snakepit

defmodule Mix.Tasks.Diagnose.Scaling do
  @moduledoc """
  Comprehensive scaling diagnostics to find the bottleneck preventing >105 workers.

  Usage:
    mix diagnose.scaling
  """

  use Mix.Task
  require Logger

  @shortdoc "Find scaling bottleneck preventing >105 workers"

  @impl Mix.Task
  def run(_args) do
    Mix.Task.run("app.start")

    IO.puts("\n" <> String.duplicate("=", 80))
    IO.puts("🔬 SNAKEPIT SCALING DIAGNOSTICS")
    IO.puts(String.duplicate("=", 80) <> "\n")

    # Run all diagnostics
    check_erlang_limits()
    check_erlang_process_limits()
    check_dets_contention()
    check_python_spawn_rate()
    check_grpc_limits()
    find_exact_breaking_point()

    IO.puts("\n" <> String.duplicate("=", 80))
    IO.puts("✅ DIAGNOSTICS COMPLETE")
    IO.puts(String.duplicate("=", 80) <> "\n")
  end

  defp check_erlang_limits do
    IO.puts("📊 TEST 1: ERLANG PORT LIMITS")
    IO.puts(String.duplicate("-", 80))

    port_limit = :erlang.system_info(:port_limit)
    port_count = :erlang.system_info(:port_count)
    port_available = port_limit - port_count

    IO.puts("Port Limit:     #{format_number(port_limit)}")
    IO.puts("Current Ports:  #{format_number(port_count)}")
    IO.puts("Available:      #{format_number(port_available)}")
    IO.puts("Usage:          #{Float.round(port_count / port_limit * 100, 2)}%")

    # Estimate ports per worker
    if port_count > 0 do
      # Try to count actual workers
      worker_count = count_workers()

      if worker_count > 0 do
        ports_per_worker = port_count / worker_count
        IO.puts("\nEstimated ports per worker: #{Float.round(ports_per_worker, 2)}")
        max_workers = floor(port_limit / ports_per_worker)
        IO.puts("Theoretical max workers: #{max_workers}")

        if max_workers < 110 do
          IO.puts("\n⚠️  WARNING: Port limit would prevent 110 workers!")

          IO.puts(
            "    At #{Float.round(ports_per_worker, 2)} ports/worker, you can only spawn ~#{max_workers} workers"
          )
        end
      end
    end

    # Check if we're close to the limit
    if port_available < 1000 do
      IO.puts("\n🔴 CRITICAL: Less than 1000 ports available!")
    else
      if port_count / port_limit > 0.8 do
        IO.puts("\n⚠️  WARNING: Using >80% of port limit")
      else
        IO.puts("\n✅ Port usage looks healthy")
      end
    end

    IO.puts("\n")
  end

  defp check_erlang_process_limits do
    IO.puts("📊 TEST 2: ERLANG PROCESS LIMITS")
    IO.puts(String.duplicate("-", 80))

    process_limit = :erlang.system_info(:process_limit)
    process_count = :erlang.system_info(:process_count)
    process_available = process_limit - process_count

    IO.puts("Process Limit:  #{format_number(process_limit)}")
    IO.puts("Current Procs:  #{format_number(process_count)}")
    IO.puts("Available:      #{format_number(process_available)}")
    IO.puts("Usage:          #{Float.round(process_count / process_limit * 100, 2)}%")

    # Count snakepit-related processes
    snakepit_procs =
      Process.list()
      |> Enum.count(fn pid ->
        case Process.info(pid, :registered_name) do
          {:registered_name, name} ->
            name_str = to_string(name)
            String.contains?(name_str, "snakepit") || String.contains?(name_str, "Snakepit")

          _ ->
            # Check initial call
            case Process.info(pid, :initial_call) do
              {:initial_call, {mod, _fun, _arity}} ->
                mod_str = to_string(mod)
                String.contains?(mod_str, "Snakepit")

              _ ->
                false
            end
        end
      end)

