[LAD] "enhanced event port" LV2 extension proposal

Dave Robillard drobilla at connect.carleton.ca
Thu Nov 29 00:02:50 UTC 2007

On Wed, 2007-11-28 at 23:12 +0100, David Olofson wrote:
> On Wednesday 28 November 2007, Dave Robillard wrote:
> [...]
> > > Obviously, this effect is less visible with non-trivial plugins -
> > > but even how many plugins in a "normal" graph are actually CPU
> > > bound? In my experience, the effect is very pronounced (several
> > > times higher CPU load with 512 samples/buffer than with 32) when
> > > using a bunch of sampleplayer voices (cubic interpolation +
> > > oversampling) and some simple effects. You have to do some pretty
> > > serious processing to go CPU bound on a modern PC...
> > 
> > Not really true for linear scans through memory, which reading
> > plugin buffers usually is.  This is the ideal cache situation, and
> > you can definitely get full utilization sweeping through things much
> > larger than the cache this way (this is exactly what the cache is
> > designed to do, and it does it well; try it).
> I did, and that's what I'm drawing my conclusions from. ;-)
> No cache controller logic in the world can avoid the bottleneck that 
> is memory bandwidth. If your data has been kicked from the cache, it 
> needs to get back in, and the only time you're not taking a hit from 
> that is when you're doing sequential access *and* your code is CPU 
> bound, rather than memory bound.

Well, sure, but big data is big data.  In the typical case plugin
buffers are much smaller than the cache (especially on recent CPUs) -
crunching away on plain old audio here is definitely CPU bound (with
properly written RT safe host+plugins anyway).

No matter how you slice it if you want to play with a massive chunk of
data you're bound by the memory bandwidth*, sure.  So what? :)


(* until everything is parallel like it really should be, and this
problem goes away...)

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