[PD] pd, openCV, pointers and indirection.

[Mathieu Bouchard]

On Mon, 5 Oct 2009, Loic Kessous wrote:

> thanks Mathieu, it is still not clear for me what make things faster in 
> one case or another but it helps.

1. data spacing: the more your data is spaced in memory, the more the 
cache has to load lots of data, because it assumes that the data is not 
very fragmented. Pd in 64-bit mode spends half of the argv space on 
padding. Here by spacing I mean the difference of starting position of two 
elements next to each other (e.g. where are $1 and $2 in RAM).

2. data element size: when the data doesn't have padding, this is the same 
as data spacing. Pd in in any mode spends half of the nonpadding argv 
space on type information.

3. type checking: if you have to check that every element of an argv is 
indeed a float, you need to use twice more data, and it's twice more 
spacing, but on top of that you need one conditional per element, just in 
case it isn't a float, and conditionals are getting comparatively slow on 
modern CPUs because they're harder to accelerate than the rest.

4. time fragmentation: a low block size may mean the CPU has to reload 
things in the cache more often, if the CPU's other tasks need the same 
cache for other purposes between the processing of two blocks. Bigger 
blocks mean that the CPU can "concentrate". Having to repeatedly call, 
init, deinit, return, is also something that can take time.

5. cache fitting: repeatedly making long sweeps on very long arrays can 
make the cache completely useless. it's better to do as many things as 
possible on a small area of RAM at a time.

Based on those five criteria, we could compare various storage and 
computation strategies of various internals and externals of pd, provided 
that we get a bit more precise on some things. There may also be 
additional criteria.

> loic PS: what do you call Martin's strings ?

I thought I knew, but I borked that. Martin's strings are [mrpeach/str], 
but they don't use pd lists of floats, they use a custom atom type called 
BLOB, which is essentially a form of double-indirection. (POINTER is also 
a double-indirection, but it was meant for DS, though it's often hijacked 
to be used in other ways.)

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| Mathieu Bouchard, Montréal, Québec. téléphone: +1.514.383.3801