    IO.puts("\nSnakepit processes: #{snakepit_procs}")

    worker_count = count_workers()

    if worker_count > 0 do
      procs_per_worker = process_count / worker_count
      IO.puts("Estimated procs per worker: #{Float.round(procs_per_worker, 2)}")
      max_workers = floor(process_limit / procs_per_worker)
      IO.puts("Theoretical max workers: #{max_workers}")

      if max_workers < 110 do
        IO.puts("\n⚠️  WARNING: Process limit would prevent 110 workers!")
      end
    end

    if process_count / process_limit > 0.8 do
      IO.puts("\n⚠️  WARNING: Using >80% of process limit")
    else
      IO.puts("\n✅ Process usage looks healthy")
    end

    IO.puts("\n")
  end

  defp check_dets_contention do
    IO.puts("📊 TEST 3: DETS LOCK CONTENTION")
    IO.puts(String.duplicate("-", 80))

    # Check if DETS table exists
    dets_table = :snakepit_process_registry_dets

    case :dets.info(dets_table) do
      :undefined ->
        IO.puts("⚠️  DETS table not initialized yet")

      _info ->
        size = :dets.info(dets_table, :size)
        IO.puts("DETS table size: #{size} entries")

        # Test concurrent write performance
        IO.puts("\nTesting concurrent DETS writes (simulating 110 workers)...")

        {time_us, results} =
          :timer.tc(fn ->
            1..110
            |> Task.async_stream(
              fn i ->
                start = System.monotonic_time(:microsecond)

                result =
                  :dets.insert(
                    dets_table,
                    {"test_worker_#{i}_#{:rand.uniform(999_999)}",
                     %{pid: self(), started_at: System.system_time(:second)}}
                  )

                elapsed = System.monotonic_time(:microsecond) - start
                {result, elapsed}
              end,
              max_concurrency: 110,
              timeout: 30_000
            )
            |> Enum.to_list()
          end)

        time_s = time_us / 1_000_000

        successes =
          Enum.count(results, fn
            {:ok, {:ok, _}} -> true
            _ -> false
          end)

        failures = 110 - successes

        # Get individual write times
        write_times =
          Enum.map(results, fn
            {:ok, {_result, elapsed}} -> elapsed
            _ -> nil
          end)
          |> Enum.reject(&is_nil/1)

        if length(write_times) > 0 do
          avg_write_us = Enum.sum(write_times) / length(write_times)
          max_write_us = Enum.max(write_times)
          min_write_us = Enum.min(write_times)

          IO.puts("\nResults:")
          IO.puts("  Total time:     #{Float.round(time_s, 3)}s")
          IO.puts("  Successes:      #{successes}/110")
          IO.puts("  Failures:       #{failures}/110")
          IO.puts("  Avg write time: #{Float.round(avg_write_us / 1000, 2)}ms")
          IO.puts("  Min write time: #{Float.round(min_write_us / 1000, 2)}ms")
          IO.puts("  Max write time: #{Float.round(max_write_us / 1000, 2)}ms")

          # Diagnosis
          if time_s > 5.0 do
            IO.puts("\n🔴 CRITICAL: DETS serialization is likely your bottleneck!")
            IO.puts("    110 concurrent writes took #{Float.round(time_s, 1)}s")
            IO.puts("    DETS locks on every write, causing extreme serialization")
            IO.puts("\n💡 SOLUTION: Replace DETS with ETS or remove registry entirely")
          else
            if time_s > 2.0 do
              IO.puts("\n⚠️  WARNING: DETS writes are slow (#{Float.round(time_s, 1)}s)")
              IO.puts("    This could contribute to timeouts during worker startup")
            else
              IO.puts("\n✅ DETS performance acceptable (#{Float.round(time_s, 3)}s)")
            end
          end
        end
    end

    IO.puts("\n")
  end

  defp check_python_spawn_rate do
    IO.puts("📊 TEST 4: PYTHON PROCESS SPAWN RATE")
    IO.puts(String.duplicate("-", 80))

    IO.puts("Testing Python process spawn rate (110 processes)...")

