diff --git a/pkgs/racket-doc/scribblings/foreign/derived.scrbl b/pkgs/racket-doc/scribblings/foreign/derived.scrbl
index 74557bbc84..e3d9f61ab3 100644
--- a/pkgs/racket-doc/scribblings/foreign/derived.scrbl
+++ b/pkgs/racket-doc/scribblings/foreign/derived.scrbl
@@ -14,6 +14,7 @@
 @include-section["custodian.scrbl"]
 @include-section["atomic.scrbl"]
 @include-section["try-atomic.scrbl"]
+@include-section["schedule.scrbl"]
 @include-section["global.scrbl"]
 @include-section["objc.scrbl"]
 @include-section["ns.scrbl"]
diff --git a/pkgs/racket-doc/scribblings/foreign/schedule.scrbl b/pkgs/racket-doc/scribblings/foreign/schedule.scrbl
new file mode 100644
index 0000000000..e8a8dad361
--- /dev/null
+++ b/pkgs/racket-doc/scribblings/foreign/schedule.scrbl
@@ -0,0 +1,105 @@
+#lang scribble/doc
+@(require "utils.rkt"
+          (for-label ffi/unsafe/schedule))
+
+@title{Thread Scheduling}
+
+@defmodule[ffi/unsafe/schedule]{The
+@racketmodname[ffi/unsafe/schedule] library provides functions for
+cooperating with the thread scheduler and manipulating it.}
+
+@history[#:added "6.11.0.1"]
+
+@defproc[(unsafe-poller [poll (evt? (or/c #f any/c) . -> . (values (or/c #f list?) evt?))])
+         any/c]{
+
+Produces a @deftech{poller} value that is allowed as a
+@racket[prop:evt] value, even though it is not a procedure or itself
+an @racket[evt?]. The @racket[poll] callback is called in @tech{atomic
+mode} to check whether the event is ready or to allow it to register a
+wakeup trigger.
+
+The first argument to @racket[poll] is always the object that is used
+as a @tech[#:doc reference.scrbl]{synchronizable event} with the
+@tech{poller} as its @racket[prop:evt] value. Let's call that value
+@racket[_evt].
+
+The second argument to @racket[poll] is @racket[#f] when @racket[poll]
+is called to check whether the event is ready. The result must be two
+values. The first result value is a list of results if @racket[_evt]
+is ready, or it is @racket[#f] if @racket[_evt] is not ready. The
+second result value is @racket[#f] if @racket[_evt] is ready, or it is
+an event to replace @racket[_evt] (often just @racket[_evt] itself) if
+@racket[_evt] is not ready.
+
+When the thread scheduler has determined that the Racket process
+should sleep until an external event or timeout, then @racket[poll] is
+called with a non-@racket[#f] second argument, @racket[_wakeups]. In
+that case, the result must always be @racket[(values #f _evt)]. In
+addition to returning the requires values, @racket[poll] can call
+@racket[unsafe-poll-ctx-fd-wakeup],
+@racket[unsafe-poll-ctx-eventmask-wakeup], and/or
+@racket[unsafe-poll-ctx-milliseconds-wakeup] on @racket[_wakeups] to
+register wakeup triggers.}
+
+
+@defproc[(unsafe-poll-ctx-fd-wakeup [wakeups any/c]
+                                    [fd fixnum?]
+                                    [mode '(read write error)])
+         void?]{
+
+Registers a file descriptor (Unix and Mac OS) or socket (all
+platforms) to cause the Racket process to wake up and resume polling
+if the file descriptor or socket becomes ready for reading, writing,
+or error reporting, as selected by @racket[mode]. The @racket[wakeups]
+argument must be a non-@racket[#f] value that is passed by the
+scheduler to a @racket[unsafe-poller]-wrapped procedure.}
+
+
+@defproc[(unsafe-poll-ctx-eventmask-wakeup [wakeups any/c]
+                                           [mask fixnum?])
+         void?]{
+
+On Windows, registers an eventmask to cause the Racket process to wake
+up and resume polling if an event selected by the mask becomes
+available.}
+
+
+@defproc[(unsafe-poll-ctx-milliseconds-wakeup [wakeups any/c]
+                                              [msecs flonum?])
+         void?]{
+
+Causes the Racket process will wake up and resume polling at the point
+when @racket[(current-inexact-milliseconds)] starts returning a value
+that is @racket[msecs] or greater.}
+
+
+Registers a file descriptor (Unix and Mac OS) or socket (all
+platforms) to cause the Racket process to wake up if the file
+descriptor or socket becomes ready for reading, writing, or error
+reporting, as selected by @racket[mode]. The @racket[wakeups] argument
+must be a non-@racket[#f] value that is passed by the scheduler to a
+@racket[unsafe-poller]-wrapped procedure.}
+
+@defproc[(unsafe-set-sleep-in-thread! [foreground-sleep (-> any/c)]
+                                      [fd fixnum?])
+         void?]{
+
+Registers @racket[foreground-sleep] as a procedure to implement
+sleeping for the Racket process when the thread scheduler determines
+at the process will sleep. Meanwhile, during a call to
+@racket[foreground-sleep], the scheduler's default sleeping function
+will run in a separate OS-level thread. When that default sleeping
+function wakes up, a byte is written to @racket[fd] as a way of
+notifying @racket[foreground-sleep] that it should return
+immediately.
+
+This function works on when OS-level threads are available within the
+Racket implementation. It always works for Mac OS.}
+
+@defproc[(unsafe-signal-received) void?]{
+
+For use with @racket[unsafe-set-sleep-in-thread!] by
+@racket[foreground-sleep] or something that it triggers, causes the
+default sleeping function to request @racket[foreground-sleep] to
+return.}
diff --git a/pkgs/racket-doc/scribblings/foreign/try-atomic.scrbl b/pkgs/racket-doc/scribblings/foreign/try-atomic.scrbl
index 9e5a892c5e..0b0f3a903f 100644
--- a/pkgs/racket-doc/scribblings/foreign/try-atomic.scrbl
+++ b/pkgs/racket-doc/scribblings/foreign/try-atomic.scrbl
@@ -5,7 +5,7 @@
 @title{Speculatively Atomic Execution}
 
