can only make suggestions, e.g.
adding the following doxygen snippet at line 264/265 in seqmid.c
(alsa-lib-1.0.14rc2) for snd_seq_set_client_pool_output:

 * The pool size is measured in events, not in bytes. The maximum size
 * is 2000, values greater than that are ignored.

Thx & Regards,
Gerald

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