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wasapi: Promote/demote render thread on start/stop. #843

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Merged
padenot merged 1 commit into from
May 6, 2026
Merged

wasapi: Promote/demote render thread on start/stop. #843

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64 changes: 50 additions & 14 deletions src/cubeb_wasapi.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -437,6 +437,10 @@ struct cubeb_stream {
* practice, we read from the input stream in the output callback, so
* this is not used, but it is necessary to start getting input data. */
HANDLE input_available_event = 0;
/* Signaled by stream_start/stream_stop when the active state changes,
so the render thread can promote/demote its MMCSS task accordingly
instead of remaining promoted while idle. */
HANDLE mmcss_event = 0;
/* Each cubeb_stream has its own thread. */
HANDLE thread = 0;
/* The lock protects all members that are touched by the render thread or
Expand Down Expand Up @@ -1503,8 +1507,9 @@ static unsigned int __stdcall wasapi_stream_render_loop(LPVOID stream)
} com;

bool is_playing = true;
HANDLE wait_array[4] = {stm->shutdown_event, stm->reconfigure_event,
stm->refill_event, stm->input_available_event};
HANDLE wait_array[5] = {stm->shutdown_event, stm->mmcss_event,
stm->reconfigure_event, stm->refill_event,
stm->input_available_event};
HANDLE mmcss_handle = NULL;
HRESULT hr = 0;
DWORD mmcss_task_index = 0;
Expand All @@ -1517,15 +1522,6 @@ static unsigned int __stdcall wasapi_stream_render_loop(LPVOID stream)
return 0;
}

/* We could consider using "Pro Audio" here for WebAudio and
maybe WebRTC. */
mmcss_handle = AvSetMmThreadCharacteristicsA("Audio", &mmcss_task_index);
if (!mmcss_handle) {
/* This is not fatal, but we might glitch under heavy load. */
LOG("Unable to use mmcss to bump the render thread priority: %lx",
GetLastError());
}

while (is_playing) {
DWORD waitResult = WaitForMultipleObjects(ARRAY_LENGTH(wait_array),
wait_array, FALSE, INFINITE);
Expand All @@ -1539,7 +1535,30 @@ static unsigned int __stdcall wasapi_stream_render_loop(LPVOID stream)
}
continue;
}
case WAIT_OBJECT_0 + 1: { /* reconfigure */
case WAIT_OBJECT_0 + 1: { /* mmcss: active state changed */
/* stm->active was set by wasapi_stream_start/_stop before signaling,
and SetEvent provides the necessary memory barrier. */
if (stm->active && !mmcss_handle) {
/* We could consider using "Pro Audio" here for WebAudio and
maybe WebRTC. */
mmcss_handle =
AvSetMmThreadCharacteristicsA("Audio", &mmcss_task_index);
if (!mmcss_handle) {
/* This is not fatal, but we might glitch under heavy load. */
LOG("Unable to use mmcss to bump the render thread priority: %lx",
GetLastError());
} else {
LOG("MMCSS render thread promoted (task index %lu)",
mmcss_task_index);
}
} else if (!stm->active && mmcss_handle) {
AvRevertMmThreadCharacteristics(mmcss_handle);
mmcss_handle = NULL;
LOG("MMCSS render thread demoted");
}
continue;
}
case WAIT_OBJECT_0 + 2: { /* reconfigure */
auto_lock lock(stm->stream_reset_lock);
if (!stm->active) {
/* Avoid reconfiguring, stream start will handle it. */
Expand Down Expand Up @@ -1592,12 +1611,12 @@ static unsigned int __stdcall wasapi_stream_render_loop(LPVOID stream)
}
break;
}
case WAIT_OBJECT_0 + 2: /* refill */
case WAIT_OBJECT_0 + 3: /* refill */
XASSERT((has_input(stm) && has_output(stm)) ||
(!has_input(stm) && has_output(stm)));
is_playing = stm->refill_callback(stm);
break;
case WAIT_OBJECT_0 + 3: { /* input available */
case WAIT_OBJECT_0 + 4: { /* input available */
bool rv = get_input_buffer(stm);
if (!rv) {
is_playing = false;
Expand Down Expand Up @@ -2946,6 +2965,12 @@ wasapi_stream_init(cubeb * context, cubeb_stream ** stream,
return CUBEB_ERROR;
}

stm->mmcss_event = CreateEvent(NULL, 0, 0, NULL);
if (!stm->mmcss_event) {
LOG("Can't create the mmcss event, error: %lx", GetLastError());
return CUBEB_ERROR;
}

stm->thread_ready_event = CreateEvent(NULL, 0, 0, NULL);
if (!stm->thread_ready_event) {
LOG("Can't create the thread ready event, error: %lx", GetLastError());
Expand Down Expand Up @@ -3071,6 +3096,7 @@ wasapi_stream_release(cubeb_stream * stm)
CloseHandle(stm->reconfigure_event);
CloseHandle(stm->refill_event);
CloseHandle(stm->input_available_event);
CloseHandle(stm->mmcss_event);

CloseHandle(stm->thread);

Expand Down Expand Up @@ -3167,6 +3193,11 @@ wasapi_stream_start(cubeb_stream * stm)

stm->active = true;

if (!SetEvent(stm->mmcss_event)) {
LOG("wasapi_stream_start: SetEvent(mmcss_event) failed: %lx",
GetLastError());
}

stm->state_callback(stm, stm->user_ptr, CUBEB_STATE_STARTED);

return CUBEB_OK;
Expand Down Expand Up @@ -3199,6 +3230,11 @@ wasapi_stream_stop(cubeb_stream * stm)

stm->active = false;

if (!SetEvent(stm->mmcss_event)) {
LOG("wasapi_stream_stop: SetEvent(mmcss_event) failed: %lx",
GetLastError());
}

wasapi_state_callback(stm, stm->user_ptr, CUBEB_STATE_STOPPED);
}

Expand Down
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