 @defmodule[ffi/unsafe/try-atomic]{The
-@racketmodname[ffi/unsafe/try-atomic] supports atomic execution that
+@racketmodname[ffi/unsafe/try-atomic] library supports atomic execution that
 can be suspended and resumed in non-atomic mode if it takes too long
 or if some external event causes the attempt to be abandoned.}
 
diff --git a/racket/collects/ffi/unsafe/schedule.rkt b/racket/collects/ffi/unsafe/schedule.rkt
new file mode 100644
index 0000000000..dc83c5a9e3
--- /dev/null
+++ b/racket/collects/ffi/unsafe/schedule.rkt
@@ -0,0 +1,9 @@
+#lang racket/base
+(require (only-in '#%unsafe
+                  unsafe-poller
+                  unsafe-poll-ctx-fd-wakeup
+                  unsafe-poll-ctx-eventmask-wakeup
+                  unsafe-poll-ctx-milliseconds-wakeup
+                  unsafe-signal-received
+                  unsafe-set-sleep-in-thread!))
+(provide (all-from-out '#%unsafe))
diff --git a/racket/collects/racket/unsafe/ops.rkt b/racket/collects/racket/unsafe/ops.rkt
index c92cc9eba2..050428fd77 100644
--- a/racket/collects/racket/unsafe/ops.rkt
+++ b/racket/collects/racket/unsafe/ops.rkt
@@ -15,6 +15,11 @@
                      unsafe-start-atomic unsafe-end-atomic
                      unsafe-start-breakable-atomic unsafe-end-breakable-atomic
                      unsafe-in-atomic?
+                     unsafe-poller
+                     unsafe-poll-ctx-fd-wakeup
+                     unsafe-poll-ctx-eventmask-wakeup
+                     unsafe-poll-ctx-milliseconds-wakeup
+                     unsafe-signal-received unsafe-set-sleep-in-thread!
                      unsafe-thread-at-root
                      unsafe-make-custodian-at-root
                      unsafe-custodian-register
diff --git a/racket/src/racket/include/schthread.h b/racket/src/racket/include/schthread.h
index ca6153b417..35d2217e6a 100644
--- a/racket/src/racket/include/schthread.h
+++ b/racket/src/racket/include/schthread.h
@@ -363,6 +363,8 @@ typedef struct Thread_Local_Variables {
   struct Scheme_Hash_Table *loaded_extensions_;
   struct Scheme_Hash_Table *fullpath_loaded_extensions_;
   Scheme_Sleep_Proc scheme_place_sleep_;
+  struct Scheme_Object *thread_sleep_callback_;
+  int thread_sleep_callback_fd_;
   struct GHBN_Thread_Data *ghbn_thread_data_;
   Scheme_On_Atomic_Timeout_Proc on_atomic_timeout_;
   void *on_atomic_timeout_data_;
@@ -759,6 +761,8 @@ XFORM_GC_VARIABLE_STACK_THROUGH_THREAD_LOCAL;
 #define loaded_extensions XOA (scheme_get_thread_local_variables()->loaded_extensions_)
 #define fullpath_loaded_extensions XOA (scheme_get_thread_local_variables()->fullpath_loaded_extensions_)
 #define scheme_place_sleep XOA (scheme_get_thread_local_variables()->scheme_place_sleep_)
+#define thread_sleep_callback XOA (scheme_get_thread_local_variables()->thread_sleep_callback_)
+#define thread_sleep_callback_fd XOA (scheme_get_thread_local_variables()->thread_sleep_callback_fd_)
 #define ghbn_thread_data XOA (scheme_get_thread_local_variables()->ghbn_thread_data_)
 #define on_atomic_timeout XOA (scheme_get_thread_local_variables()->on_atomic_timeout_)
 #define on_atomic_timeout_data XOA (scheme_get_thread_local_variables()->on_atomic_timeout_data_)
diff --git a/racket/src/racket/src/cstartup.inc b/racket/src/racket/src/cstartup.inc
index 6df0afe921..13ef650b67 100644
--- a/racket/src/racket/src/cstartup.inc
+++ b/racket/src/racket/src/cstartup.inc
@@ -1,5 +1,5 @@
   {
-    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,48,46,51,84,0,0,0,0,0,0,0,0,0,
+    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,49,46,51,84,0,0,0,0,0,0,0,0,0,
 0,0,0,0,0,0,0,0,0,0,0,54,0,0,0,1,0,0,8,0,18,
 0,22,0,26,0,31,0,38,0,42,0,47,0,59,0,66,0,69,0,82,0,
 89,0,94,0,103,0,109,0,123,0,137,0,140,0,146,0,157,0,159,0,173,
@@ -102,7 +102,7 @@
     EVAL_ONE_SIZED_STR((char *)expr, 2091);
   }
   {
-    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,48,46,51,84,0,0,0,0,0,0,0,0,0,
+    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,49,46,51,84,0,0,0,0,0,0,0,0,0,
 0,0,0,0,0,0,0,0,0,0,0,183,0,0,0,1,0,0,8,0,16,
 0,29,0,34,0,51,0,63,0,85,0,114,0,158,0,164,0,178,0,193,0,
 211,0,223,0,239,0,253,0,19,1,39,1,73,1,90,1,107,1,130,1,145,
@@ -1011,7 +1011,7 @@
     EVAL_ONE_SIZED_STR((char *)expr, 19016);
   }
   {
-    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,48,46,51,84,0,0,0,0,0,0,0,0,0,
+    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,49,46,51,84,0,0,0,0,0,0,0,0,0,
 0,0,0,0,0,0,0,0,0,0,0,15,0,0,0,1,0,0,8,0,23,
 0,48,0,65,0,83,0,105,0,128,0,149,0,171,0,181,0,191,0,199,0,
 209,0,217,0,0,0,253,1,0,0,3,1,5,105,110,115,112,48,76,35,37,
@@ -1042,7 +1042,7 @@
     EVAL_ONE_SIZED_STR((char *)expr, 582);
   }
   {
-    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,48,46,51,84,0,0,0,0,0,0,0,0,0,
+    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,49,46,51,84,0,0,0,0,0,0,0,0,0,
 0,0,0,0,0,0,0,0,0,0,0,102,0,0,0,1,0,0,8,0,15,
 0,26,0,53,0,59,0,73,0,86,0,112,0,129,0,151,0,159,0,171,0,
 186,0,202,0,220,0,241,0,253,0,13,1,36,1,60,1,72,1,103,1,108,
@@ -1538,7 +1538,7 @@
     EVAL_ONE_SIZED_STR((char *)expr, 10344);
   }
   {
-    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,48,46,51,84,0,0,0,0,0,0,0,0,0,
+    SHARED_OK static MZCOMPILED_STRING_FAR unsigned char expr[] = {35,126,8,54,46,49,48,46,49,46,51,84,0,0,0,0,0,0,0,0,0,
 0,0,0,0,0,0,0,0,0,0,0,18,0,0,0,1,0,0,8,0,18,
 0,22,0,28,0,42,0,56,0,68,0,88,0,102,0,117,0,130,0,135,0,
 139,0,151,0,235,0,242,0,20,1,0,0,224,1,0,0,3,1,5,105,110,
diff --git a/racket/src/racket/src/port.c b/racket/src/racket/src/port.c
index ca3cea7b77..8a4ceddac0 100644
--- a/racket/src/racket/src/port.c
+++ b/racket/src/racket/src/port.c
@@ -6610,100 +6610,18 @@ void scheme_kill_green_thread_timer()
 
