diff --git a/src/mzscheme/include/scheme.h b/src/mzscheme/include/scheme.h
index 4e5c881b68..8e406c1121 100644
--- a/src/mzscheme/include/scheme.h
+++ b/src/mzscheme/include/scheme.h
@@ -1491,7 +1491,7 @@ typedef void (*Scheme_Invoke_Proc)(Scheme_Env *env, long phase_shift,
 #  define scheme_fuel_counter (*scheme_fuel_counter_ptr)
 # endif
 #else
-MZ_EXTERN volatile int scheme_fuel_counter;
+MZ_EXTERN THREAD_LOCAL volatile int scheme_fuel_counter;
 #endif
 
 #ifdef FUEL_AUTODECEREMENTS
diff --git a/src/mzscheme/src/eval.c b/src/mzscheme/src/eval.c
index 2b54ef1e19..b73df666e8 100644
--- a/src/mzscheme/src/eval.c
+++ b/src/mzscheme/src/eval.c
@@ -147,7 +147,7 @@
 #define EMBEDDED_DEFINES_START_ANYWHERE 0
 
 /* globals */
-volatile int scheme_fuel_counter;
+THREAD_LOCAL volatile int scheme_fuel_counter;
 
 int scheme_startup_use_jit = 1;
 void scheme_set_startup_use_jit(int v) { scheme_startup_use_jit =  v; }
diff --git a/src/mzscheme/src/port.c b/src/mzscheme/src/port.c
index 83cd58d4ce..b63e5c4b34 100644
--- a/src/mzscheme/src/port.c
+++ b/src/mzscheme/src/port.c
@@ -8422,7 +8422,7 @@ static long ITimer(void)
 END_XFORM_SKIP;
 #endif
 
-void scheme_start_itimer_thread(long usec)
+static void scheme_start_itimer_thread(long usec)
 {
   DWORD id;
 
@@ -8441,65 +8441,137 @@ void scheme_start_itimer_thread(long usec)
 #ifdef USE_PTHREAD_THREAD_TIMER
 
 #include <pthread.h>
+typedef struct ITimer_Data {
+  int itimer;
+  int state;
+  pthread_t thread;
+  pthread_mutex_t mutex;
+  pthread_cond_t cond;
+  int delay;
+  volatile int * fuel_counter_ptr;
+  volatile unsigned long * jit_stack_boundary_ptr;
+} ITimer_Data;
 
-static int itimer = 0, itimer_continue = 0;
-static pthread_mutex_t itimer_mutex;
-static pthread_cond_t itimer_cond;
-static volatile long itimer_delay;
+static THREAD_LOCAL ITimer_Data itimerdata;
 
 #ifdef MZ_XFORM
 START_XFORM_SKIP;
 #endif
-static void *run_itimer(void *p)
-{
-  while (1) {
-    usleep(itimer_delay);
-    scheme_fuel_counter = 0;
-    scheme_jit_stack_boundary = (unsigned long)-1;
 
-    pthread_mutex_lock(&itimer_mutex);
-    if (itimer_continue) {
-      itimer_continue = 0;
+static void *green_thread_timer(void *data)
+{
+  ITimer_Data *itimer_data;
+  itimer_data = (ITimer_Data *)data;
+  
+  while (1) {
+    usleep(itimer_data->delay);
+    *(itimer_data->fuel_counter_ptr) = 0;
+    *(itimer_data->jit_stack_boundary_ptr) = (unsigned long)-1;
+
+    pthread_mutex_lock(&itimer_data->mutex);
+    if (itimer_data->state) {
+      itimer_data->state = 0;
     } else {
-      itimer_continue = -1;
-      pthread_cond_wait(&itimer_cond, &itimer_mutex);
+      itimer_data->state = -1;
+      pthread_cond_wait(&itimer_data->cond, &itimer_data->mutex);
     }
-    pthread_mutex_unlock(&itimer_mutex);
+    pthread_mutex_unlock(&itimer_data->mutex);
   }
+  return NULL;
 }
+
 #ifdef MZ_XFORM
 END_XFORM_SKIP;
 #endif
 
-void scheme_start_itimer_thread(long usec)
-{
-  itimer_delay = usec;
+static void start_green_thread_timer(long usec) {
+  itimerdata.delay = usec;
+  itimerdata.fuel_counter_ptr = &scheme_fuel_counter;
+  itimerdata.jit_stack_boundary_ptr = &scheme_jit_stack_boundary;
+  pthread_mutex_init(&itimerdata.mutex, NULL);
+  pthread_cond_init(&itimerdata.cond, NULL);
+  pthread_create(&itimerdata.thread, NULL, green_thread_timer, &itimerdata);
+  itimerdata.itimer = 1;
+}
 
