[Jack-Devel] jack dsp load calculation

Hi all,

I'd like to propose changing the algorithm how jack calculates the DSP
load from averaging to worst-case.

The main reason for having DSP load is to know how much more processing
one can safely add without getting x-runs. What matters in this case is
the worst-case value.



It looks like there are some prior attempts in the code [1], but the
current averaging DSP load is pretty much useless for this. As-is, it
mainly lends itself to compare different machines.

Short spikes, which are in most cases the cause for xruns, are simply
ignored in the calculation: For them to show up, currently three
criteria must be met: The spike must be in the last 32 jack-cycles, be >
95% and the control application must read the value near the end of
those 32 cycles (see [1], very high LPF).

At 1024fpp / 48KHz, 32 cycles corresponds to ~680msec. Most control
applications only update the value every second or so.

Apart from that, I'm at a loss how to interpret the current value in a
meaningful way.


The proposed change is to:
 - jump to the worst-case value if it is larger than the current load
 - slow fall-off (1st order low-pass), 4-5 seconds (not cycle dependent)

I'm happy to submit a patch for this.

The only downside I can think of is that the reading will be higher and
some users will complain about this being a regression.

thoughts, comments?
robin


[1]
https://github.com/jackaudio/jack2/blob/master/common/JackEngineControl.cpp#L66

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