 #ifdef OS_X
 
-/* Sleep-in-thread support needed for GUIs Mac OS X.
-   To merge waiting on a CoreFoundation event with a select(), an embedding
-   application can attach a single socket to an event callback, and then
-   create a Mac thread to call the usual sleep and write to the socket when
-   data is available. */
-
-#ifdef MZ_PRECISE_GC
-START_XFORM_SKIP;
-#endif
-
-typedef struct {
-  pthread_mutex_t lock;
-  pthread_cond_t cond;
-  int count;
-} pt_sema_t;
-
-void pt_sema_init(pt_sema_t *sem)
+void scheme_start_sleeper_thread(void (*ignored_sleep)(float seconds, void *fds), float secs, void *fds, int hit_fd)
+  XFORM_SKIP_PROC
 {
-  pthread_mutex_init(&sem->lock, NULL);
-  pthread_cond_init(&sem->cond, NULL);
-  sem->count = 0;
-}
-
-void pt_sema_wait(pt_sema_t *sem)
-{
-  pthread_mutex_lock(&sem->lock);
-  while (sem->count <= 0)
-    pthread_cond_wait(&sem->cond, &sem->lock);
-  sem->count--;
-  pthread_mutex_unlock(&sem->lock);
-}
-
-void pt_sema_post(pt_sema_t *sem)
-{
-  pthread_mutex_lock(&sem->lock);
-  sem->count++;
-  if (sem->count > 0)
-    pthread_cond_signal(&sem->cond);
-  pthread_mutex_unlock(&sem->lock);
-}
-
-static pthread_t watcher;
-static pt_sema_t sleeping_sema, done_sema;
-static float sleep_secs;
-static int slept_fd;
-static void *sleep_fds;
-static void (*sleep_sleep)(float seconds, void *fds);
-
-static void *do_watch(void *other)
-{
-  scheme_init_os_thread_like(other);
-  while (1) {
-    pt_sema_wait(&sleeping_sema);
-
-    sleep_sleep(sleep_secs, sleep_fds);
-    write(slept_fd, "y", 1);
-
-    pt_sema_post(&done_sema);
-  }
-  return NULL;
-}
-
-void scheme_start_sleeper_thread(void (*given_sleep)(float seconds, void *fds), float secs, void *fds, int hit_fd)
-{
-  if (!watcher) {
-    pt_sema_init(&sleeping_sema);
-    pt_sema_init(&done_sema);
-
-    if (pthread_create(&watcher, NULL, do_watch, scheme_get_os_thread_like())) {
-      scheme_log_abort("pthread_create failed");
-      abort();
-    }
-  }
-
-  sleep_sleep = given_sleep;
-  sleep_fds = fds;
-  sleep_secs = secs;
-  slept_fd = hit_fd;
-  pt_sema_post(&sleeping_sema);
+  rktio_start_sleep(scheme_rktio, secs, fds, scheme_semaphore_fd_set, hit_fd);
 }
 
 void scheme_end_sleeper_thread()
+  XFORM_SKIP_PROC
 {
-  scheme_signal_received();
-  pt_sema_wait(&done_sema);
-
-  /* Clear external event flag */
-  rktio_flush_signals_received(scheme_rktio);
+  rktio_end_sleep(scheme_rktio);
 }
 
