diff -ruN kozos28/thread.c kozos29/thread.c
--- kozos28/thread.c	Wed Dec 12 20:44:00 2007
+++ kozos29/thread.c	Wed Dec 12 21:42:51 2007
@@ -25,6 +25,7 @@
   kz_thread *head;
   kz_thread *tail;
 } readyque[PRI_NUM];
+static unsigned int readyque_bitmap;
 static kz_timebuf *timers;
 static kz_thread *sigcalls[SIG_NUM];
 static int debug_sockt = 0;
@@ -36,8 +37,10 @@
 static void getcurrent()
 {
   readyque[current->pri].head = current->next;
-  if (readyque[current->pri].head == NULL)
+  if (readyque[current->pri].head == NULL) {
     readyque[current->pri].tail = NULL;
+    readyque_bitmap &= ~(1 << current->pri);
+  }
   current->flags &= ~KZ_THREAD_FLAG_RUNNING;
   current->next = NULL;
 }
@@ -55,6 +58,7 @@
     readyque[current->pri].head = current;
   }
   readyque[current->pri].tail = current;
+  readyque_bitmap |= (1 << current->pri);
   current->flags |= KZ_THREAD_FLAG_RUNNING;
 
   return 0;
@@ -420,15 +424,34 @@
 
 static void schedule()
 {
-  int i;
-  for (i = 0; i < PRI_NUM; i++) {
-    if (readyque[i].head) break;
+#if PRI_NUM > 32
+#error ビットマップを配列化する必要あり
+#endif
+  unsigned int bitmap = readyque_bitmap;
+  int n = 0;
+  static int bitmap2num[16] = {
+    -32, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
+  };
+
+  if (!(bitmap & 0xffff)) {
+    bitmap >>= 16;
+    n += 16;
+  }
+  if (!(bitmap & 0xff)) {
+    bitmap >>= 8;
+    n += 8;
+  }
+  if (!(bitmap & 0xf)) {
+    bitmap >>= 4;
+    n += 4;
   }
-  if (i == PRI_NUM) {
+  n += bitmap2num[bitmap & 0xf];
+  if (n < 0) {
     /* 実行可能なスレッドが存在しないので，終了する */
     exit(0);
   }
-  current = readyque[i].head;
+
+  current = readyque[n].head;
 }
 
 static void thread_intrvec(int signo)
@@ -500,6 +523,7 @@
   memset(readyque, 0, sizeof(readyque));
   memset(sigcalls, 0, sizeof(sigcalls));
 
+  readyque_bitmap = 0;
   timers = NULL;
 
   sigstack.ss_sp = malloc(SIGSTKSZ);