Hi,
On Tue, 23 Oct 2018 at 02:40, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, 22 Oct 2018, Pierre-Louis Bossart wrote:
>
> > 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
>
> I'm with Pierre-Louis on this; I can't make heads or tails out of these
> formulas.
>
> To begin with, I'm accustomed to talking about frames, periods, and
> buffers. What are "block"s? Are they the same as buffers?
>
> What do these formulas mean? Is the first supposed to be a definition
> of round_trip_latency? If it isn't, then how do you define or measure
> round_trip_latency?
>
> What is additional_delay? How is it measured or calculated?
See below.
>
> > > Linux 4.17.14, Class Compliant Mode (snd-usb-audio, ALSA backend):
> > > 16/2 32 + 80 ~ 2.333 ms
>
> What are these numbers? Are these lines supposed to in the format
> expressed by the first formula above? If they are, how come
> "block_size/periods" shows up as a pair of numbers "16/2" but
> "block_size*periods" shows up as a single number "32"?
>
To interpret "16/2 32 + 80 ~ 2.333 ms"
Block size: 16 samples
Periods: 2 (one period for playback + one period for recording when
determining round trip latency)
The minimum round trip latency is: 16 * 2 = 32 samples
However, I measured 112 samples round trip latency which is an
additional delay of 80 samples (32 + 80 = 112).
112 samples at 48000 Hz is 112 / 48000 * 1000 is approximately 2.333
ms measured round trip latency.
> > > 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
>
> What are the USB parameters for these tests? How many bytes/frame?
> What is the endpoint's maxpacket size? What is the speed of the USB
> bus?
>
How would I determine the USB parameters and bytes/frame?
USB port is Intel USB 3.0 port. Device is USB 2.0 high speed (480 Mbps).
Here is the lsusb output:
Bus 001 Device 004: ID 2a39:3fb0
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 2.00
bDeviceClass 239 Miscellaneous Device
bDeviceSubClass 2
bDeviceProtocol 1 Interface Association
bMaxPacketSize0 64
idVendor 0x2a39
idProduct 0x3fb0
bcdDevice 0.01
iManufacturer 1 RME
iProduct 2 Babyface Pro (71964099)
iSerial 3 EF72ADBCCECA4C8
bNumConfigurations 1
Configuration Descriptor:
bLength 9
bDescriptorType 2
wTotalLength 0x01a7
bNumInterfaces 4
bConfigurationValue 1
iConfiguration 0
bmAttributes 0x80
(Bus Powered)
MaxPower 100mA
Interface Association:
bLength 8
bDescriptorType 11
bFirstInterface 0
bInterfaceCount 4
bFunctionClass 1 Audio
bFunctionSubClass 0
bFunctionProtocol 32
iFunction 0
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 0
bAlternateSetting 0
bNumEndpoints 0
bInterfaceClass 1 Audio
bInterfaceSubClass 1 Control Device
bInterfaceProtocol 32
iInterface 0
AudioControl Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 1 (HEADER)
bcdADC 2.00
bCategory 8
wTotalLength 0x0055
bmControls 0x00
AudioControl Interface Descriptor:
bLength 8
bDescriptorType 36
bDescriptorSubtype 10 (CLOCK_SOURCE)
bClockID 1
bmAttributes 3 Internal programmable clock
bmControls 0x03
Clock Frequency Control (read/write)
bAssocTerminal 0
iClockSource 0
AudioControl Interface Descriptor:
bLength 17
bDescriptorType 36
bDescriptorSubtype 2 (INPUT_TERMINAL)
bTerminalID 3
wTerminalType 0x0101 USB Streaming
bAssocTerminal 0
bCSourceID 1
bNrChannels 12
bmChannelConfig 0x00000000
iChannelNames 0
bmControls 0x0000
iTerminal 0
AudioControl Interface Descriptor:
bLength 17
bDescriptorType 36
bDescriptorSubtype 2 (INPUT_TERMINAL)
bTerminalID 5
wTerminalType 0x0201 Microphone
bAssocTerminal 0
bCSourceID 1
bNrChannels 12
bmChannelConfig 0x00000000
iChannelNames 0
bmControls 0x0000
iTerminal 0
AudioControl Interface Descriptor:
bLength 12
bDescriptorType 36
bDescriptorSubtype 3 (OUTPUT_TERMINAL)
bTerminalID 4
wTerminalType 0x0301 Speaker
bAssocTerminal 0
bSourceID 2
bCSourceID 1
bmControls 0x0000
iTerminal 0
AudioControl Interface Descriptor:
bLength 12
bDescriptorType 36
bDescriptorSubtype 3 (OUTPUT_TERMINAL)
bTerminalID 6
wTerminalType 0x0101 USB Streaming
bAssocTerminal 0
bSourceID 5
bCSourceID 1
bmControls 0x0000
iTerminal 0
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 0
bNumEndpoints 0
