Well it is a little more complicated actually. Julia is only copying references, while Matlab seems to lazily copy the whole array <http://www.mathworks.com/help/matlab/matlab_prog/memory-allocation.html>. Doing stuff you did is bad for two reasons. Consider: *function test(n)* * a = rand(10^5)* * for i=1:n* * a = a[:] # or e.g. rand(10^5) or a+1, anything that creates a new array with the same type* * end* * a* *end*
*a[:]* is a getindex operation which currently copies. This means you're overwriting the reference from* a *with a new reference. Now it's the job of the garbage collector to figure things out. This puts a lot of pressure on it, since you're constantly allocating new memory and freeing up old one (you're overwriting the one and only reference to old *a*). >From what I know, this could be handled better with some cleverness, but isn't right now. It's not entirely clear where the best place is to optimize this. One way would be to use in place operators, which is not as straight forward as one might expect: https://github.com/JuliaLang/julia/issues/249 For immutables this is handled pretty well by LLVM, though. Because it's Julia, it's pretty straightforward to find out: https://gist.github.com/SimonDanisch/c614016d9f9551f8f64c#file-jl When in doubt, @allocated, @time and @code_llvm and @code_native are your friends to find out what's going on under the hood. Am Mittwoch, 7. Oktober 2015 22:25:00 UTC+2 schrieb Patrick Kofod Mogensen: > > So I asked a question over at > https://github.com/lindahua/Devectorize.jl/issues/48#issuecomment-146307811 > and it seems that I have got something completely wrong. > > It seems that the following > > index = rand(8000) > phat = zeros(8000) > > phat = 1./(1+exp(-index)) > > binds the output of the calculation on the rhs to a _new_ vector phat, not > the original vector. This really means I have misunderstood > preallocation/re-using memory in Julia completely! @blakejohnson suggests > that I use > > phat[:] = 1./1+(exp(-index)) > > instead. As this puts the calculations in the already preallocated vector > phat. > > Question 1) Can I learn more about this in the documentation ? I'm having > a hard time finding anything, probably because [:] is hard to use as a > search phrase, and I'm not sure what concepts to search for. > > > > I often wrap my variables in a type, but this seems like an extremely bad > idea in combination with devec. I have taken Blake Johnsons examples from > github, and added a few > https://gist.github.com/pkofod/fb6c4b8ffcca1a056363 > > Question 2) What is it that makes test4() and test6() run so slowly? > test1() and test3() seems to perform equivalently fast. I use this kind of > setup all the time, and I am surprised that this method allocates so much > memory. > > Bad news: I've been doing Julia ALL wrong! > Good news: Time to learn some more! >
