From edc69377fa53a1cfc5db87f3e8cf4f01a6d19cae Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Axel=20D=C3=B6rfler?= <axeld@pinc-software.de>
Date: Fri, 11 Dec 2009 17:40:15 +0000
Subject: [PATCH] * Changed the simple scheduler's next thread selection in the
 same way as the   affine scheduler to avoid possible latency issues.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@34634 a95241bf-73f2-0310-859d-f6bbb57e9c96
---
 .../kernel/scheduler/scheduler_simple.cpp     | 43 +++++++++++--------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/src/system/kernel/scheduler/scheduler_simple.cpp b/src/system/kernel/scheduler/scheduler_simple.cpp
index f63c98ed92..48c6d08e37 100644
--- a/src/system/kernel/scheduler/scheduler_simple.cpp
+++ b/src/system/kernel/scheduler/scheduler_simple.cpp
@@ -1,6 +1,6 @@
 /*
  * Copyright 2008, Ingo Weinhold, ingo_weinhold@gmx.de.
- * Copyright 2002-2007, Axel Dörfler, axeld@pinc-software.de.
+ * Copyright 2002-2009, Axel Dörfler, axeld@pinc-software.de.
  * Copyright 2002, Angelo Mottola, a.mottola@libero.it.
  * Distributed under the terms of the MIT License.
  *
@@ -239,23 +239,32 @@ simple_reschedule(void)
 			if (nextThread->priority >= B_FIRST_REAL_TIME_PRIORITY)
 				break;
 
-			// never skip last non-idle normal thread
-			if (nextThread->queue_next
-				&& nextThread->queue_next->priority == B_IDLE_PRIORITY)
-				break;
-
-			// skip normal threads sometimes (roughly 20%)
-			if (_rand() > 0x1a00)
-				break;
-
-			// skip until next lower priority
+			// find next thread with lower priority
+			struct thread *lowerNextThread = nextThread->queue_next;
+			struct thread *lowerPrevThread = nextThread;
 			int32 priority = nextThread->priority;
-			do {
-				prevThread = nextThread;
-				nextThread = nextThread->queue_next;
-			} while (nextThread->queue_next != NULL
-				&& priority == nextThread->queue_next->priority
-				&& nextThread->queue_next->priority > B_IDLE_PRIORITY);
+
+			while (lowerNextThread != NULL
+				&& priority == lowerNextThread->priority) {
+				lowerPrevThread = lowerNextThread;
+				lowerNextThread = lowerNextThread->queue_next;
+			}
+			// never skip last non-idle normal thread
+			if (lowerNextThread == NULL
+				|| lowerNextThread->priority == B_IDLE_PRIORITY)
+				break;
+
+			int32 priorityDiff = priority - lowerNextThread->priority;
+			if (priorityDiff > 15)
+				break;
+
+			// skip normal threads sometimes
+			// (twice as probable per priority level)
+			if ((_rand() >> (15 - priorityDiff)) != 0)
+				break;
+
+			nextThread = lowerNextThread;
+			prevThread = lowerPrevThread;
 		}
 
 		break;