map is horrible for small and fast functions like sin, because Julia
currently doesn't generate specialized code, so method dispatch for both
sin and the resulting setindex! call needs to be performed runtime (for
every element of the array). There are open issues about fixing this.
Ivar
kl. 18:52:02 UTC+1 søndag 30. november 2014 skrev Deniz Yuret følgende:
>
> Thanks John. Here is the results when wrapped in a function body. The
> for loop wins as expected, map! still looks horrible...
>
> julia> a=rand(10000,10000)
> julia> f1(a::Array{Float64})=map!(sin,a)
> julia> @time f1(a);
> elapsed time: 7.344967598 seconds (4799991904 bytes allocated, 21.07% gc
> time)
> julia> function g1(a::Array{Float64}) b=sin(a); end
> julia> @time g1(a);
> elapsed time: 0.886401569 seconds (800000128 bytes allocated)
> julia> function h1(a::Array{Float64}) for i=1:length(a); a[i]=sin(a[i]);
> end; end
> julia> @time h1(a);
> elapsed time: 0.70801756 seconds (80 bytes allocated)
>
> On Sunday, November 30, 2014 7:26:15 PM UTC+2, John Myles White wrote:
>>
>> Hi Deniz,
>>
>> If you time things in the global scope, you will come to incorrect
>> conclusions about performance. If you want to do performance comparisons,
>> you need to do them inside of a function body to get meaningful results.
>>
>> — John
>>
>> On Nov 30, 2014, at 9:24 AM, Deniz Yuret <deniz...@gmail.com> wrote:
>>
>> I am trying to figure out the most efficient way to perform in-place
>> array operations in Julia.
>>
>> julia> a=rand(10000,10000)
>>
>> julia> tic(); b=sin(a); toc();
>> elapsed time: 1.25738114 seconds
>>
>> julia> tic(); map!(sin,a); toc();
>> elapsed time: 7.821650464 seconds
>>
>> julia> tic(); for i=1:length(a); a[i]=sin(a[i]); end; toc();
>> elapsed time: 24.993171377 seconds
>>
>> Nothing seems faster than creating a new array, which I'd like to avoid
>> without slowing down the code if possible.
>>
>> Any ideas?
>>
>>
>>