-  if (!itimer) {
-    pthread_t t;
-    pthread_mutex_init(&itimer_mutex, NULL);
-    pthread_cond_init(&itimer_cond, NULL);
-    pthread_create(&t, NULL, run_itimer,  NULL);
-    itimer = 1;
-  } else {
-    pthread_mutex_lock(&itimer_mutex);
-    if (!itimer_continue) {
+static void kickoff_green_thread_timer(long usec) {
+    pthread_mutex_lock(&itimerdata.mutex);
+    itimerdata.delay = usec;
+    if (!itimerdata.state) {
       /* itimer thread is currently running working */
-      itimer_continue = 1;
-    } else if (itimer_continue < 0) {
+      itimerdata.state = 1;
+    } else if (itimerdata.state < 0) {
       /* itimer thread is waiting on cond */
-      itimer_continue = 0;
-      pthread_cond_signal(&itimer_cond);
+      itimerdata.state = 0;
+      pthread_cond_signal(&itimerdata.cond);
     } else {
       /* itimer thread is working, and we've already
          asked it to continue */
     }
-    pthread_mutex_unlock(&itimer_mutex);
+    pthread_mutex_unlock(&itimerdata.mutex);
+}
+
+static void scheme_start_itimer_thread(long usec)
+{
+  if (!itimerdata.itimer) {
+    start_green_thread_timer(usec);
+  } else {
+    kickoff_green_thread_timer(usec);
   }
 }
 
 #endif
 
+#ifdef USE_ITIMER
+
+#ifdef MZ_XFORM
+START_XFORM_SKIP;
+#endif
+
+static void itimer_expired(int ignored)
+{
+  scheme_fuel_counter = 0;
+  scheme_jit_stack_boundary = (unsigned long)-1;
+#  ifdef SIGSET_NEEDS_REINSTALL
+  MZ_SIGSET(SIGPROF, itimer_expired);
+#  endif
+}
+
+static void kickoff_itimer(long usec) {
+  struct itimerval t;
+  struct itimerval old;
+  static int itimer_handler_installed = 0;
+
+  if (!itimer_handler_installed) {
+    itimer_handler_installed = 1;
+    MZ_SIGSET(SIGPROF, itimer_expired);
+  }
+
+  t.it_value.tv_sec = 0;
+  t.it_value.tv_usec = usec;
+  t.it_interval.tv_sec = 0;
+  t.it_interval.tv_usec = 0;
+
+  setitimer(ITIMER_PROF, &t, &old);
+}
+
+#ifdef MZ_XFORM
+END_XFORM_SKIP;
+#endif
+
+#endif
+
+void scheme_kickoff_green_thread_time_slice_timer(long usec) {
+#ifdef USE_ITIMER
+  kickoff_itimer(usec);
+#elif defined(USE_WIN32_THREAD_TIMER)
+  scheme_start_itimer_thread(usec);
+#elif defined(USE_PTHREAD_THREAD_TIMER)
+  scheme_start_itimer_thread(usec);
+#else
+  #error scheme_start_green_thread_time_slice_timer NOT IMPLEMENTED
+#endif
+}
 
 #ifdef OS_X
 
diff --git a/src/mzscheme/src/schemef.h b/src/mzscheme/src/schemef.h
index 1f37b63ddf..36cdcae970 100644
--- a/src/mzscheme/src/schemef.h
+++ b/src/mzscheme/src/schemef.h
@@ -71,7 +71,7 @@ MZ_EXTERN Scheme_Object *scheme_current_break_cell();
 # ifndef MZ_USE_PLACES
 MZ_EXTERN THREAD_LOCAL Scheme_Thread *scheme_current_thread;
 # endif
-MZ_EXTERN volatile int scheme_fuel_counter;
+MZ_EXTERN THREAD_LOCAL volatile int scheme_fuel_counter;
 #else
 MZ_EXTERN Scheme_Thread **scheme_current_thread_ptr;
 MZ_EXTERN volatile int *scheme_fuel_counter_ptr;
diff --git a/src/mzscheme/src/schemex.h b/src/mzscheme/src/schemex.h
index 9d564c9491..6fef165afb 100644
--- a/src/mzscheme/src/schemex.h
+++ b/src/mzscheme/src/schemex.h
@@ -57,7 +57,7 @@ Scheme_Object *(*scheme_current_break_cell)();
 # ifndef MZ_USE_PLACES
 Scheme_Thread *scheme_current_thread;
 # endif
-volatile int scheme_fuel_counter;
+THREAD_LOCAL volatile int scheme_fuel_counter;
 #else
 Scheme_Thread **scheme_current_thread_ptr;
 volatile int *scheme_fuel_counter_ptr;
diff --git a/src/mzscheme/src/schpriv.h b/src/mzscheme/src/schpriv.h
index 227fbe7b73..6410ac4851 100644
--- a/src/mzscheme/src/schpriv.h
+++ b/src/mzscheme/src/schpriv.h
@@ -365,7 +365,7 @@ extern THREAD_LOCAL MZ_MARK_POS_TYPE scheme_current_cont_mark_pos;
 # define MZ_CONT_MARK_POS (scheme_current_thread->cont_mark_pos)
 #endif
 
