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
> > > >
