Unexpected multithreading behavior

[t0ralf]

I'm not sure if this is a bug or expected behavior. If the latter is the case I would like to hear an explanation.

Consider the following sample code:

using Oxygen
using HTTP

example_vector = rand(1:100,500000000)

function single_thread_calc(v)
  s = 0.0
  for i = 1:length(v)
      s += sin(v[i])
  end
  return s
end

@get "/calc" function(req::HTTP.Request) 
    @show Threads.threadid()
    sum=single_thread_calc(example_vector)
    return "hello, your sum is $sum"
end

serveparallel()

The execution of this endpoint will take about 7s on my PC (Windows 11, 8 Cores, 16 Threads, Julia 1.10.3, Oxygen 1.5.11, HTTP 1.10.8)

Now, if I run this code in a julia session with 4 threads assigned (i.e. julia --threads 4) and call the endpoint 4 times in quick succession (for example via swagger in 4 different browser tabs), I would expect a parallel execution of these 4 requests. What happens though is that the first request is executed and awaited and then the other 3 requests are run in parallel. This is repeatable behavior. Why does it always wait until the first request is finished before running a parallel execution? This would not be great behavior for a production scenario with a decent amount of traffic.

Thanks for the help!