Merge in the new GIL.

Include/ceval.h

@@ -112,10 +112,6 @@ PyAPI_FUNC(PyObject *) PyEval_GetCallStats(PyObject *);
 PyAPI_FUNC(PyObject *) PyEval_EvalFrame(struct _frame *);
 PyAPI_FUNC(PyObject *) PyEval_EvalFrameEx(struct _frame *f, int exc);
 
-/* this used to be handled on a per-thread basis - now just two globals */
-PyAPI_DATA(volatile int) _Py_Ticker;
-PyAPI_DATA(int) _Py_CheckInterval;
-
 /* Interface for threads.
 
    A module that plans to do a blocking system call (or something else
@@ -174,6 +170,9 @@ PyAPI_FUNC(void) PyEval_AcquireThread(PyThreadState *tstate);
 PyAPI_FUNC(void) PyEval_ReleaseThread(PyThreadState *tstate);
 PyAPI_FUNC(void) PyEval_ReInitThreads(void);
 
+PyAPI_FUNC(void) _PyEval_SetSwitchInterval(unsigned long microseconds);
+PyAPI_FUNC(unsigned long) _PyEval_GetSwitchInterval(void);
+
 #define Py_BEGIN_ALLOW_THREADS { \
 			PyThreadState *_save; \
 			_save = PyEval_SaveThread();
@@ -192,6 +191,7 @@ PyAPI_FUNC(void) PyEval_ReInitThreads(void);
 #endif /* !WITH_THREAD */
 
 PyAPI_FUNC(int) _PyEval_SliceIndex(PyObject *, Py_ssize_t *);
+PyAPI_FUNC(void) _PyEval_SignalAsyncExc(void);
 
 
 #ifdef __cplusplus

Include/pystate.h

@@ -88,6 +88,8 @@ typedef struct _ts {
 
     PyObject *dict;  /* Stores per-thread state */
 
+    /* XXX doesn't mean anything anymore (the comment below is obsolete)
+       => deprecate or remove? */
     /* tick_counter is incremented whenever the check_interval ticker
      * reaches zero. The purpose is to give a useful measure of the number
      * of interpreted bytecode instructions in a given thread.  This

Include/sysmodule.h

@@ -18,7 +18,6 @@ PyAPI_FUNC(void) PySys_WriteStderr(const char *format, ...)
 			Py_GCC_ATTRIBUTE((format(printf, 1, 2)));
 
 PyAPI_DATA(PyObject *) _PySys_TraceFunc, *_PySys_ProfileFunc;
-PyAPI_DATA(int) _PySys_CheckInterval;
 
 PyAPI_FUNC(void) PySys_ResetWarnOptions(void);
 PyAPI_FUNC(void) PySys_AddWarnOption(const wchar_t *);

Lib/test/test_sys.py

@@ -154,6 +154,21 @@ class SysModuleTest(unittest.TestCase):
             sys.setcheckinterval(n)
             self.assertEquals(sys.getcheckinterval(), n)
 
+    def test_switchinterval(self):
+        self.assertRaises(TypeError, sys.setswitchinterval)
+        self.assertRaises(TypeError, sys.setswitchinterval, "a")
+        self.assertRaises(ValueError, sys.setswitchinterval, -1.0)
+        self.assertRaises(ValueError, sys.setswitchinterval, 0.0)
+        orig = sys.getswitchinterval()
+        # sanity check
+        self.assertTrue(orig < 0.5, orig)
+        try:
+            for n in 0.00001, 0.05, 3.0, orig:
+                sys.setswitchinterval(n)
+                self.assertAlmostEquals(sys.getswitchinterval(), n)
+        finally:
+            sys.setswitchinterval(orig)
+
     def test_recursionlimit(self):
         self.assertRaises(TypeError, sys.getrecursionlimit, 42)
         oldlimit = sys.getrecursionlimit()

Makefile.pre.in

@@ -596,7 +596,7 @@ Objects/unicodeobject.o: $(srcdir)/Objects/unicodeobject.c \
 $(OPCODETARGETS_H): $(OPCODETARGETGEN_FILES)
 	$(OPCODETARGETGEN) $(OPCODETARGETS_H)
 
