Thread: Avoid setting realtime priority on Thread instances by default on POSIX systems

Author: reuk

BREAKING-CHANGES.txt

@@ -4,6 +4,33 @@ JUCE breaking changes
 Develop
 =======
 
+Change
+------
+Thread::setPriority() will no longer set a realtime scheduling policy for all
+threads with non-zero priorities on POSIX systems.
+
+Possible Issues
+---------------
+Threads that implicitly relied on using a realtime policy will no longer
+request a realtime policy if their priority is 7 or lower.
+
+Workaround
+----------
+For threads that require a realtime policy on POSIX systems, request a priority
+of 8 or higher by calling Thread::setPriority() or
+Thread::setCurrentThreadPriority().
+
+Rationale
+---------
+By default, new Thread instances have a priority of 5. Previously, non-zero
+priorities corresponded to realtime scheduling policies, meaning that new
+Threads would use the realtime scheduling policy unless they explicitly
+requested a priority of 0. However, most threads do not and should not require
+realtime scheduling. Setting a realtime policy on all newly-created threads may
+degrade performance, as multiple realtime threads will end up fighting for
+limited resources.
+
+
 Change
 ------
 The JUCE_GLSL_VERSION preprocessor definition has been removed.

modules/juce_core/native/juce_posix_SharedCode.h

@@ -953,22 +953,31 @@ void JUCE_CALLTYPE Thread::setCurrentThreadName (const String& name)
 
 bool Thread::setThreadPriority (void* handle, int priority)
 {
+    constexpr auto maxInputPriority = 10;
+    constexpr auto lowestRealtimePriority = 8;
+
     struct sched_param param;
     int policy;
-    priority = jlimit (0, 10, priority);
 
     if (handle == nullptr)
         handle = (void*) pthread_self();
 
     if (pthread_getschedparam ((pthread_t) handle, &policy, &param) != 0)
         return false;
 
-    policy = priority == 0 ? SCHED_OTHER : SCHED_RR;
+    policy = priority < lowestRealtimePriority ? SCHED_OTHER : SCHED_RR;
+
+    const auto minPriority = sched_get_priority_min (policy);
+    const auto maxPriority = sched_get_priority_max (policy);
+
+    param.sched_priority = [&]
+    {
+        if (policy == SCHED_OTHER)
+            return 0;
 
-    const int minPriority = sched_get_priority_min (policy);
-    const int maxPriority = sched_get_priority_max (policy);
+        return jmap (priority, lowestRealtimePriority, maxInputPriority, minPriority, maxPriority);
+    }();
 
-    param.sched_priority = ((maxPriority - minPriority) * priority) / 10 + minPriority;
     return pthread_setschedparam ((pthread_t) handle, policy, &param) == 0;
 }