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Subject: Re: [boost] interest in structure of arrays container?
From: Oswin Krause (Oswin.Krause_at_[hidden])
Date: 2016-10-16 06:34:20

On 2016-10-16 08:59, degski wrote:
> On 16 October 2016 at 08:36, Michael Marcin <mike.marcin_at_[hidden]>
> wrote:
> You state that the example is a toy example. To me the example shows
> that
> iterating over a vector of smaller objects (pods) is faster than
> iterating
> over a vector of larger objects, duh. The real use case might be more
> interesting, maybe you can describe it.


I gave a real use-case in a slightly more complicated setting. We had to
face the problem of "how can we align data in a way that we can unify a
set of slow matrix-vector multiplications into one fast matrix-matrix
multiplication". We usually have to traverse the same dataset several
100-1000 times in order to do our computations, so some overhead in the
data setup phase (e.g. cost of insertion) is okay and we would sacrifice
even more performance in that phase if we got in return more performance
in the following three days of computations. Having said that, I agree
that just reordering does not give much in many cases. But the real gain
is given when data can be aligned better or used more independently.
e.g. if the struct contains a std::vector or std::string we can gain a
lot by using a data structure which stores the contents of the vectors
consecutively, thus removing one level of indirection.

Note that this pattern is also common in game industry, where objects
are made up of data structures which are used in (nearly) independent
parts of the game and where data has to be copied to and from the gpu.


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