-Python/ceval.o: $(OPCODETARGETS_H)
+Python/ceval.o: $(OPCODETARGETS_H) Python/ceval_gil.h
 
 Python/formatter_unicode.o: $(srcdir)/Python/formatter_unicode.c \
 				$(BYTESTR_DEPS) \

Objects/longobject.c

@@ -96,10 +96,7 @@ maybe_small_long(PyLongObject *v)
 #define MIN(x, y) ((x) > (y) ? (y) : (x))
 
 #define SIGCHECK(PyTryBlock) \
-	if (--_Py_Ticker < 0) { \
-		_Py_Ticker = _Py_CheckInterval; \
 	if (PyErr_CheckSignals()) PyTryBlock \
-	}
 
 /* forward declaration */
 static int bits_in_digit(digit d);

Python/ceval.c

@@ -216,6 +216,28 @@ PyEval_GetCallStats(PyObject *self)
 #endif
 
 
+#define COMPUTE_EVAL_BREAKER() \
+	(eval_breaker = gil_drop_request | pendingcalls_to_do | pending_async_exc)
+
+#define SET_GIL_DROP_REQUEST() \
+	do { gil_drop_request = 1; eval_breaker = 1; } while (0)
+
+#define RESET_GIL_DROP_REQUEST() \
+	do { gil_drop_request = 0; COMPUTE_EVAL_BREAKER(); } while (0)
+
+#define SIGNAL_PENDING_CALLS() \
+	do { pendingcalls_to_do = 1; eval_breaker = 1; } while (0)
+
+#define UNSIGNAL_PENDING_CALLS() \
+	do { pendingcalls_to_do = 0; COMPUTE_EVAL_BREAKER(); } while (0)
+
+#define SIGNAL_ASYNC_EXC() \
+	do { pending_async_exc = 1; eval_breaker = 1; } while (0)
+
+#define UNSIGNAL_ASYNC_EXC() \
+	do { pending_async_exc = 0; COMPUTE_EVAL_BREAKER(); } while (0)
+
+
 #ifdef WITH_THREAD
 
 #ifdef HAVE_ERRNO_H
@@ -223,36 +245,55 @@ PyEval_GetCallStats(PyObject *self)
 #endif
 #include "pythread.h"
 
-static PyThread_type_lock interpreter_lock = 0; /* This is the GIL */
 static PyThread_type_lock pending_lock = 0; /* for pending calls */
 static long main_thread = 0;
+/* This single variable consolidates all requests to break out of the fast path
+   in the eval loop. */
+static volatile int eval_breaker = 0;
+/* Request for droppping the GIL */
+static volatile int gil_drop_request = 0;
+/* Request for running pending calls */
+static volatile int pendingcalls_to_do = 0; 
+/* Request for looking at the `async_exc` field of the current thread state */
+static volatile int pending_async_exc = 0;
+
+#include "ceval_gil.h"
 
 int
 PyEval_ThreadsInitialized(void)
 {
-	return interpreter_lock != 0;
+	return gil_created();
 }
 
 void
 PyEval_InitThreads(void)
 {
-	if (interpreter_lock)
+	if (gil_created())
 		return;
-	interpreter_lock = PyThread_allocate_lock();
+	create_gil();
-	PyThread_acquire_lock(interpreter_lock, 1);
+	take_gil(PyThreadState_GET());
 	main_thread = PyThread_get_thread_ident();
+	if (!pending_lock)
+		pending_lock = PyThread_allocate_lock();
 }
 
 void
 PyEval_AcquireLock(void)
 {
-	PyThread_acquire_lock(interpreter_lock, 1);
+	PyThreadState *tstate = PyThreadState_GET();
+	if (tstate == NULL)
+		Py_FatalError("PyEval_AcquireLock: current thread state is NULL");
+	take_gil(tstate);
 }
 
