Re: [Alsa-devel] Re: [OT] ALSA userspace API complexity

[David Lang <dlang@digitalinsight.com>]

On Mon, 9 Jan 2006, John Rigg wrote:

> On Mon, Jan 09, 2006 at 01:58:00PM -0800, David Lang wrote:
> > On Mon, 9 Jan 2006, René Rebe wrote:
> > > > > Also, when the data is already available as single streams in a
> > > > > user-space
> > > > > multi track application, why should it be forced interleaved, when the
> > > > > hardware
> > > > > could handle the format just fine?
> > > > Because the conversion doesn't cost anything. Trying to avoid it by
> > > > making the API more complicated (I would even say confusing) is extreme
> > > > overkill.
> > >
> > > Since when doesn't cost convesion anything? I'm able to count a lot of
> > > wasted
> > > CPU cycles in there ...
> >
> > if the data needed to be accessed by the CPU anyway it's free becouse
> > otherwise the CPU would stall waiting for the next chunk of memory. you
> > can do quite a bit of work on data in cache while you are waiting for the
> > next cache line to load.
> >
> > in this same way, checksumming a network packet is free if the CPU needs
> > to copy the data anway, it only costs something if the data could bypass
> > the CPU.
>
> Yes, but the CPU has plenty of other work to do. The sound cards that
> would be worst affected by this are the big RME cards (non-interleaved) and
> multiple ice1712 cards (non-interleaved blocks of interleaved data),
> which AFAIK are the only cards capable of handling serious professional audio.
> This could represent 48 or more channels of 96kHz audio, which
> doesn't leave a lot of spare CPU capacity for running X, for example.

does the CPU touch the data for these, or do you DMA directly from 
userspace (i.e. "zero-copy")?

if the cpu touches this data on it's way in and out of the system then you 
are going to have a time period where you are maxing out the memory bus of 
your CPU (this may be a short time, but since the bus is either active or 
not there will be a time when it's active transfering audio data :-). 
while the memory bus is busy transfering the audio data your cpu can only 
work on data that's in the cache.

remember that as you keep reading the data from memory it will push other 
stuff out of your cache.

what magic do you pull to have the CPU busy on other things while the 
cache (and memory bus) is being occupied by the audio data transfers?

David Lang

--
There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies.
 -- C.A.R. Hoare

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