Subject: Re: [boost] interest in structure of arrays container?
From: Andreas Schäfer (gentryx_at_[hidden])
Date: 2016-10-16 06:59:26
Hi,
On 09:59 Sun 16 Oct , degski wrote:
> In your example, which I think is flawed as an example, you create in
> memory a 20+ GB data structure (hope I got the maths right).
300M * 4 * 4B = 2.4GB. IMHO not too large. Even 20GB isn't that much
in HPC.
> This data structure optimizes one operation (calculating the average) at
> the cost of pessimising almost any other operation (relocation if the
> vector needs to grow, insert, delete, push_back etc are all done 4 times in
> your example)
Keep in mind that this is a sketch of what he would like to use, not a
polished solution. The point is: with a Struct-of-Arrays (SoA) memory
layout you can implement kernels operating on that data that can be
vectorized. Depending on your CPU, vectorization can yield you a
speedup of 2x - 8x. If implemented properly then insert, delete,
push_back etc. can also be efficient for SoA.
> while iterating over the records will land you right into
> cache-miss-heaven.
Depends. If your access pattern is very random and your algorithm
needs to access every member of an object, then a std::vector might be
faster. But if your algorithm has some (piecewise) continuity, then
the SoA structure will benefit just as well from the CPU's caches.
Cheers
-Andreas
--
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Andreas Schäfer
HPC and Supercomputing
Institute for Multiscale Simulation
Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
+49 9131 85-20866
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