 void
 PyEval_ReleaseLock(void)
 {
-	PyThread_release_lock(interpreter_lock);
+	/* This function must succeed when the current thread state is NULL.
+	   We therefore avoid PyThreadState_GET() which dumps a fatal error
+	   in debug mode.
+	*/
+	drop_gil(_PyThreadState_Current);
 }
 
 void
@@ -261,8 +302,8 @@ PyEval_AcquireThread(PyThreadState *tstate)
 	if (tstate == NULL)
 		Py_FatalError("PyEval_AcquireThread: NULL new thread state");
 	/* Check someone has called PyEval_InitThreads() to create the lock */
-	assert(interpreter_lock);
+	assert(gil_created());
-	PyThread_acquire_lock(interpreter_lock, 1);
+	take_gil(tstate);
 	if (PyThreadState_Swap(tstate) != NULL)
 		Py_FatalError(
 			"PyEval_AcquireThread: non-NULL old thread state");
@@ -275,7 +316,7 @@ PyEval_ReleaseThread(PyThreadState *tstate)
 		Py_FatalError("PyEval_ReleaseThread: NULL thread state");
 	if (PyThreadState_Swap(NULL) != tstate)
 		Py_FatalError("PyEval_ReleaseThread: wrong thread state");
-	PyThread_release_lock(interpreter_lock);
+	drop_gil(tstate);
 }
 
 /* This function is called from PyOS_AfterFork to ensure that newly
@@ -287,17 +328,17 @@ void
 PyEval_ReInitThreads(void)
 {
 	PyObject *threading, *result;
-	PyThreadState *tstate;
+	PyThreadState *tstate = PyThreadState_GET();
 
-	if (!interpreter_lock)
+	if (!gil_created())
 		return;
 	/*XXX Can't use PyThread_free_lock here because it does too
 	  much error-checking.  Doing this cleanly would require
 	  adding a new function to each thread_*.h.  Instead, just
 	  create a new lock and waste a little bit of memory */
-	interpreter_lock = PyThread_allocate_lock();
+	recreate_gil();
 	pending_lock = PyThread_allocate_lock();
-	PyThread_acquire_lock(interpreter_lock, 1);
+	take_gil(tstate);
 	main_thread = PyThread_get_thread_ident();
 
 	/* Update the threading module with the new state.
@@ -317,7 +358,21 @@ PyEval_ReInitThreads(void)
 		Py_DECREF(result);
 	Py_DECREF(threading);
 }
-#endif
+
+#else
+static int eval_breaker = 0;
+static int gil_drop_request = 0;
+static int pending_async_exc = 0;
+#endif /* WITH_THREAD */
+
+/* This function is used to signal that async exceptions are waiting to be
+   raised, therefore it is also useful in non-threaded builds. */
+
+void
+_PyEval_SignalAsyncExc(void)
+{
+	SIGNAL_ASYNC_EXC();
+}
 
 /* Functions save_thread and restore_thread are always defined so
    dynamically loaded modules needn't be compiled separately for use
@@ -330,8 +385,8 @@ PyEval_SaveThread(void)
 	if (tstate == NULL)
 		Py_FatalError("PyEval_SaveThread: NULL tstate");
 #ifdef WITH_THREAD
-	if (interpreter_lock)
+	if (gil_created())
-		PyThread_release_lock(interpreter_lock);
+		drop_gil(tstate);
 #endif
 	return tstate;
 }
@@ -342,9 +397,9 @@ PyEval_RestoreThread(PyThreadState *tstate)
 	if (tstate == NULL)
 		Py_FatalError("PyEval_RestoreThread: NULL tstate");
 #ifdef WITH_THREAD
-	if (interpreter_lock) {
+	if (gil_created()) {
 		int err = errno;
-		PyThread_acquire_lock(interpreter_lock, 1);
+		take_gil(tstate);
 		errno = err;
 	}
 #endif
@@ -390,7 +445,6 @@ static struct {
 } pendingcalls[NPENDINGCALLS];
 static int pendingfirst = 0;
 static int pendinglast = 0;
-static volatile int pendingcalls_to_do = 1; /* trigger initialization of lock */
 static char pendingbusy = 0;
 