-extern volatile int scheme_fuel_counter;
+extern THREAD_LOCAL volatile int scheme_fuel_counter;
 
 extern THREAD_LOCAL Scheme_Thread *scheme_main_thread;
 
@@ -419,9 +419,8 @@ void scheme_forget_subthread(struct Scheme_Thread_Memory *);
 void scheme_suspend_remembered_threads(void);
 void scheme_resume_remembered_threads(void);
 #endif
-#if defined(USE_WIN32_THREAD_TIMER) || defined(USE_PTHREAD_THREAD_TIMER)
-void scheme_start_itimer_thread(long usec);
-#endif
+
+void scheme_kickoff_green_thread_time_slice_timer(long usec);
 
 #ifdef UNIX_PROCESSES
 void scheme_block_child_signals(int block);
diff --git a/src/mzscheme/src/thread.c b/src/mzscheme/src/thread.c
index 72b4258534..2654994753 100644
--- a/src/mzscheme/src/thread.c
+++ b/src/mzscheme/src/thread.c
@@ -54,11 +54,6 @@
 #  include <be/net/socket.h>
 # endif
 #endif
-#ifdef USE_ITIMER
-# include <sys/types.h>
-# include <sys/time.h>
-# include <signal.h>
-#endif
 #ifdef USE_WINSOCK_TCP
 # ifdef USE_TCP
 #  include <winsock.h>
@@ -3636,28 +3631,6 @@ void scheme_out_of_fuel(void)
   scheme_current_thread->ran_some = 1;
 }
 
-#ifdef USE_ITIMER
-static int itimer_handler_installed = 0;
-
-#ifdef MZ_XFORM
-START_XFORM_SKIP;
-#endif
-
-static void timer_expired(int ignored)
-{
-  scheme_fuel_counter = 0;
-  scheme_jit_stack_boundary = (unsigned long)-1;
-#  ifdef SIGSET_NEEDS_REINSTALL
-  MZ_SIGSET(SIGPROF, timer_expired);
-#  endif
-}
-
-#ifdef MZ_XFORM
-END_XFORM_SKIP;
-#endif
-
-#endif
-
 static void init_schedule_info(Scheme_Schedule_Info *sinfo, Scheme_Thread *false_pos_ok, 
 			       double sleep_end)
 {
@@ -4251,26 +4224,7 @@ void scheme_thread_block(float sleep_time)
   scheme_fuel_counter = p->engine_weight;
   scheme_jit_stack_boundary = scheme_stack_boundary;
 
-#ifdef USE_ITIMER
-  {
-    struct itimerval t, old;
-
-    if (!itimer_handler_installed) {
-      itimer_handler_installed = 1;
-      MZ_SIGSET(SIGPROF, timer_expired);
-    }
-
-    t.it_value.tv_sec = 0;
-    t.it_value.tv_usec = MZ_THREAD_QUANTUM_USEC;
-    t.it_interval.tv_sec = 0;
-    t.it_interval.tv_usec = 0;
-
-    setitimer(ITIMER_PROF, &t, &old);
-  }
-#endif
-#if defined(USE_WIN32_THREAD_TIMER) || defined(USE_PTHREAD_THREAD_TIMER)
-  scheme_start_itimer_thread(MZ_THREAD_QUANTUM_USEC);
-#endif
+  scheme_kickoff_green_thread_time_slice_timer(MZ_THREAD_QUANTUM_USEC);
 
   /* Check scheduled_kills early and often. */
   check_scheduled_kills();
diff --git a/src/mzscheme/uconfig.h b/src/mzscheme/uconfig.h
index 12610b9bf0..463f2ad6f8 100644
--- a/src/mzscheme/uconfig.h
+++ b/src/mzscheme/uconfig.h
@@ -36,4 +36,9 @@
 
 #define USE_GETRUSAGE
 
-#define USE_ITIMER
+#if defined MZ_USE_PLACES
+# undef USE_ITIMER
+# define USE_PTHREAD_THREAD_TIMER
+#else
+# define USE_ITIMER
+#endif