D-oh. Sometimes its the simple things.
Thanks Yichao
Jared Crean
On 03/08/2016 11:34 AM, Yichao Yu wrote:
On Mar 8, 2016 11:32 AM, "Yichao Yu" <[email protected]
<mailto:[email protected]>> wrote:
>
> On Tue, Mar 8, 2016 at 11:17 AM, Jared Crean <[email protected]
<mailto:[email protected]>> wrote:
> > Hello,
> > I did a micro-brenchmark about using Ints or UInt8s as indices for
> > randomly reading an array, and discovered that when using Ints
(but not
> > UInt8s), there is an unexpected memory allocation. More
interestingly, the
> > size of the memory allocation does not depend on the size of the
array (n in
> > the code below:
> >
> > function sumjc(val_arr, idx_arr)
> > # random read sum
> > sum = 0.0
> > for i=1:length(idx_arr)
> > idx = idx_arr[i]
> > sum += val_arr[idx]
> > end
> >
> > return sum
> > end
> >
> > function sumjc2(val_arr)
> > # sequential read sum
> > sum = 0.0
> > for i=1:length(val_arr)
> > sum += val_arr[i]
> > end
> >
> > return sum
> > end
> >
> > n = 200
> > a = round(Int, n*rand(n)) + 1 # Int indices
> > b = UInt8[ i for i in a] # UInt8 indices
> > vals = rand(n + 1)
> >
> > # warm up
> > sumjc(vals, a)
> > sumjc(vals, b)
> > sumjc2(vals)
> >
> > # results
> > @time sumjc(vals, a)
> > println("Int random read @time printed above")
> > @time sumjc(vals, b)
> > println("UInt8 random read @time printed above")
> > @time sumjc2(vals)
> > println("standard sum @time printed above")
> >
> > The output I get is:
> > 4.083 microseconds (156 allocations: 10861 bytes) # ???
>
> Functions used by @time itself needs to be compiled and causes memory
> allocation.
And also other initialization needed when you call it the first time
in global scope
>
> > Int random read @time printed above
> > 2.716 microseconds (5 allocations: 160 bytes)
> > UInt8 random read @time printed above
> > 1.715 microseconds (5 allocations: 160 bytes)
> > standard sum @time printed above
> >
> >
> > Is this a bug that should be reported on Github, or is this expected
> > behavior? I am using the 0.4 release version of Julia.
> >
> > Jared Crean