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 32
iInterface 0
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 1
bNumEndpoints 2
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 32
iInterface 0
AudioStreaming Interface Descriptor:
bLength 16
bDescriptorType 36
bDescriptorSubtype 1 (AS_GENERAL)
bTerminalLink 3
bmControls 0x00
bFormatType 1
bmFormats 0x00000001
PCM
bNrChannels 2
bmChannelConfig 0x00000003
Front Left (FL)
Front Right (FR)
iChannelNames 0
AudioStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (FORMAT_TYPE)
bFormatType 1 (FORMAT_TYPE_I)
bSubslotSize 3
bBitResolution 24
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x03 EP 3 OUT
bmAttributes 5
Transfer Type Isochronous
Synch Type Asynchronous
Usage Type Data
wMaxPacketSize 0x0096 1x 150 bytes
bInterval 1
AudioStreaming Endpoint Descriptor:
bLength 8
bDescriptorType 37
bDescriptorSubtype 1 (EP_GENERAL)
bmAttributes 0x00
bmControls 0x00
bLockDelayUnits 0 Undefined
wLockDelay 0x0000
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x83 EP 3 IN
bmAttributes 17
Transfer Type Isochronous
Synch Type None
Usage Type Feedback
wMaxPacketSize 0x0004 1x 4 bytes
bInterval 4
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 1
bAlternateSetting 2
bNumEndpoints 2
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 32
iInterface 0
AudioStreaming Interface Descriptor:
bLength 16
bDescriptorType 36
bDescriptorSubtype 1 (AS_GENERAL)
bTerminalLink 3
bmControls 0x00
bFormatType 1
bmFormats 0x00000001
PCM
bNrChannels 12
bmChannelConfig 0x00000000
iChannelNames 0
AudioStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (FORMAT_TYPE)
bFormatType 1 (FORMAT_TYPE_I)
bSubslotSize 3
bBitResolution 24
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x03 EP 3 OUT
bmAttributes 5
Transfer Type Isochronous
Synch Type Asynchronous
Usage Type Data
wMaxPacketSize 0x0384 1x 900 bytes
bInterval 1
AudioStreaming Endpoint Descriptor:
bLength 8
bDescriptorType 37
bDescriptorSubtype 1 (EP_GENERAL)
bmAttributes 0x00
bmControls 0x00
bLockDelayUnits 0 Undefined
wLockDelay 0x0000
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x83 EP 3 IN
bmAttributes 17
Transfer Type Isochronous
Synch Type None
Usage Type Feedback
wMaxPacketSize 0x0004 1x 4 bytes
bInterval 4
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 2
bAlternateSetting 0
bNumEndpoints 0
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 32
iInterface 0
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 2
bAlternateSetting 1
bNumEndpoints 1
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 32
iInterface 0
AudioStreaming Interface Descriptor:
bLength 16
bDescriptorType 36
bDescriptorSubtype 1 (AS_GENERAL)
bTerminalLink 6
bmControls 0x00
bFormatType 1
bmFormats 0x00000001
PCM
bNrChannels 12
bmChannelConfig 0x00000000
iChannelNames 0
AudioStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (FORMAT_TYPE)
bFormatType 1 (FORMAT_TYPE_I)
bSubslotSize 3
bBitResolution 24
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x84 EP 4 IN
bmAttributes 5
Transfer Type Isochronous
Synch Type Asynchronous
Usage Type Data
wMaxPacketSize 0x0384 1x 900 bytes
bInterval 1
AudioStreaming Endpoint Descriptor:
bLength 8
bDescriptorType 37
bDescriptorSubtype 1 (EP_GENERAL)
bmAttributes 0x00
bmControls 0x00
bLockDelayUnits 0 Undefined
wLockDelay 0x0000
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 2
bAlternateSetting 2
bNumEndpoints 1
bInterfaceClass 1 Audio
bInterfaceSubClass 2 Streaming
bInterfaceProtocol 32
iInterface 0
AudioStreaming Interface Descriptor:
bLength 16
bDescriptorType 36
bDescriptorSubtype 1 (AS_GENERAL)
bTerminalLink 6
bmControls 0x00
bFormatType 1
bmFormats 0x00000001
PCM
bNrChannels 2
bmChannelConfig 0x00000003
Front Left (FL)
Front Right (FR)
iChannelNames 0
AudioStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (FORMAT_TYPE)
bFormatType 1 (FORMAT_TYPE_I)
bSubslotSize 3
bBitResolution 24
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x84 EP 4 IN
bmAttributes 5
Transfer Type Isochronous
Synch Type Asynchronous
Usage Type Data
wMaxPacketSize 0x0096 1x 150 bytes
bInterval 1
AudioStreaming Endpoint Descriptor:
bLength 8
bDescriptorType 37
bDescriptorSubtype 1 (EP_GENERAL)
bmAttributes 0x00
bmControls 0x00
bLockDelayUnits 0 Undefined
wLockDelay 0x0000
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 3
bAlternateSetting 0
bNumEndpoints 2
bInterfaceClass 1 Audio
bInterfaceSubClass 3 MIDI Streaming
bInterfaceProtocol 0
iInterface 2 Babyface Pro (71964099)
MIDIStreaming Interface Descriptor:
bLength 7
bDescriptorType 36
bDescriptorSubtype 1 (HEADER)
bcdADC 1.00
wTotalLength 0x0061
MIDIStreaming Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 3 (MIDI_OUT_JACK)
bJackType 1 Embedded
bJackID 3
bNrInputPins 1
baSourceID( 0) 2
BaSourcePin( 0) 1
iJack 4 Port 1
MIDIStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (MIDI_IN_JACK)
bJackType 2 External
bJackID 2
iJack 4 Port 1
MIDIStreaming Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 3 (MIDI_OUT_JACK)
bJackType 1 Embedded
bJackID 7
bNrInputPins 1
baSourceID( 0) 6
BaSourcePin( 0) 1
iJack 5 Port 2
MIDIStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (MIDI_IN_JACK)
bJackType 2 External
bJackID 6
iJack 5 Port 2
MIDIStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (MIDI_IN_JACK)
bJackType 1 Embedded
bJackID 1
iJack 4 Port 1
MIDIStreaming Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 3 (MIDI_OUT_JACK)
bJackType 2 External
bJackID 4
bNrInputPins 1
baSourceID( 0) 1
BaSourcePin( 0) 1
iJack 4 Port 1
MIDIStreaming Interface Descriptor:
bLength 6
bDescriptorType 36
bDescriptorSubtype 2 (MIDI_IN_JACK)
bJackType 1 Embedded
bJackID 5
iJack 5 Port 2
MIDIStreaming Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 3 (MIDI_OUT_JACK)
bJackType 2 External
bJackID 8
bNrInputPins 1
baSourceID( 0) 5
BaSourcePin( 0) 1
iJack 5 Port 2
Endpoint Descriptor:
bLength 9
bDescriptorType 5
bEndpointAddress 0x07 EP 7 OUT
bmAttributes 2
Transfer Type Bulk
Synch Type None
Usage Type Data
wMaxPacketSize 0x0200 1x 512 bytes
bInterval 0
bRefresh 0
bSynchAddress 0
MIDIStreaming Endpoint Descriptor:
bLength 6
bDescriptorType 37
bDescriptorSubtype 1 (GENERAL)
bNumEmbMIDIJack 2
baAssocJackID( 0) 1
baAssocJackID( 1) 5
Endpoint Descriptor:
bLength 9
bDescriptorType 5
bEndpointAddress 0x86 EP 6 IN
bmAttributes 2
Transfer Type Bulk
Synch Type None
Usage Type Data
wMaxPacketSize 0x0200 1x 512 bytes
bInterval 0
bRefresh 0
bSynchAddress 0
MIDIStreaming Endpoint Descriptor:
bLength 6
bDescriptorType 37
bDescriptorSubtype 1 (GENERAL)
bNumEmbMIDIJack 2
baAssocJackID( 0) 3
baAssocJackID( 1) 7
Device Qualifier (for other device speed):
bLength 10
bDescriptorType 6
bcdUSB 2.00
bDeviceClass 239 Miscellaneous Device
bDeviceSubClass 2
bDeviceProtocol 1 Interface Association
bMaxPacketSize0 64
bNumConfigurations 0
Device Status: 0x0e00
(Bus Powered)
> > 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.
>
> In theory the number of zero-filled URBs could be reduced, maybe even
> eliminated.
>
> > 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.
>
> Agreed.
>
> > 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=v3.13&id=976b6c064a957445eb0573b270f2d0282630e9b9
> > > - additional_delay is not constant as block_size is increased like on
> > > macOS and Windows
>
> Perhaps this additional_delay is caused by the zero-filled URBs
> mentioned earlier.
>
> We can't say anything about the effect of setting MAX_PACKS to 1
> without knowing how the driver is currently fitting packets into frames
> and URBs. In any case, you should be able to reduce the number of
> packets in each URB simply by reducing the period size, since the
> driver strives to keep each URB not much larger than a period (as I
> recall -- it's been a long time since I worked on this (2013)).
>
> > > 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-audio-lowlatency-dkms
> > >
> > > 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?
>
> You can reduce the CPU load. :-)
>
> Seriously, how can you compare loads between different operating
> systems?
>
> Also, note the Linux's scheduler has a number of adjustable parameters,
> which I am not familiar with.
>
> Alan Stern
>
Regards,
Jonathan
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