-#ifdef MZ_PRECISE_GC
-END_XFORM_SKIP;
-#endif
-
 #else
 
 void scheme_start_sleeper_thread(void (*given_sleep)(float seconds, void *fds), float secs, void *fds, int hit_fd)
diff --git a/racket/src/racket/src/schminc.h b/racket/src/racket/src/schminc.h
index 65f993268e..b37cab18dd 100644
--- a/racket/src/racket/src/schminc.h
+++ b/racket/src/racket/src/schminc.h
@@ -15,7 +15,7 @@
 #define USE_COMPILED_STARTUP 1
 
 #define EXPECTED_PRIM_COUNT 1159
-#define EXPECTED_UNSAFE_COUNT 142
+#define EXPECTED_UNSAFE_COUNT 148
 #define EXPECTED_FLFXNUM_COUNT 69
 #define EXPECTED_EXTFL_COUNT 45
 #define EXPECTED_FUTURES_COUNT 15
diff --git a/racket/src/racket/src/schpriv.h b/racket/src/racket/src/schpriv.h
index 6975dd521b..b1efac1107 100644
--- a/racket/src/racket/src/schpriv.h
+++ b/racket/src/racket/src/schpriv.h
@@ -602,6 +602,8 @@ extern Scheme_Object *scheme_equal_proc;
 
 extern Scheme_Object *scheme_def_exit_proc;
 
+extern Scheme_Object *scheme_unsafe_poller_proc;
+
 THREAD_LOCAL_DECL(extern Scheme_Object *scheme_orig_stdout_port);
 THREAD_LOCAL_DECL(extern Scheme_Object *scheme_orig_stdin_port);
 THREAD_LOCAL_DECL(extern Scheme_Object *scheme_orig_stderr_port);
diff --git a/racket/src/racket/src/schvers.h b/racket/src/racket/src/schvers.h
index 68a466a75c..3d8ed9af40 100644
--- a/racket/src/racket/src/schvers.h
+++ b/racket/src/racket/src/schvers.h
@@ -13,7 +13,7 @@
    consistently.)
 */
 
-#define MZSCHEME_VERSION "6.11.0.1"
+#define MZSCHEME_VERSION "6.11.0.2"
 
 #define MZSCHEME_VERSION_X 6
 #define MZSCHEME_VERSION_Y 11
diff --git a/racket/src/racket/src/struct.c b/racket/src/racket/src/struct.c
index add5d064a0..1b757b6197 100644
--- a/racket/src/racket/src/struct.c
+++ b/racket/src/racket/src/struct.c
@@ -51,8 +51,10 @@ READ_ONLY Scheme_Object *scheme_liberal_def_ctx_type;;
 READ_ONLY Scheme_Object *scheme_object_name_property;
 READ_ONLY Scheme_Object *scheme_struct_to_vector_proc;
 READ_ONLY Scheme_Object *scheme_authentic_property;
+READ_ONLY Scheme_Object *scheme_unsafe_poller_proc;
 
 READ_ONLY static Scheme_Object *location_struct;
+READ_ONLY static Scheme_Object *poller_struct;
 READ_ONLY static Scheme_Object *write_property;
 READ_ONLY static Scheme_Object *print_attribute_property;
 READ_ONLY static Scheme_Object *evt_property;
@@ -168,6 +170,7 @@ static void get_struct_type_info(int argc, Scheme_Object *argv[], Scheme_Object
 
 
 static int evt_struct_is_ready(Scheme_Object *o, Scheme_Schedule_Info *sinfo);
+static void evt_struct_needs_wakeup(Scheme_Object *o, void *fds);
 static int is_evt_struct(Scheme_Object *);
 
 static int wrapped_evt_is_ready(Scheme_Object *o, Scheme_Schedule_Info *sinfo);
@@ -221,6 +224,9 @@ static void register_traversers(void);
 THREAD_LOCAL_DECL(static Scheme_Bucket_Table *prefab_table);
 static Scheme_Object *make_prefab_key(Scheme_Struct_Type *type);
 
+#define SCHEME_POLLERP(v) (SCHEME_STRUCTP(v) && scheme_is_struct_instance(poller_struct, v))
+#define SCHEME_POLLER_PROC(o) (((Scheme_Structure *)(o))->slots[0])
+
 #define cons scheme_make_pair
 #define icons scheme_make_pair
 #define _intern scheme_intern_symbol
@@ -337,6 +343,11 @@ scheme_init_struct (Scheme_Env *env)
 			       env);
   }
 
