Re: [Boost-users] R: R: [multi-array] Performance? Difference from GIL or uBLAS?

From: Andrea Denzler (andrea_at_[hidden])
Date: 2008-08-08 05:44:59

Yes, iterators if available make our code clean and elegant and without
overhead. We use iterators even on STL::vector. They are great.

The minimum overhead you have with indexes is to calculate the address 
   (char *)array + i*sizeof(row) + j*sizeof(item)
And to double the increment and loop check since you have two variables
   ++i, i<h   +    ++j, j<w

But if the underlying structure is not a plain array but a class you may
have also boundary checks on the indexes. All this together can cost several
CPU cycles. 
Also with a iterator/pointer you may use only a register (not a memory
location) improving even more the speed.

It depends on the cost of the inner code, if it is just a simple arithmetic
calculation (1 cpu cycle) then I expect something like 10 times slower.

All the GIL examples I saw use iterators and what I liked is that
complicated image calculations can be stacked. If I have to flip the an
image and convert it to B/W then traditionally I would do that in two steps,
with Gil you do that in one step only, with nearly double performance.

-----Messaggio originale-----
The standard algorithms use iterators; An array implementation that 
translates iterators to indexes and expects the indexes to be converted 
back into pointers is questionable to me. Especially when my trivial 
test code showed the multi-array implementation was 10-20 times slower.  
This slow down is way more than I expected.

The multi_array implementation is extremely scary to me because it does 
not seem to be low overhead at all. 

-- John
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