- 28,78% 0,00% rawmidi_alsa [snd_rawmidi] [k]
snd_rawmidi_drain_output
- snd_rawmidi_drain_output
- 26,59% msleep
schedule_timeout
schedule
+ __schedule
+ 2,13% schedule_timeout
Seems to be because of:
https://github.com/torvalds/linux/blob/master/sound/core/rawmidi.c#L244
if (substream->ops->drain)
substream->ops->drain(substream);
else
msleep(50);
It see what fileops have no drain impl:
https://github.com/torvalds/linux/blob/master/sound/core/rawmidi.c#L1708
and I see
https://github.com/torvalds/linux/blob/master/sound/core/seq/seq_virmidi.c#L329
that define no drain op. Not which ones are actually used here :/
The docs confirm teh 50 ms wait though:
https://github.com/torvalds/linux/blob/master/Documentation/sound/kernel-api/writing-an-alsa-driver.rst#drain-callback
would it make sense to have a dummy drain function for the seq_virmidi
output ops?
Stefan
Am Di., 4. Jan. 2022 um 16:54 Uhr schrieb Takashi Iwai <tiwai@xxxxxxx>:
> On Sat, 01 Jan 2022 12:49:13 +0100,
> Stefan Sauer wrote:
> >
> > hi,
> >
> > I've tried to link BitwigStudio to the webapp cables.gl over virmidi.
> > Unfortunately Bitwig Studio only supports rawmidi. What I discovered is
> > that there is a strange slowness when sending data to virmidi caused
> > by snd_rawmidi_drain().
> >
> > I've posted two tiny, self-contained c apps to:
> > https://gist.github.com/ensonic/c7588b87fa6c1fa94a8f753b1e0aa394
> > See some examples below. 2 observations:
> > * snd_rawmidi_type() is *not* reporting virmidi as VIRTUAL
> > * snd_rawmidi_drain() takes about 60ms! on virtual vs. less that 0.1 ms
> on
> > usb midi (I checked all my hw midi and the worst was avg=1ms on physical
> > midi image unitor8)
> >
> > When comparing the implementations:
> >
> https://github.com/alsa-project/alsa-lib/blob/master/src/rawmidi/rawmidi_virt.c#L173
> >
> https://github.com/alsa-project/alsa-lib/blob/master/src/rawmidi/rawmidi_hw.c#L164
> > I see that the hw one results in an IOCTL which I can see when striking
> the
> > code and I wonder if this is the root cause? Why is rawmidi_virt.c not
> used
> > for virmidi?
> > >From poking at snd_rawmidi_open_conf() I have not yet figured where
> this is
> > decided ....
> >
> > Stefan
> >
> > > amidi -l
> > Dir Device Name
> > IO hw:0,0,0 Scarlett 18i20 USB MIDI 1
> > IO hw:3,0,0 nanoKEY2 nanoKEY2 _ KEYBOARD
> > IO hw:5,0,0 nanoKONTROL nanoKONTROL _ SLIDE
> > IO hw:10,0 Virtual Raw MIDI (16 subdevices)
> > IO hw:11,0 Virtual Raw MIDI (16 subdevices)
> >
> > # using direct i/o to virmidi - all good
> > > ./rawmidi_oss /dev/midi11 0
> > Using device '/dev/midi11' without draining
> > write took min= 0.0015 ms, avg= 0.0016 ms, max= 0.0110 ms
> > > ./rawmidi_oss /dev/midi11 1
> > Using device '/dev/midi11' with draining
> > write took min= 0.0015 ms, avg= 0.0017 ms, max= 0.0101 ms
> > drain took min= 0.0001 ms, avg= 0.0001 ms, max= 0.0008 ms
> >
> > # using snd_rawmidi to virmidi - slow drain operations
> > > ./rawmidi_alsa hw:11,0 0
> > Using device 'hw:11,0' without draining
> > SND_RAWMIDI_TYPE_HW
> > write took min= 0.0010 ms, avg= 0.0011 ms, max= 0.0056 ms
> > > ./rawmidi_alsa hw:11,0 1
> > Using device 'hw:11,0' with draining
> > SND_RAWMIDI_TYPE_HW
> > write took min= 0.0016 ms, avg= 0.0040 ms, max= 0.0077 ms
> > drain took min= 55.9951 ms, avg= 60.4330 ms, max= 64.0653 ms
> >
> > # using snd_rawmidi to usb hw - all good
> > > ./rawmidi_alsa hw:3,0 0
> > Using device 'hw:3,0' without draining
> > SND_RAWMIDI_TYPE_HW
> > write took min= 0.0012 ms, avg= 0.0015 ms, max= 0.0121 ms
> > > ./rawmidi_alsa hw:3,0 1
> > Using device 'hw:3,0' with draining
> > SND_RAWMIDI_TYPE_HW
> > write took min= 0.0024 ms, avg= 0.0032 ms, max= 0.0110 ms
> > drain took min= 0.0293 ms, avg= 0.0636 ms, max= 0.2277 ms
>
> This kind of thing needs profiling. You can try perf or whatever
> available, and identify which call takes long. My wild guess is
> something about snd_seq_sync_output_queue(), maybe poll syscall takes
> unexpected long.
>
>
> thanks,
>
> Takashi
>
