This is not a new issue.
You are simply bumping into the problem that passing functions as arguments
incur a cost every time the function is called.
If you want to compare it with map! in base you should do the following:
function mape4a(f, A, F)
tmp = similar(A)
for i in eachindex(A)
tmp[i] = f(A[i], F[i])
end
100 * sumabs(tmp) / length(A)
end
@time mape4a(_f A,F)
0.348988 seconds (20.00 M allocations: 343.323 MB, 8.25% gc time)
There are plans on fixing this,
see https://github.com/JuliaLang/julia/pull/13412
On Friday, October 23, 2015 at 10:58:06 AM UTC+2, Ján Dolinský wrote:
>
> versioninfo()
> Julia Version 0.4.0
> Commit 0ff703b* (2015-10-08 06:20 UTC)
> Platform Info:
> System: Linux (x86_64-linux-gnu)
> CPU: Intel(R) Core(TM) i5-4300U CPU @ 1.90GHz
> WORD_SIZE: 64
> BLAS: libopenblas (NO_LAPACK NO_LAPACKE DYNAMIC_ARCH NO_AFFINITY Haswell
> )
> LAPACK: liblapack.so.3
> LIBM: libopenlibm
> LLVM: libLLVM-3.3
>
> Hi Milan,
>
> The above is the versioninfo() output. I am exploring this further, using
> map() instead of map!() give me 3 time 5 million allocations as opposed to
> map!() with 4 times 5 million allocations. The "for" cycle in either map or
> map!() should not allocate that much memory. See my devectorized example
> in the previous post.
>
> Shall I file an issue, please advise me on how to do it. In general, I
> think map() and broadcast() should have about the same performance in the
> example given in the beginning of this thread.
>
> Thanks,
> Jan
>
> Dňa piatok, 23. októbra 2015 10:44:01 UTC+2 Milan Bouchet-Valat
> napísal(-a):
>>
>> This sounds suspicious to me. If you can file an issue with a
>> reproducible example, you'll soon get feedback about what's going on
>> here.
>>
>> Please report the output of versioninfo() there too. I assume this is
>> on 0.4?
>>
>>
>> Regards
>>
>> Le vendredi 23 octobre 2015 à 00:42 -0700, Ján Dolinský a écrit :
>> > ## 2 argument
>> > function map!{F}(f::F, dest::AbstractArray, A::AbstractArray,
>> > B::AbstractArray)
>> > for i = 1:length(A)
>> > dest[i] = f(A[i], B[i])
>> > end
>> > return dest
>> > end
>> >
>> > The above is the map!() implementation in abstractarray.jl. Should it
>> > return "dest" if it is an in-place function ? Is there any
>> > fundamental difference between my mape4a() and map!() in
>> > abstractarray.jl ?
>> >
>> > Thanks,
>> > Jan
>> >
>> > Dňa piatok, 23. októbra 2015 9:30:36 UTC+2 Ján Dolinský napísal(-a):
>> > > Hi Glen,
>> > >
>> > > Thanks for the investigation. I am afraid the for loop in map!() is
>> > > not the source of the issue. Consider the folowing:
>> > >
>> > > _f(a,f) = (a - f) / a
>> > >
>> > > function mape4(A, F)
>> > > # A - actual target values
>> > > # F - forecasts (model estimations)
>> > >
>> > > tmp = similar(A)
>> > > map!(_f, tmp, A, F)
>> > > 100 * sumabs(tmp) / length(A)
>> > >
>> > > end
>> > >
>> > > function mape4a(A, F)
>> > >
>> > > tmp = similar(A)
>> > > for i in eachindex(A)
>> > > tmp[i] = _f(A[i], F[i])
>> > > end
>> > > 100 * sumabs(tmp) / length(A)
>> > > end
>> > >
>> > > @time mape4(A,F)
>> > > 0.452273 seconds (20.00 M allocations: 343.323 MB, 9.80% gc time)
>> > > 832.852597807525
>> > >
>> > > @time mape4a(A,F)
>> > > 0.040240 seconds (7 allocations: 38.147 MB, 1.93% gc time)
>> > > 832.852597807525
>> > >
>> > > The for loop in mape4a() does not do 4 * 5 milion allocations,
>> > > neither should do the loop in map!(). Is this possibly a bug ?
>> > >
>> > > Thanks,
>> > > Jan
>> > >
>> > > Dňa štvrtok, 22. októbra 2015 19:43:31 UTC+2 Glen O napísal(-a):
>> > > > I'm uncertain, but I think I may have figured out what's going
>> > > > on.
>> > > >
>> > > > The hint lies in the number of allocations - map! has 20 million
>> > > > allocations, while broadcast! has just 5. So I had a look at how
>> > > > the two functions are implemented.
>> > > >
>> > > > map! is implemented in perhaps the simplest way you can think of
>> > > > - for i=1:length(A) dest[i]=f(A[i],B[i]); end - which means that
>> > > > it has to store four values per iteration - i, A[i], B[i], and
>> > > > f(A[i],B[i]). Thus, 4 times 5 million allocations.
>> > > >
>> > > > broadcast! is using a cache to store values, instead, and I
>> > > > believe it's generating instructions using a macro instead of a
>> > > > regular loop, thus avoiding the assignments for i. As such, it
>> > > > doesn't need to store anything except for the initial caches, and
>> > > > after that it just overwrites the existing values. Unfortunately,
>> > > > that's as much as I can figure out from broadcast!, because it
>> > > > uses a lot of macros and a lot of relatively opaque structure.
>> > > >
>> > > > I'm also not entirely sure how it avoids the assignments
>> > > > necessary in the function call.
>> > > >
>> > > > On Friday, 23 October 2015 01:54:14 UTC+10, Ján Dolinský wrote:
>> > > > > Hi,
>> > > > >
>> > > > > I am exploring Julia's map() and broadcast() functions. I did a
>> > > > > simple implementation of MAPE (mean absolute percentage error)
>> > > > > using broadcast() and map(). Interestingly, the difference in
>> > > > > performance was huge.
>> > > > >
>> > > > > A = rand(5_000_000)
>> > > > > F = rand(5_000_000)
>> > > > >
>> > > > > _f(a,f) = (a - f) / a
>> > > > >
>> > > > > function mape3(A, F)
>> > > > > # A - actual target values
>> > > > > # F - forecasts (model estimations)
>> > > > >
>> > > > > tmp = similar(A)
>> > > > > broadcast!(_f, tmp, A, F)
>> > > > > 100 * sumabs(tmp) / length(A)
>> > > > >
>> > > > > end
>> > > > >
>> > > > > function mape4(A, F)
>> > > > > # A - actual target values
>> > > > > # F - forecasts (model estimations)
>> > > > >
>> > > > > tmp = similar(A)
>> > > > > map!(_f, tmp, A, F)
>> > > > > 100 * sumabs(tmp) / length(A)
>> > > > >
>> > > > > end
>> > > > >
>> > > > > @time mape3(A,F) # after JIT warm-up
>> > > > > 0.038686 seconds (8 allocations: 38.147 MB, 2.25% gc time)
>> > > > > 876.4813057521973
>> > > > >
>> > > > > @time mape4(A,F) # after JIT warm-up
>> > > > > 0.457771 seconds (20.00 M allocations: 343.323 MB, 11.29% gc
>> > > > > time)
>> > > > > 876.4813057521973
>> > > > >
>> > > > > I wonder why map() is so much slower ?
>> > > > >
>> > > > > Thanks,
>> > > > > Jan
>> > > > >
>>
>
