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
