[Bug] Python process consuming ~98% CPU on macOS — tight async loop with TypeError in bytes.contains

1

Hi,

I noticed that PlatformIO’s background Python process was consuming ~98% CPU on my Mac, causing significant fan noise and heat. After stopping all but one process, the issue subsided.

I captured a macOS sample profile of the process (pid 83226, python3.11) and analysed the call graph. Here’s what I found:

Environment

  • macOS 15.4 (Apple Silicon, ARM64)
  • Python 3.11
  • PlatformIO (started via the IDE extension)

Root cause (from call graph analysis)

Two hotspots account for most of the CPU usage:

  1. ~25% of samples — _Py_bytes_contains → _PyNumber_Index → PyErr_Format
    An async task is using the in operator on a bytes object while passing a non-integer type. This raises a TypeError internally, which is caught and ignored — but because this runs in a tight asyncio loop with no sleep(), it fires hundreds of thousands of times per second. The cost of building the exception string via PyUnicode_FromFormatV, allocating and deallocating exception objects, and clearing the error repeatedly adds up to significant CPU load.

  2. ~21% of samples — time.time() → clock_gettime → mach_absolute_time
    Very frequent calls to time.time() suggest a polling loop running without any asyncio.sleep() or equivalent yield, keeping the event loop spinning continuously.

The affected modules visible in the binary images are md.cpython-311-darwin.so and md__mypyc.cpython-311-darwin.so.

Suggested fix

  • Add a type check before the bytes.__contains__ call to avoid the repeated TypeError.
  • Add await asyncio.sleep(0) or a small sleep in the polling loop to yield control and reduce CPU usage to near 0% when idle.

Impact

On Apple Silicon this causes sustained high CPU load, elevated temperatures, and loud fan noise even when no build or upload is running — just having PlatformIO open in the background is enough to trigger it.

I’m happy to share the full sample output if that would help with debugging.

Thanks!

2

What is the exact process name and arguments? Is it really platformio / pio getting run in the python process or is it the VSCode C/C++ Intellisense process? That makes a huge difference.