    # Test python3 spawn rate
    {time_us, {output, exit_code}} =
      :timer.tc(fn ->
        System.cmd(
          "sh",
          [
            "-c",
            """
            for i in {1..110}; do
              python3 -c "import sys; sys.exit(0)" &
            done
            wait
            """
          ],
          stderr_to_stdout: true
        )
      end)

    time_s = time_us / 1_000_000

    IO.puts("Time to spawn 110 Python processes: #{Float.round(time_s, 2)}s")
    IO.puts("Exit code: #{exit_code}")

    if exit_code != 0 do
      IO.puts("Output: #{String.slice(output, 0, 500)}")
    end

    if time_s > 30.0 do
      IO.puts("\n🔴 CRITICAL: Python spawn rate is very slow!")
      IO.puts("    System may be CPU or fork() constrained")
    else
      if time_s > 10.0 do
        IO.puts("\n⚠️  WARNING: Python spawn rate is slow (#{Float.round(time_s, 1)}s)")
      else
        IO.puts("\n✅ Python spawn rate looks good (#{Float.round(time_s, 2)}s)")
      end
    end

    IO.puts("\n")
  end

  defp check_grpc_limits do
    IO.puts("📊 TEST 5: GRPC CONNECTION LIMITS")
    IO.puts(String.duplicate("-", 80))

    # Check for Ranch (used by GRPC.Server)
    try do
      ranch_info = :ranch.info()
      IO.puts("Ranch info:")
      IO.inspect(ranch_info, limit: :infinity, pretty: true)
    rescue
      _ ->
        IO.puts("Ranch not available or not running")
    end

    # Count TCP connections
    case System.cmd("sh", ["-c", "ss -tan | grep ESTAB | wc -l"], stderr_to_stdout: true) do
      {output, 0} ->
        tcp_count = String.trim(output) |> String.to_integer()
        IO.puts("\nEstablished TCP connections: #{tcp_count}")

        worker_count = count_workers()

        if worker_count > 0 do
          tcp_per_worker = tcp_count / worker_count
          IO.puts("TCP connections per worker: #{Float.round(tcp_per_worker, 2)}")
        end

      {output, _} ->
        IO.puts("Could not count TCP connections: #{output}")
    end

    # Check gRPC port status
    case System.cmd("sh", ["-c", "ss -tlnp | grep -E ':(50051|50052)'"], stderr_to_stdout: true) do
      {output, 0} ->
        IO.puts("\ngRPC port status:")
        IO.puts(output)

      {_output, _} ->
        IO.puts("\ngRPC ports not listening or ss command failed")
    end

    IO.puts("\n")
  end

  defp find_exact_breaking_point do
    IO.puts("📊 TEST 6: BINARY SEARCH FOR EXACT BREAKING POINT")
    IO.puts(String.duplicate("-", 80))
    IO.puts("\n⚠️  This test spawns real workers and may fail!")
    IO.puts("Testing from 100 to 400 workers in increments...\n")

    # Capture baseline metrics
    baseline = capture_metrics()
    IO.puts("Baseline metrics:")
    print_metrics(baseline)

    # Test increasing worker counts
    results =
      for count <- [100, 250, 400, 500] do
        IO.puts("\n" <> String.duplicate("-", 60))
        IO.puts("🔬 Testing #{count} workers...")

        # Small delay between tests
        :timer.sleep(2000)

        before_metrics = capture_metrics()

        # Try to start a worker
        worker_id = "diagnostic_#{count}_#{:rand.uniform(999_999)}"

        start_time = System.monotonic_time(:millisecond)

        result =
          try do
            # Check if WorkerSupervisor exists
            case Process.whereis(Snakepit.Pool.WorkerSupervisor) do
              nil ->
                {:error, :supervisor_not_running}

              _pid ->
                Snakepit.Pool.WorkerSupervisor.start_worker(
                  worker_id,
                  Snakepit.GRPCWorker,
                  Snakepit.Adapters.GRPCPython,
                  Snakepit.Pool
                )
            end
          rescue
            e -> {:error, e}
          catch
            :exit, reason -> {:error, {:exit, reason}}
          end

        elapsed = System.monotonic_time(:millisecond) - start_time

        after_metrics = capture_metrics()
        delta = calculate_delta(before_metrics, after_metrics)

        status =
          case result do
            {:ok, _pid} -> :success
            {:error, reason} -> {:failed, reason}
          end

