Zoom Performance

This means: avoid variables defined in the global environment.

Definitions

Scope of variables:   Environment within which a variables exist

Global scope:    Global environment of a module

Local scope:     Environment within a function, a loop, a struct, a macro…

Why avoid global variables?

The Julia compiler is not good at optimizing code using global variables.

Part of the reason is that their type can change.

Example

We will use the @time macro to time a loop …

… in the global environment:

total = 0
n = 1e6

@time for i in 1:n
    global total += i
end

  0.121428 seconds (4.00 M allocations: 76.286 MiB, 2.79% gc time)

… in a local environment (a function):

function local_loop(total, n)
    total = total
    @time for i in 1:n
        global total += i
    end
end

local_loop(0, 1e6)

  0.024386 seconds (2.00 M allocations: 30.518 MiB, 5.47% gc time)

For more accurate performance measurements, you should use the @btime macro from the BenchmarkTools package which excludes compilation time from the timing, averages metrics over multiple runs, and is highly customizable.

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