Global variable in Julia makes code an order of magnitude slower - julia

Question

I have the following code that runs in about 8 seconds:
using Random
DEBUG = false
PRISONERS = 100
ATTEMPTS = 50
function tryit()
a = [1:1:PRISONERS;]
a = shuffle(a)
for i in 1:PRISONERS
if DEBUG
println("prisoner $i")
end
count = 0
check = i
while count <= ATTEMPTS
count += 1
if a[check] == i
if DEBUG
println("Attempt $count: Checking box $check and found my number")
end
break
else
if DEBUG
println("Attempt $count: Checking box $check and found $(a[check])")
end
check = a[check]
end
end
if count > ATTEMPTS
if DEBUG
println("Prisoner $i failed to find his number in 50 attempts")
end
return false
end
end
return true
end
function main()
tries = 100000
success = 0.0
for i in 1:tries
if tryit()
success += 1
end
end
println("Ratio of success = $(success / tries)")
end
main()
Moving the global variables inside the function cuts the runtime to a tenth:
using Random
function tryit()
DEBUG = false
PRISONERS = 100
ATTEMPTS = 50
a = [1:1:PRISONERS;]
a = shuffle(a)
for i in 1:PRISONERS
if DEBUG
println("prisoner $i")
end
count = 0
check = i
while count <= ATTEMPTS
count += 1
if a[check] == i
if DEBUG
println("Attempt $count: Checking box $check and found my number")
end
break
else
if DEBUG
println("Attempt $count: Checking box $check and found $(a[check])")
end
check = a[check]
end
end
if count > ATTEMPTS
if DEBUG
println("Prisoner $i failed to find his number in 50 attempts")
end
return false
end
end
return true
end
function main()
tries = 100000
success = 0.0
for i in 1:tries
if tryit()
success += 1
end
end
println("Ratio of success = $(success / tries)")
end
main()
I could not reproduce the problem with a smaller code example. I am using Julia version 1.7.2 on Linux Mint.

Answer 1

Why globals? If you are concerned because you want to keep the variables' values global at function call for some reason, aliasing them in your function signature can fix the slowdown problem:
function tryit(PRISONERS = PRISONERS, ATTEMPTS = ATTEMPTS, DEBUG = DEBUG)
...

Answer 2

Declaring the global variables as constants solved the problem.
using Random
const DEBUG = false
const PRISONERS = 100
const ATTEMPTS = 50
function tryit()
a = [1:1:PRISONERS;]
a = shuffle(a)
for i in 1:PRISONERS
if DEBUG
println("prisoner $i")
end
count = 0
check = i
while count <= ATTEMPTS
count += 1
if a[check] == i
if DEBUG
println("Attempt $count: Checking box $check and found my number")
end
break
else
if DEBUG
println("Attempt $count: Checking box $check and found $(a[check])")
end
check = a[check]
end
end
if count > ATTEMPTS
if DEBUG
println("Prisoner $i failed to find his number in 50 attempts")
end
return false
end
end
return true
end
function main()
tries = 100000
success = 0.0
for i in 1:tries
if tryit()
success += 1
end
end
println("Ratio of success = $(success / tries)")
end
main()
See Why is there a performance difference between global and local variables in non-interactive Julia programs? for technical explanations.