slow snd_rawmidi_drain() for VirMidi devcies

[Stefan Sauer]

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