Class WindowsLoadTracker

Lib/test/libregrtest/win_utils.py:19–128

This class asynchronously reads the performance counters to calculate the system load on Windows. A "raw" thread is used here to prevent interference with the test suite's cases for the threading module.

class WindowsLoadTracker():
    """
    This class asynchronously reads the performance counters to calculate
    the system load on Windows.  A "raw" thread is used here to prevent
    interference with the test suite's cases for the threading module.
    """

    def __init__(self):
        # make __del__ not fail if pre-flight test fails
        self._running = None
        self._stopped = None

        # Pre-flight test for access to the performance data;
        # `PermissionError` will be raised if not allowed
        winreg.QueryInfoKey(winreg.HKEY_PERFORMANCE_DATA)

        self._values = []
        self._load = None
        self._running = _overlapped.CreateEvent(None, True, False, None)
        self._stopped = _overlapped.CreateEvent(None, True, False, None)

        _thread.start_new_thread(self._update_load, (), {})

    def _update_load(self,
                    # localize module access to prevent shutdown errors
                     _wait=_winapi.WaitForSingleObject,
                     _signal=_overlapped.SetEvent):
        # run until signaled to stop
        while _wait(self._running, 1000):
            self._calculate_load()
        # notify stopped
        _signal(self._stopped)

    def _calculate_load(self,
                        # localize module access to prevent shutdown errors
                        _query=winreg.QueryValueEx,
                        _hkey=winreg.HKEY_PERFORMANCE_DATA,
                        _unpack=struct.unpack_from):
        # get the 'System' object
        data, _ = _query(_hkey, '2')
        # PERF_DATA_BLOCK {
        #   WCHAR Signature[4]      8 +
        #   DWOWD LittleEndian      4 +
        #   DWORD Version           4 +
        #   DWORD Revision          4 +
        #   DWORD TotalByteLength   4 +
        #   DWORD HeaderLength      = 24 byte offset
        #   ...
        # }
        obj_start, = _unpack('L', data, 24)
        # PERF_OBJECT_TYPE {
        #   DWORD TotalByteLength
        #   DWORD DefinitionLength
        #   DWORD HeaderLength
        #   ...
        # }
        data_start, defn_start = _unpack('4xLL', data, obj_start)
        data_base = obj_start + data_start