Re: Re: [Alsa-user] AD1985 full-duplex(?)

[Takashi Iwai <tiwai@suse.de>]

At Mon, 23 Aug 2004 13:36:02 +0200,
Adam Tlałka wrote:
> 
> > On Thu, 19 Aug 2004, Takashi Iwai wrote:
> >>Can "network sound device" work with the mmap, too?
> > I think yes, it can, but the real output will be more delayd. So, it won't 
> > be much useable for the strict real-time applications.
> But old quake and new quake3 and some OSS apps use mmap mode for better 
> timing and accurate mixing

I doubt it.

Mmap doesn't provide better nor accurate "timing" although it provides
more efficient data transfer method.  The accuracy depends on the
fineness of DMA interrupts and the scheduler latency.

I don't see so big requirement of mmap in this regard.  If you need a
free-running ring buffer, you can do it simply by setting the proper
stop_threshold for disabling XRUN detection.  The perfomance win by
mmap is far little in comparison with computations of mixing and
effects.  Hence, what I see here is only the demand of
"compatibility".  From the performance perspective, there is no reason
to force mmap.

Note that I don't mention here about apps with high bandwidth like
HDR but apps with normal 16bit/48k sounds with at most 6 voices in the
background.

Also, I don't mean that network system can provide as good response as
the normal system.  What I mean is that the discussion of latency
should be indepdent from necessity of mmap implementaion.


Takashi


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