- 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#L... 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...
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@suse.de:
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_vir...
https://github.com/alsa-project/alsa-lib/blob/master/src/rawmidi/rawmidi_hw....
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