+  /* Add poller structure: */
+  REGISTER_SO(poller_struct);
+  poller_struct = scheme_make_struct_type_from_string("unsafe-poller", NULL, 1, NULL, NULL, 1);
+  scheme_unsafe_poller_proc = make_struct_proc((Scheme_Struct_Type *)poller_struct, "unsafe-poller", SCHEME_CONSTR, 1);
+
   REGISTER_SO(write_property);
   {
     Scheme_Object *a[2], *pred, *access;
@@ -887,7 +898,7 @@ void scheme_init_struct_wait()
 {
   scheme_add_evt(scheme_structure_type,
                  (Scheme_Ready_Fun)evt_struct_is_ready,
-                 NULL,
+                 (Scheme_Needs_Wakeup_Fun)evt_struct_needs_wakeup,
                  is_evt_struct, 1);
   scheme_add_evt(scheme_proc_struct_type,
                  (Scheme_Ready_Fun)evt_struct_is_ready,
@@ -1612,11 +1623,20 @@ static Scheme_Object *check_indirect_property_value_ok(const char *name, Check_V
   return v;
 }
 
+static int is_evt_or_poller(Scheme_Object *v)
+{
+  if (scheme_is_evt(v))
+    return 1;
+  if (SCHEME_POLLERP(v))
+    return 1;
+  return 0;
+}
+
 static Scheme_Object *check_evt_property_value_ok(int argc, Scheme_Object *argv[])
 /* This is the guard for prop:evt */
 {
   return check_indirect_property_value_ok("guard-for-prop:evt", 
-                                          scheme_is_evt, 1,
+                                          is_evt_or_poller, 1,
                                           "(or/c evt? (any/c . -> . any) exact-nonnegative-integer?)",
                                           argc, argv);
 }
@@ -1682,9 +1702,68 @@ static int evt_struct_is_ready(Scheme_Object *o, Scheme_Schedule_Info *sinfo)
     }
   }
 
+  if (SCHEME_POLLERP(v)) {
+    Scheme_Thread *p;
+    Scheme_Object *a[2];
+    int done;
+    
+    scheme_start_in_scheduler();
+    a[0] = o;
+    a[1] = scheme_false;
+    v = _scheme_apply_multi(SCHEME_POLLER_PROC(v), 2, a);
+
+    p = scheme_current_thread;
+    if ((v == SCHEME_MULTIPLE_VALUES)
+        && (p->ku.multiple.count == 2)) {
+      if (SCHEME_FALSEP(p->ku.multiple.array[0])) {
+        v = p->ku.multiple.array[1];
+        if (v == o) v = NULL;
+        done = 0;
+      } else {
+        v = p->ku.multiple.array[0];
+        done = 1;
+      }
+    } else {
+      /* wrong number of results => treat as not ready */
+      v = NULL;
+      done = 0;
+    }
+    scheme_end_in_scheduler();
+
+    if (v) {
+      if (done && SCHEME_PROCP(v)) {
+        v = scheme_make_closed_prim_w_arity(return_wrapped, (void *)v, "wrapper", 1, 1);
+      }
+      scheme_set_sync_target(sinfo, v, (done ? v : NULL), NULL, 0, 0, NULL);
+      return 1;
+    }
+  }
+
   return 0;
 }
 
+void evt_struct_needs_wakeup(Scheme_Object *o, void *fds)
+{
+  Scheme_Object *v;
+  
+  /* Check for wakeup only if the struct has an immediate `unsafe-poller` */
+
+  if (SCHEME_CHAPERONEP(o))
+    return;
+  
+  v = scheme_struct_type_property_ref(evt_property, o);
+  if (SCHEME_POLLERP(v)) {
+    Scheme_Object *a[2], *e;
+    
+    scheme_start_in_scheduler();
+    a[0] = o;
+    e = scheme_make_cptr(fds, scheme_false);
+    a[1] = e;
+    _scheme_apply_multi(SCHEME_POLLER_PROC(v), 2, a);
+    scheme_end_in_scheduler();
+  }
+}
+
 static int is_evt_struct(Scheme_Object *o)
 {
   if (scheme_struct_type_property_ref(evt_property, o))
diff --git a/racket/src/racket/src/thread.c b/racket/src/racket/src/thread.c
index 17e4742713..6846e0c21f 100644
--- a/racket/src/racket/src/thread.c
+++ b/racket/src/racket/src/thread.c
@@ -183,6 +183,8 @@ THREAD_LOCAL_DECL(static volatile short delayed_break_ready);
 THREAD_LOCAL_DECL(static Scheme_Thread *main_break_target_thread);
 
 THREAD_LOCAL_DECL(Scheme_Sleep_Proc scheme_place_sleep);
+THREAD_LOCAL_DECL(Scheme_Object *thread_sleep_callback);
+THREAD_LOCAL_DECL(int thread_sleep_callback_fd);
 HOOK_SHARED_OK void (*scheme_sleep)(float seconds, void *fds);
 HOOK_SHARED_OK void (*scheme_notify_multithread)(int on);
 HOOK_SHARED_OK void (*scheme_wakeup_on_input)(void *fds);
@@ -401,6 +403,12 @@ static Scheme_Object *unsafe_start_breakable_atomic(int argc, Scheme_Object **ar
 static Scheme_Object *unsafe_end_breakable_atomic(int argc, Scheme_Object **argv);
 static Scheme_Object *unsafe_in_atomic_p(int argc, Scheme_Object **argv);
 
