[pulseaudio-discuss] [PATCH v4] Make module loopback honor requested latency

Alexander E. Patrakov patrakov at gmail.com
Thu Feb 5 07:59:42 PST 2015

01.02.2015 03:43, Georg Chini wrote:
> This is the final version of my patch for module-loopback. It is on top of the
> patch I sent about an hour ago and contains a lot more changes than the previous
> versions:
> - Honor specified latency if possible, if not adjust to the lowest possible value
> - Smooth switching from fixed latency to dynamic latency source or sink and vice versa
> - good rate and latency stability, no rate oscillation
> - adjusts latency as good as your setup allows
> - fast regulation of latency offsets, adjusts 100 ms offset within 22 seconds (adjust
>    time=1) to 60 seconds (adjust_time=10)
> - usable latency range 4 - 30000 ms
> - Avoid rewinds and "cannot peek into queue" messages during startup and switching
> - works with rates between 200 and 190000 Hz
> - maximum latency offset after source/sink switch or at startup around is 200 ms
> I also introduced a new parameter, buffer_latency_msec which can be used together
> with latency_msec. If buffer_latency_msec is specified, the resulting latency
> will be latency_msec + buffer_latency_msec. Latency_msec then refers only to
> the source/sink latency while buffer_latency_msec specifies the buffer part.
> This can be used to save a lot of CPU at low latencies, running 10 ms latency
> with latency_msec=6 buffer_latency_msec=4 gives 8% CPU on my system compared to
> 12% when I only specify latency_msec=10.

Is there any more detailed profiling data for it? E.g., have you tried 
running perf record / perf report?

I am not quoting the patch, but my opinion is that it needs either 
comments or some text in the commit message on the following topics:

Why does it use hand-crafted heuristics instead of a PID-controller? Or, 
is this "Low pass filtered difference between expectation value and 
observed latency" a PI controller in disguise?

How was the lowpass filter tuned?

Alexander E. Patrakov

More information about the pulseaudio-discuss mailing list