I am running an empty double loop in Julia
Ngal = 16000000
function get_vinz()
for i in 1:5
print(i, " ")
for j in i:Ngal
end
end
end
and the outcome of @time vinz() gives me
1 2 3 4 5 5.332660 seconds (248.94 M allocations: 4.946 GiB, 7.12% gc time)
What is the 5GB of memory allocated for?
the culprit is the use of global variables. your function calls the global variable, and with each call, a Int64 is allocated (64 bits). 64*16000000*5/1024/1024 = 4882.8125 MiB, that seems like the culprit your function doesn't know the size of the inner loop, and does a lookup on the global scope to check Ngal. It does that every single loop. compare that with this implementation:
function get_vinz(Ngal)
for i in 1:5
print(i, " ")
for j in i:Ngal
end
end
end
julia> @time get_vinz(Ngal)
1 2 3 4 5 0.043481 seconds (53.67 k allocations: 2.776 MiB)
also, the first time a function is called in julia, is compiled to machine code, so the subsecuent runs are fast. measuring time again:
julia> @time get_vinz(Ngal)
1 2 3 4 5 0.000639 seconds (50 allocations: 1.578 KiB)
The use of global variables is a bad practice in general. the recommended way is to pass those values to the function