        # Clean up if successful
        if status == :success do
          try do
            Snakepit.Pool.WorkerSupervisor.stop_worker(worker_id)
          rescue
            _ -> :ok
          end
        end

        result_data = %{
          count: count,
          status: status,
          elapsed_ms: elapsed,
          before: before_metrics,
          after: after_metrics,
          delta: delta
        }

        print_test_result(result_data)

        result_data
      end

    # Analyze results
    IO.puts("\n" <> String.duplicate("=", 60))
    IO.puts("📈 ANALYSIS")
    IO.puts(String.duplicate("=", 60))

    first_failure =
      Enum.find(results, fn r ->
        match?({:failed, _}, r.status)
      end)

    case first_failure do
      nil ->
        IO.puts("\n✅ All tests passed up to 500 workers!")
        IO.puts("   The bottleneck may be higher than 500.")

      %{count: count, status: {:failed, reason}} ->
        IO.puts("\n🔴 FIRST FAILURE AT #{count} WORKERS")
        IO.puts("\nReason: #{inspect(reason, pretty: true)}")

        # Check what metric grew most
        analyze_metric_growth(results, count)
    end

    IO.puts("\n")
  end

  defp capture_metrics do
    %{
      erlang_ports: :erlang.system_info(:port_count),
      erlang_procs: :erlang.system_info(:process_count),
      tcp_connections: count_tcp_connections(),
      worker_count: count_workers(),
      memory_bytes: :erlang.memory(:total)
    }
  end

  defp calculate_delta(before, after_metrics) do
    %{
      ports: after_metrics.erlang_ports - before.erlang_ports,
      procs: after_metrics.erlang_procs - before.erlang_procs,
      tcp: after_metrics.tcp_connections - before.tcp_connections,
      workers: after_metrics.worker_count - before.worker_count,
      memory_mb: (after_metrics.memory_bytes - before.memory_bytes) / (1024 * 1024)
    }
  end

  defp print_metrics(metrics) do
    IO.puts("  Erlang ports: #{metrics.erlang_ports}")
    IO.puts("  Erlang procs: #{metrics.erlang_procs}")
    IO.puts("  TCP conns:    #{metrics.tcp_connections}")
    IO.puts("  Workers:      #{metrics.worker_count}")
    IO.puts("  Memory:       #{Float.round(metrics.memory_bytes / (1024 * 1024), 1)} MB")
  end

  defp print_test_result(result) do
    case result.status do
      :success ->
        IO.puts("✅ SUCCESS (#{result.elapsed_ms}ms)")

      {:failed, reason} ->
        IO.puts("🔴 FAILED (#{result.elapsed_ms}ms)")
        IO.puts("   Reason: #{inspect(reason)}")
    end

    IO.puts("\nMetric deltas:")
    IO.puts("  Ports:   #{format_delta(result.delta.ports)}")
    IO.puts("  Procs:   #{format_delta(result.delta.procs)}")
    IO.puts("  TCP:     #{format_delta(result.delta.tcp)}")
    IO.puts("  Workers: #{format_delta(result.delta.workers)}")
    IO.puts("  Memory:  #{format_delta(result.delta.memory_mb)} MB")
  end

  defp analyze_metric_growth(results, failure_count) do
    successful = Enum.filter(results, fn r -> r.status == :success end)

    if length(successful) > 0 do
      # Calculate average resource usage per worker from successful runs
      avg_delta = %{
        ports: avg(Enum.map(successful, & &1.delta.ports)),
        procs: avg(Enum.map(successful, & &1.delta.procs)),
        tcp: avg(Enum.map(successful, & &1.delta.tcp))
      }

      IO.puts("\nAverage resource usage per worker (from successful runs):")
      IO.puts("  Ports per worker:  #{Float.round(avg_delta.ports, 2)}")
      IO.puts("  Procs per worker:  #{Float.round(avg_delta.procs, 2)}")
      IO.puts("  TCP per worker:    #{Float.round(avg_delta.tcp, 2)}")

