On 10/17/18 7:58 AM, Jonathan Liu wrote:

Hi,

I want to start a discussion regarding round trip latency for class compliant USB audio interfaces on Linux. In particular, I am noticing with my USB 2.0 RME Babyface Pro audio interface that the round trip latency is considerably higher on Linux than on macOS High Sierra and Windows 10.

I tested the round trip latency using a loopback audio cable and the ReaInsert plugin included with Reaper DAW (www.reaper.fm) that can be downloaded for Windows/macOS/Linux to calculate the additional delay.

Here are the results for 48000 Hz, 24-bit on my RME Babyface Pro:

block_size/periods block_size*periods + additional_delay ~ round_trip_latency round_trip_latency = (block_size*periods + additional_delay) / 48000 * 1000

Linux 4.17.14, Class Compliant Mode (snd-usb-audio, ALSA backend): 16/2 32 + 80 ~ 2.333 ms 16/3 48 + 109 ~ 3.271 ms 32/2 64 + 129 ~ 4.021 ms 32/3 96 + 166 ~ 5.458 ms 64/2 128 + 205 ~ 6.938 ms 64/3 192 + 242 ~ 9.042 ms 128/2 256 + 352 ~ 12.667 ms 128/3 384 + 496 ~ 18.334 ms 256/2 512 + 650 ~ 24.208 ms 256/3 768 + 650 ~ 29.542 ms 512/2 1024 + 634 ~ 34.542 ms 512/3 1536 + 634 ~ 45.208 ms 1024/2 2048 + 650 ~ 56.208 ms 1024/3 3072 + 650 ~ 77.542 ms 2048/2 4096 + 633 ~ 98.521 ms 2048/3 6144 + 633 ~ 141.188 ms

macOS High Sierra, Class Compliant Mode (Apple Driver): 16/2 32 + 205 ~ 4.938 ms 32/2 64 + 205 ~ 5.604 ms 64/2 128 + 205 ~ 6.938 ms 128/2 256 + 205 ~ 9.604 ms 256/2 512 + 205 ~ 14.938 ms 512/2 1024 + 205 ~ 25.604 ms 1024/2 2048 + 205 ~ 46.938 ms 2048/2 4096 + 205 ~ 89.604 ms

I couldn't figure out how to analyze your data, not sure what the extra delays mean nor how you conclude that Linux is worse than MacOS or Windows10 for small buffers?

At any rate, I looked into this some time back but had to put the work on the back burner due to other priorities. What I do remember is that there is a built-in latency due to the fact that on playback the driver submits a number of zero-filled URBs and will only add valid audio data when the first URB is retired, which means you get a constant startup latency you will never be able to catch up.

I also vaguely remember that at some point the buffer/period sizes don't matter, each period will be broken up in a series of URBs and hence you will have more wake-ups than what is configured by the period size. In short I would look into the way the data is spread on multiple URBs and check how latency is impacted by the software design.

the last thing I have in mind is that for latency analysis and comparisons, using simple devices make sense. Latency can be affected by extra processing that might be enabled in the USB device depending on user configurations or parameters. Ideally to focus on the ALSA/xHCI interaction/latency we'd want to look at really dumb devices with just an input and output terminal and no processing.

-Pierre

macOS High Sierra, PC Mode (RME Driver v3.08): 16/2 32 + 59 ~ 1.896 ms 32/2 64 + 59 ~ 2.563 ms 64/2 128 + 59 ~ 3.896 ms 128/2 256 + 59 ~ 6.563 ms 256/2 512 + 59 ~ 11.596 ms 512/2 1024 + 59 ~ 22.563 ms 1024/2 2048 + 59 ~ 43.896 ms 2048/2 4096 + 59 ~ 86.563 ms

Windows 10, PC Mode (RME Driver 1.099): 48/2 96 + 63 ~ 3.313 ms 64/2 128 + 63 ~ 3.979 ms 96/2 192 + 63 ~ 5.313 ms 128/2 256 + 63 ~ 6.646 ms 256/2 512 + 63 ~ 11.979 ms 512/2 1024 + 63 ~ 22.646 ms 1024/2 2048 + 63 ~ 43.979 ms 2048/2 4096 + 63 ~ 86.646 ms ===

Some things in particular I noticed on Linux:

• additional_delay varies a bit if I close and open the audio device again
• additional_delay seems to increase as the block_size increases. I

can make the additional_delay stay about the same rather than increasing by setting MAX_PACKS and MAX_PACKS_HS to 1 in sound/usb/card.h. In Linux versions before 3.13 there was a nrpacks parameter for snd-usb-audio to control this but it was removed with commit https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h...

• additional_delay is not constant as block_size is increased like on

macOS and Windows

I made a patch to snd-usb-audio to expose the snd-usb-audio constants as runtime adjustable module parameters (/sys/module/snd_usb_audio/parameters/) for testing (takes effect when the device is disconnected+reconnected and logs the parameter values to dmesg): https://aur.archlinux.org/cgit/aur.git/plain/parameters.patch?h=snd-usb-audi...

The patch is used in my Arch Linux AUR package for convenience (using DKMS to avoid having to recompile entire kernel): https://aur.archlinux.org/packages/snd-usb-audio-lowlatency-dkms/

Can snd-usb-audio be improved so the additional_delay is always the same when closing/opening/reconfiguring the audio device and does not increase as the block_size increases?

I noticed using USB audio on Linux at lower latencies (block_size <= 128) is more prone to audio dropouts under load compared to macOS and Windows, even with CPU power management disabled (writing 0 to /dev/cpu_dma_latency). What can be done about this?

Thanks.

Regards, Jonathan _______________________________________________ Alsa-devel mailing list Alsa-devel@alsa-project.org http://mailman.alsa-project.org/mailman/listinfo/alsa-devel