+static Scheme_Object *unsafe_poll_ctx_fd_wakeup(int argc, Scheme_Object **argv);
+static Scheme_Object *unsafe_poll_ctx_eventmask_wakeup(int argc, Scheme_Object **argv);
+static Scheme_Object *unsafe_poll_ctx_time_wakeup(int argc, Scheme_Object **argv);
+static Scheme_Object *unsafe_signal_received(int argc, Scheme_Object **argv);
+static Scheme_Object *unsafe_set_sleep_in_thread(int argc, Scheme_Object **argv);
+
 static void make_initial_config(Scheme_Thread *p);
 
 static int do_kill_thread(Scheme_Thread *p);
@@ -645,6 +653,13 @@ scheme_init_unsafe_thread (Scheme_Env *env)
   GLOBAL_PRIM_W_ARITY("unsafe-set-on-atomic-timeout!", unsafe_set_on_atomic_timeout, 1, 1, env);
 
   GLOBAL_PRIM_W_ARITY("unsafe-make-security-guard-at-root", unsafe_make_security_guard_at_root, 0, 3, env);
+
+  scheme_add_global_constant("unsafe-poller", scheme_unsafe_poller_proc, env);
+  GLOBAL_PRIM_W_ARITY("unsafe-poll-ctx-fd-wakeup", unsafe_poll_ctx_fd_wakeup, 3, 3, env);
+  GLOBAL_PRIM_W_ARITY("unsafe-poll-ctx-eventmask-wakeup", unsafe_poll_ctx_eventmask_wakeup, 2, 2, env);
+  GLOBAL_PRIM_W_ARITY("unsafe-poll-ctx-milliseconds-wakeup", unsafe_poll_ctx_time_wakeup, 2, 2, env);
+  GLOBAL_PRIM_W_ARITY("unsafe-signal-received", unsafe_signal_received, 0, 0, env);
+  GLOBAL_PRIM_W_ARITY("unsafe-set-sleep-in-thread!", unsafe_set_sleep_in_thread, 2, 2, env);
 }
 
 void scheme_init_thread_places(void) {
@@ -5313,6 +5328,88 @@ sch_sleep(int argc, Scheme_Object *args[])
   return scheme_void;
 }
 
+Scheme_Object *unsafe_poll_ctx_fd_wakeup(int argc, Scheme_Object **argv)
+{
+  if (SCHEME_TRUEP(argv[0])) {
+    void *fds = SCHEME_CPTR_VAL(argv[0]);
+    intptr_t fd;
+    int m;
+
+    if (SCHEME_INTP(argv[1]))
+      fd = SCHEME_INT_VAL(argv[1]);
+    else
+      fd = rktio_fd_system_fd(scheme_rktio, (rktio_fd_t *)SCHEME_CPTR_VAL(argv[1]));
+
+    if (SAME_OBJ(argv[2], read_symbol))
+      m = 0;
+    else if (SAME_OBJ(argv[2], write_symbol))
+      m = 1;
+    else
+      m = 2;
+    
+    scheme_fdset(scheme_get_fdset(fds, m), fd);
+  }
+
+  return scheme_void;
+}
+
+Scheme_Object *unsafe_poll_ctx_eventmask_wakeup(int argc, Scheme_Object **argv)
+{
+  if (SCHEME_TRUEP(argv[0])) {
+    void *fds = SCHEME_CPTR_VAL(argv[0]);
+    intptr_t mask = SCHEME_INT_VAL(argv[1]);
+
+    scheme_add_fd_eventmask(fds, mask);
+  }
+
+  return scheme_void;
+}
+
+Scheme_Object *unsafe_poll_ctx_time_wakeup(int argc, Scheme_Object **argv)
+{
+  if (SCHEME_TRUEP(argv[0])) {
+    void *fds = SCHEME_CPTR_VAL(argv[0]);
+    double msecs = SCHEME_DBL_VAL(argv[1]);
+
+    scheme_set_wakeup_time(fds, msecs);
+  }
+
+  return scheme_void;
+}
+
+Scheme_Object *unsafe_signal_received(int argc, Scheme_Object **argv)
+{
+  scheme_signal_received();
+  return scheme_void;
+}
+
+static void sleep_via_thread(float seconds, void *fds)
+{
+#ifdef OS_X
+  scheme_start_sleeper_thread(scheme_sleep, seconds, fds, thread_sleep_callback_fd);
+  scheme_start_in_scheduler();
+  _scheme_apply_multi(thread_sleep_callback, 0, NULL);
+  scheme_end_in_scheduler();
+  scheme_end_sleeper_thread();
+#endif
+}
+
+Scheme_Object *unsafe_set_sleep_in_thread(int argc, Scheme_Object **argv)
+{
+  if (!thread_sleep_callback)
+    REGISTER_SO(thread_sleep_callback);
+
+  thread_sleep_callback = argv[0];
+  if (SCHEME_INTP(argv[1]))
+    thread_sleep_callback_fd = SCHEME_INT_VAL(argv[1]);
+  else
+    thread_sleep_callback_fd = rktio_fd_system_fd(scheme_rktio, (rktio_fd_t *)SCHEME_CPTR_VAL(argv[1]));
+
+  scheme_place_sleep = sleep_via_thread;
+
+  return scheme_void;
+}
+
 static Scheme_Object *break_thread(int argc, Scheme_Object *args[])
 {
   Scheme_Thread *p;
diff --git a/racket/src/rktio/Makefile.in b/racket/src/rktio/Makefile.in
index ded7a7bd1d..53ee91789a 100644
--- a/racket/src/rktio/Makefile.in
+++ b/racket/src/rktio/Makefile.in
@@ -28,6 +28,7 @@ OBJS = rktio_fs.@LTO@ \
        rktio_file.@LTO@ \
        rktio_poll_set.@LTO@ \
        rktio_ltps.@LTO@ \
+       rktio_sleep.@LTO@ \
        rktio_network.@LTO@ \
        rktio_pipe.@LTO@ \
        rktio_process.@LTO@ \
@@ -76,6 +77,9 @@ rktio_poll_set.@LTO@: $(srcdir)/rktio_poll_set.c $(RKTIO_HEADERS)
 rktio_ltps.@LTO@: $(srcdir)/rktio_ltps.c $(RKTIO_HEADERS)
 	$(CC) $(CFLAGS) -I$(srcdir) -I. -o rktio_ltps.@LTO@ -c $(srcdir)/rktio_ltps.c
 
