Issue: Response count mismatch in decode function under concurrent load

Setup: 4-node cluster, 500 concurrent HTTP requests dispatched via Task.async. All requests return within 2ms max latency.

Problem: Instrumented the decode function with entry/exit log markers. Entry log shows all 500 invocations as expected, but exit log shows only ~179 completions — roughly 64% of decode calls are not reaching the exit log point.

I am using Task.yield_many(spec.kill_cutoff_time_ms) with 4.5 seconds as timeout here. Each task will make http request and decode call Jason.decode

I have tried with Nimble ParserPool but I could reach only 232 at maximum on 64 Core system with pool size of
200

Here is the ParserPool.

defmodule GrncHub.ParserPool do
  @behaviour NimblePool
  require Logger
  
  @pool_size 500
  
  def child_spec(opts) do
    %{
      id: **MODULE**,
      start: {**MODULE**, :start_link, \[opts\]},
      type: :worker,
      restart: :permanent,
      shutdown: 500
    }
      end
  
  def start_link(\_) do
    NimblePool.start_link(worker: {**MODULE**, nil}, pool_size: @pool_size, name: **MODULE**)
  end
  
  @impl true
  def init_worker(pool_state), do: {:ok, nil, pool_state}
  
  @impl true
  def handle_checkout(:parse, \_from, worker_state, pool_state) do
    {:ok, worker_state, worker_state, pool_state}
  end
  
  @impl true
  def terminate_worker(\_reason, _worker_state, pool_state) do
    {:ok, pool_state}
  end
  
  @doc """
  Returns {:ok, decoded} | {:error, :invalid_json} | {:error, :worker_crashed} | {:error, :invalid_input}
  """
  def parse(payload, content_type, parser) when is_binary(payload) do
    parser = default_parser(content_type, parser)
    
    try do
      NimblePool.checkout!(
        __MODULE__,
        :parse,
        fn _from, worker_state ->
          {GrncHub.Parser.decode(payload, content_type, parser), worker_state}
        end,
        :infinity
      )
    catch
      :exit, reason ->
        Logger.error("ParsePool worker crashed: #{inspect(reason)}")
        {:error, %{"error" => "worker_crashed"}}
    end
  end
  
  def parse(\_invalid), do: {:error, %{"error" => "worker_crashed"}}
  
  defp default_parser(:json, nil), do: Jason
  defp default_parser(\_content_type, parser), do: parser
end