Your friendly neighbourhood multithreaded Spiderman

New to Julia
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Hi I am your friendly neighborhood multithreaded spiderman

Today, you have gain a new superpower with your new suit called julia-1.3

Before you go swinging high above the building tops, I want to talk about thread safety.

Let’s jump straight into thread safety

UNSAFE THREAD USAGE

julia> using Base.Threads

julia> total=0
0

julia> Threads.@threads for k = 1:1_000_000
                global total += 1
         end

julia> total
170321

Opps, instead of one million, we only get 170321. You may think that is because the variable total is not thread safe. So let’s use an atomic powered variable

STILL UNSAFE THREAD USAGE

julia> using Base.Threads

julia> atomic_total = Threads.Atomic{Int}(0)
Atomic{Int64}(0)

julia> Threads.@threads for k = 1:1_000_000
                global atomic_total[] = atomic_total[] + 1
          end

julia> atomic_total[]
144247

Wow! Even atomic powered variable does not provide thread safety. You also need atomic functions

SAFE THREAD USAGE

julia> using Base.Threads

julia> atomic_total = Threads.Atomic{Int}(0)
Atomic{Int64}(0)

julia> Threads.@threads for k = 1:1_000_000
                       global atomic_total
                       Threads.atomic_add!(atomic_total,1)
          end

julia> atomic_total[]
1000000

Yeah now you feel like a million dollars.

Moral of the story: Everytime you swing above the rooftops and your threads depend on something that other threads also depends upon, you need to practise safe threading otherwise you will fall and hurt yourself.

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