+rktio_sleep.@LTO@: $(srcdir)/rktio_sleep.c $(RKTIO_HEADERS)
+	$(CC) $(CFLAGS) -I$(srcdir) -I. -o rktio_sleep.@LTO@ -c $(srcdir)/rktio_sleep.c
+
 rktio_network.@LTO@: $(srcdir)/rktio_network.c $(RKTIO_HEADERS)
 	$(CC) $(CFLAGS) -I$(srcdir) -I. -o rktio_network.@LTO@ -c $(srcdir)/rktio_network.c
 
diff --git a/racket/src/rktio/rktio.h b/racket/src/rktio/rktio.h
index 3533b96804..45ef2ac3cc 100644
--- a/racket/src/rktio/rktio.h
+++ b/racket/src/rktio/rktio.h
@@ -736,6 +736,20 @@ RKTIO_EXTERN void rktio_sleep(rktio_t *rktio, float nsecs, rktio_poll_set_t *fds
    something registered with `fds` or `lt` is ready, or until there's
    some other activity that sometimes causes an early wakeup. */
 
+/*************************************************/
+/* Sleeping in a background thread               */
+
+RKTIO_EXTERN rktio_ok_t rktio_start_sleep(rktio_t *rktio, float nsecs, rktio_poll_set_t *fds, rktio_ltps_t *lt,
+                                          int woke_fd);
+/* Like `rktio_sleep`, but starts a sleep in a background thread. When the
+   background thread is done sleeping, it writes a byte to `woke_fd`, but the
+   background thread can be woken up with `rktio_end_sleep`. */
+
+RKTIO_EXTERN void rktio_end_sleep(rktio_t *rktio);
+/* Ends a background sleep started with `rktio_sleep`. Call this
+   function exactly once for each successful `rktio_start_sleep`,
+   whether or not the background thread write to `woke_fd` already. */
+
 /*************************************************/
 /* Files, directories, and links                 */
 
diff --git a/racket/src/rktio/rktio_main.c b/racket/src/rktio/rktio_main.c
index 0f068d36ac..595c89f998 100644
--- a/racket/src/rktio/rktio_main.c
+++ b/racket/src/rktio/rktio_main.c
@@ -38,6 +38,7 @@ rktio_t *rktio_init(void)
 
 void rktio_destroy(rktio_t *rktio)
 {
+  rktio_stop_background(rktio);
   rktio_syslog_clean(rktio);
   rktio_dll_clean(rktio);
   rktio_error_clean(rktio);
diff --git a/racket/src/rktio/rktio_private.h b/racket/src/rktio/rktio_private.h
index ef843e5bdd..b6267b4f54 100644
--- a/racket/src/rktio/rktio_private.h
+++ b/racket/src/rktio/rktio_private.h
@@ -26,6 +26,11 @@
 # define USE_FAR_RKTIO_FDCALLS
 #endif
 
+#if defined(RKTIO_SYSTEM_UNIX) && defined(RKTIO_USE_PTHREADS)
+# define SUPPORT_BACKGROUND_SLEEP_THREAD
+struct background_sleep_t;
+#endif
+
 /*========================================================================*/
 /* Globals, as gathered into `rktio_t`                                    */
 /*========================================================================*/
@@ -107,6 +112,10 @@ struct rktio_t {
 #ifdef RKTIO_SYSTEM_UNIX
   char *dll_error;
 #endif
+
+#ifdef SUPPORT_BACKGROUND_SLEEP_THREAD
+  struct background_sleep_t *background;
+#endif
 };
 
 /*========================================================================*/
@@ -329,6 +338,8 @@ char **rktio_get_environ_array(void);
 void rktio_syslog_init(rktio_t* rktio);
 void rktio_syslog_clean(rktio_t* rktio);
 
