Re: [fluid-dev] problems with fluidsynth 1.1.6 on a raspberry pi

[David Henningsson]

On 11/19/2012 07:34 AM, Pedro Lopez-Cabanillas wrote:
> El Lunes, 19 de noviembre de 2012 06:42:38 David Henningsson escribió:
> > On 11/18/2012 07:56 PM, Pedro Lopez-Cabanillas wrote:
> > > El Domingo, 18 de noviembre de 2012 12:51:58 David Henningsson escribió:
> > > > I'll do some experiments with it when I have some more time.
> > >
> > > My bet is floating point exceptions.
> >
> > I tried to verify this theory on my Nexus 7 device. I've tried to
> > compile with -ffast-math, with according to documentation should
> > activate flush-to-zero mode, as well as activating it explicitly
>
> On my Nokia N9xx (also ARM based) the release build was made with these flags:
> QMAKE_CFLAGS_RELEASE += --mfpu=neon -ffast-math -funsafe-math-optimizations
>
> config.h:
> #define WITH_FLOAT 1

Given that floats are faster on ARM devices (and probably older x86 
devices as well), I think it makes sense to switch back to using single 
point precision by default.

> Automatic denormals to zero doesn't prevent FP operations being slower when
> such numbers are involved. The problem is the larger CPU usage if there are FP
> exceptions, which happens in all CPUs except modern Intel processors.

I'm trying to verify this theory, but I can't. I've tried putting the 
Cortex-A9 VFP into "RunFast" mode, which is supposed to treat subnormal 
and special cases without any additional latency. But there is no 
difference in throughput.

> By the way, this is the initialization code:
>
>      ::fluid_settings_setstr(m_settings, "audio.driver", "pulseaudio");
>      ::fluid_settings_setint(m_settings, "audio.period-size", 1024);
>      ::fluid_settings_setint(m_settings, "audio.periods", 2);
>      ::fluid_settings_setnum(m_settings, "synth.sample-rate", 12000.0);
>      ::fluid_settings_setint(m_settings, "synth.chorus.active", 0);
>      ::fluid_settings_setint(m_settings, "synth.reverb.active", 0);
>      ::fluid_settings_setnum(m_settings, "synth.gain", 0.4);
>      ::fluid_settings_setint(m_settings, "synth.polyphony", 16);
>          ...
>      ::fluid_synth_set_interp_method(m_synth, -1, FLUID_INTERP_LINEAR);
>
> With this setup, the synth is barely usable on the N9 for real time usage.
> Playing MIDI files on it is out of the question.

Interesting. I didn't think processors were *that* slow these days...

Maybe it's something else, like cache misses or so?

// David