      # Extrapolate to limits
      port_limit = :erlang.system_info(:port_limit)
      proc_limit = :erlang.system_info(:process_limit)

      max_by_ports = floor(port_limit / avg_delta.ports)
      max_by_procs = floor(proc_limit / avg_delta.procs)

      IO.puts("\n📊 PROJECTED MAXIMUMS:")
      IO.puts("  Max by ports:      #{max_by_ports} workers")
      IO.puts("  Max by processes:  #{max_by_procs} workers")

      bottleneck =
        cond do
          max_by_ports < failure_count ->
            IO.puts("\n🔴 SMOKING GUN: ERLANG PORT LIMIT")

            IO.puts(
              "    At #{Float.round(avg_delta.ports, 2)} ports/worker, you hit the #{port_limit} port limit at ~#{max_by_ports} workers"
            )

            :ports

          max_by_procs < failure_count ->
            IO.puts("\n🔴 SMOKING GUN: ERLANG PROCESS LIMIT")

            IO.puts(
              "    At #{Float.round(avg_delta.procs, 2)} procs/worker, you hit the #{proc_limit} process limit at ~#{max_by_procs} workers"
            )

            :procs

          true ->
            IO.puts("\n🤔 UNCLEAR: Neither port nor process limit explains the failure")
            IO.puts("    Check the error message above for clues")
            :unknown
        end

      # Suggest solutions
      suggest_solutions(bottleneck, avg_delta)
    end
  end

  defp suggest_solutions(:ports, avg_delta) do
    IO.puts("\n💡 SOLUTIONS:")
    IO.puts("  1. Increase Erlang port limit in vm.args:")
    IO.puts("     +Q 1000000")
    IO.puts("  2. Each worker uses #{Float.round(avg_delta.ports, 1)} ports - investigate why:")
    IO.puts("     - 1 Port for stdin/stdout")
    IO.puts("     - 1 TCP socket for gRPC")
    IO.puts("     - Additional ports for...?")
    IO.puts("  3. Consider reducing port usage per worker")
  end

  defp suggest_solutions(:procs, avg_delta) do
    IO.puts("\n💡 SOLUTIONS:")
    IO.puts("  1. Increase Erlang process limit in vm.args:")
    IO.puts("     +P 5000000")

    IO.puts(
      "  2. Each worker uses #{Float.round(avg_delta.procs, 1)} processes - investigate supervision tree depth"
    )

    IO.puts("  3. Consider flattening the supervision tree")
  end

  defp suggest_solutions(:unknown, _avg_delta) do
    IO.puts("\n💡 NEXT STEPS:")
    IO.puts("  1. Check the error message for clues")
    IO.puts("  2. Look for timeouts in worker startup")
    IO.puts("  3. Check DETS test results above")
    IO.puts("  4. Monitor system resources (CPU, memory, file descriptors)")
  end

  defp count_workers do
    try do
      case Process.whereis(Snakepit.Pool.Registry) do
        nil ->
          0

        _pid ->
          Registry.count(Snakepit.Pool.Registry)
      end
    rescue
      _ -> 0
    end
  end

  defp count_tcp_connections do
    case System.cmd("sh", ["-c", "ss -tan | grep ESTAB | wc -l"], stderr_to_stdout: true) do
      {output, 0} ->
        output |> String.trim() |> String.to_integer()

      _ ->
        0
    end
  end

  defp avg(list) when length(list) > 0 do
    Enum.sum(list) / length(list)
  end

  defp avg(_), do: 0.0

  defp format_number(num) when num >= 1_000_000 do
    "#{Float.round(num / 1_000_000, 2)}M"
  end

  defp format_number(num) when num >= 1_000 do
    "#{Float.round(num / 1_000, 1)}K"
  end

  defp format_number(num), do: to_string(num)

  defp format_delta(num) when num >= 0, do: "+#{Float.round(num * 1.0, 2)}"
  defp format_delta(num), do: Float.round(num * 1.0, 2) |> to_string()
end