+void rktio_stop_background(rktio_t *rktio);
+
 #ifdef USE_TRANSITIONAL_64_FILE_OPS
 # define BIG_OFF_T_IZE(n) n ## 64
 #else
diff --git a/racket/src/rktio/rktio_sleep.c b/racket/src/rktio/rktio_sleep.c
index e69de29bb2..15de939a5d 100644
--- a/racket/src/rktio/rktio_sleep.c
+++ b/racket/src/rktio/rktio_sleep.c
@@ -0,0 +1,143 @@
+#include "rktio.h"
+#include "rktio_private.h"
+#ifdef SUPPORT_BACKGROUND_SLEEP_THREAD
+# include <string.h>
+# include <stdlib.h>
+# include <unistd.h>
+# include <errno.h>
+#endif
+
+/*========================================================================*/
+/* Sleeping in a separate OS thread                                       */
+/*========================================================================*/
+
+#ifdef SUPPORT_BACKGROUND_SLEEP_THREAD
+
+/* Sleep-in-thread support needed for GUIs Mac OS.
+   To merge waiting on a CoreFoundation event with a select(), an embedding
+   application can attach a single socket to an event callback, and then
+   create a Mac thread to call the usual sleep and write to the socket when
+   data is available. */
+
+typedef struct {
+  pthread_mutex_t lock;
+  pthread_cond_t cond;
+  int count;
+} pt_sema_t;
+
+static void pt_sema_init(pt_sema_t *sem)
+{
+  pthread_mutex_init(&sem->lock, NULL);
+  pthread_cond_init(&sem->cond, NULL);
+  sem->count = 0;
+}
+
+static void pt_sema_wait(pt_sema_t *sem)
+{
+  pthread_mutex_lock(&sem->lock);
+  while (sem->count <= 0)
+    pthread_cond_wait(&sem->cond, &sem->lock);
+  sem->count--;
+  pthread_mutex_unlock(&sem->lock);
+}
+
+static void pt_sema_post(pt_sema_t *sem)
+{
+  pthread_mutex_lock(&sem->lock);
+  sem->count++;
+  if (sem->count > 0)
+    pthread_cond_signal(&sem->cond);
+  pthread_mutex_unlock(&sem->lock);
+}
+
+typedef struct background_sleep_t {
+  pthread_t th;
+  pt_sema_t sleeping_sema, done_sema;
+  int done; /* => background thread should stop */
+  float nsecs;
+  int woke_fd;
+  rktio_poll_set_t *fds;
+  rktio_ltps_t *lt;
+} background_sleep_t;
+
+static void *do_background_sleep(void *_rktio)
+{
+  rktio_t *rktio = _rktio;
+  intptr_t len;
+
+  while (1) {
+    pt_sema_wait(&rktio->background->sleeping_sema);
+
+    if (rktio->background->done)
+      break;
+
+    rktio_sleep(rktio, rktio->background->nsecs, rktio->background->fds, rktio->background->lt);
+    do {
+      len = write(rktio->background->woke_fd, "y", 1);
+    } while ((len == -1) && (errno == EINTR));
+
+    pt_sema_post(&rktio->background->done_sema);
+  }
+  
+  return NULL;
+}
+
+rktio_ok_t rktio_start_sleep(rktio_t *rktio, float nsecs, rktio_poll_set_t *fds, rktio_ltps_t *lt, int woke_fd)
+{
+  if (!rktio->background) {
+    rktio->background = malloc(sizeof(background_sleep_t));
+    memset(rktio->background, 0, sizeof(background_sleep_t));
+    pt_sema_init(&rktio->background->sleeping_sema);
+    pt_sema_init(&rktio->background->done_sema);
+    
+    if (pthread_create(&rktio->background->th, NULL, do_background_sleep, rktio)) {
+      get_posix_error();
+      return 0;
+    }
+  }
+
+  rktio->background->nsecs = nsecs;
+  rktio->background->fds = fds;
+  rktio->background->lt = lt;
+  rktio->background->woke_fd = woke_fd;
+  pt_sema_post(&rktio->background->sleeping_sema);
+
+  return 1;
+}
+
+void rktio_end_sleep(rktio_t *rktio)
+{
+  rktio_signal_received(rktio);
+  pt_sema_wait(&rktio->background->done_sema);
+
+  /* Clear external event flag */
+  rktio_flush_signals_received(rktio);
+}
+
+void rktio_stop_background(rktio_t *rktio)
+{
+  if (rktio->background) {
+    rktio->background->done = 1;
+    pt_sema_post(&rktio->background->sleeping_sema);
+    pthread_join(rktio->background->th, NULL);
+    free(rktio->background);
+  }
+}
+
+#else
+
+rktio_ok_t rktio_start_sleep(rktio_t *rktio, float nsecs, rktio_poll_set_t *fds, rktio_ltps_t *lt, int woke_fd)
+{
+  set_racket_error(RKTIO_ERROR_UNSUPPORTED);
+  return 0;
+}
+
+void rktio_end_sleep(rktio_t *rktio)
+{
+}
+
+void rktio_stop_background(rktio_t *rktio)
+{
+}
+
+#endif
diff --git a/racket/src/worksp/librktio/librktio.vcproj b/racket/src/worksp/librktio/librktio.vcproj
index fdb622825e..4ac51ca32b 100644
--- a/racket/src/worksp/librktio/librktio.vcproj
+++ b/racket/src/worksp/librktio/librktio.vcproj
@@ -130,6 +130,10 @@
 				RelativePath="..\..\rktio\rktio_ltps.c"
 				>
 			</File>
+			<File
+				RelativePath="..\..\rktio\rktio_sleep.c"
+				>
+			</File>
 			<File
 				RelativePath="..\..\rktio\rktio_network.c"
 				>
diff --git a/racket/src/worksp/librktio/librktio.vcxproj b/racket/src/worksp/librktio/librktio.vcxproj
index 702136645f..513da9e50d 100644
--- a/racket/src/worksp/librktio/librktio.vcxproj
+++ b/racket/src/worksp/librktio/librktio.vcxproj
@@ -121,6 +121,7 @@
     <ClCompile Include="..\..\rktio\rktio_file.c" />
     <ClCompile Include="..\..\rktio\rktio_poll_set.c" />
     <ClCompile Include="..\..\rktio\rktio_ltps.c" />
+    <ClCompile Include="..\..\rktio\rktio_sleep.c" />
     <ClCompile Include="..\..\rktio\rktio_network.c" />
     <ClCompile Include="..\..\rktio\rktio_pipe.c" />
     <ClCompile Include="..\..\rktio\rktio_process.c" />