On Fri, May 27, 2016 at 9:06 PM, <[email protected]> wrote: > Regarding Julia 0.4.5, I've discovered that arrays with many subscripts > (apparently 8 is enough to cause trouble) unexpectedly cause heap > allocation. See the code and timings below. This is in regard to a finite > element code in which many-way arrays are used to represent multi-index > tensors. Can anyone explain this?
I believe they are called high-dimensional arrays. This is probably caused by the splatting penalty, which appears with you are calling a vararg function (getindex in this case) with too many arguments and the compiler decided to not inline it. There is on going effort to improve this and there might already be some improvement on 0.5. > > Thanks, > Steve Vavasis > > > julia> @time test_manyway.test2way(100000) > 0.001836 seconds (6 allocations: 272 bytes) > 49837.4971725032 > > julia> @time test_manyway.test4way(100000) > 0.008931 seconds (7 allocations: 432 bytes) > 50050.05619989492 > > > julia> @time test_manyway.test8way(100000) > 49.707042 seconds (385.40 M allocations: 11.103 GB, 4.16% gc time) > 50062.84252292006 > > > > module test_manyway > > function test2way(n) > a2 = zeros(2,2) > x = 0.0 > for tr = 1 : n > for j1 = 1 : 2 > for j2 = 1 : 2 > a2[j1,j2] = rand() > end > end > x += a2[1,1] > end > x > end > > function test4way(n) > a4 = zeros(2,2,2,2) > x = 0.0 > for tr = 1 : n > for j1 = 1 : 2 > for j2 = 1 : 2 > for j3 = 1 : 2 > for j4 = 1 : 2 > a4[j1,j2,j3,j4] = rand() > end > end > end > end > x += a4[1,1,1,1] > end > x > end > > function test8way(n) > a8 = zeros(2,2,2,2,2,2,2,2) > x = 0.0 > for tr = 1 : n > for j1 = 1 : 2 > for j2 = 1 : 2 > for j3 = 1 : 2 > for j4 = 1 : 2 > for j5 = 1 : 2 > for j6 = 1 : 2 > for j7 = 1 : 2 > for j8 = 1 : 2 > a8[j1,j2,j3,j4,j5,j6,j7,j8] = rand() > end > end > end > end > end > end > end > end > x += a8[1,1,1,1,1,1,1,1] > end > x > end > > end > >
