[LAU] anyone checked out new USB reduced latency patch?

Robin Gareus robin at gareus.org
Mon Jun 7 18:15:05 CEST 2021

On 6/7/21 5:29 PM, Fons Adriaensen wrote:
> On Mon, Jun 07, 2021 at 05:11:57PM +0200, Robin Gareus wrote:
>> Does this help to reduce the random systemic latency of USB/ALSA devices?
>> Currently with USB device there is up to the buffer size (but at most
>> 20ms) additional I/O latency. The delay is different every time the
>> device is started, or after an xrun occurs. It would be great if this
>> doesn't need to be measured every time.
> Is that roundtrip delay, or input/output separately ? 

In case of Ardour, the roundtrip is measured.

For USB, there is an additional queue inside the ALSA driver. If I
understood correctly its average fill level depends on when USB endpoint
is started relative to the ALSA device start. It relaates to playback
only (not capture).

You may remember the discussion from 11 years ago where Clemens
explained this:


It seems the kernel patch mitigates this issue. Starting the playback
endpoint in sync with capture might reduce the randomness of the
driver's systemic latency.


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