 int
@@ -429,8 +483,7 @@ Py_AddPendingCall(int (*func)(void *), void *arg)
 		pendinglast = j;
 	}
 	/* signal main loop */
-	_Py_Ticker = 0;
+	SIGNAL_PENDING_CALLS();
-	pendingcalls_to_do = 1;
 	if (lock != NULL)
 		PyThread_release_lock(lock);
 	return result;
@@ -472,7 +525,10 @@ Py_MakePendingCalls(void)
 			arg = pendingcalls[j].arg;
 			pendingfirst = (j + 1) % NPENDINGCALLS;
 		}
-		pendingcalls_to_do = pendingfirst != pendinglast;
+		if (pendingfirst != pendinglast)
+			SIGNAL_PENDING_CALLS();
+		else
+			UNSIGNAL_PENDING_CALLS();
 		PyThread_release_lock(pending_lock);
 		/* having released the lock, perform the callback */
 		if (func == NULL)
@@ -538,8 +594,7 @@ Py_AddPendingCall(int (*func)(void *), void *arg)
 	pendingcalls[i].arg = arg;
 	pendinglast = j;
 
-	_Py_Ticker = 0;
+	SIGNAL_PENDING_CALLS();
-	pendingcalls_to_do = 1; /* Signal main loop */
 	busy = 0;
 	/* XXX End critical section */
 	return 0;
@@ -552,7 +607,7 @@ Py_MakePendingCalls(void)
 	if (busy)
 		return 0;
 	busy = 1;
-	pendingcalls_to_do = 0;
+	UNSIGNAL_PENDING_CALLS();
 	for (;;) {
 		int i;
 		int (*func)(void *);
@@ -565,7 +620,7 @@ Py_MakePendingCalls(void)
 		pendingfirst = (i + 1) % NPENDINGCALLS;
 		if (func(arg) < 0) {
 			busy = 0;
-			pendingcalls_to_do = 1; /* We're not done yet */
+			SIGNAL_PENDING_CALLS(); /* We're not done yet */
 			return -1;
 		}
 	}
@@ -658,10 +713,7 @@ static int unpack_iterable(PyObject *, int, int, PyObject **);
    fast_next_opcode*/
 static int _Py_TracingPossible = 0;
 
-/* for manipulating the thread switch and periodic "stuff" - used to be
+
-   per thread, now just a pair o' globals */
-int _Py_CheckInterval = 100;
-volatile int _Py_Ticker = 0; /* so that we hit a "tick" first thing */
 
 PyObject *
 PyEval_EvalCode(PyCodeObject *co, PyObject *globals, PyObject *locals)
@@ -791,10 +843,7 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 
 #define DISPATCH() \
 	{ \
-		/* Avoid multiple loads from _Py_Ticker despite `volatile` */ \
+		if (!eval_breaker) { \
-		int _tick = _Py_Ticker - 1; \
-		_Py_Ticker = _tick; \
-		if (_tick >= 0) { \
 			FAST_DISPATCH(); \
 		} \
 		continue; \
@@ -1168,13 +1217,12 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 		   async I/O handler); see Py_AddPendingCall() and
 		   Py_MakePendingCalls() above. */
 
-		if (--_Py_Ticker < 0) {
+		if (eval_breaker) {
 			if (*next_instr == SETUP_FINALLY) {
 				/* Make the last opcode before
 				   a try: finally: block uninterruptable. */
 				goto fast_next_opcode;
 			}
-			_Py_Ticker = _Py_CheckInterval;
 			tstate->tick_counter++;
 #ifdef WITH_TSC
 			ticked = 1;
@@ -1184,40 +1232,32 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 					why = WHY_EXCEPTION;
 					goto on_error;
 				}
-				if (pendingcalls_to_do)
-					/* MakePendingCalls() didn't succeed.
-					   Force early re-execution of this
-					   "periodic" code, possibly after
-					   a thread switch */
-					_Py_Ticker = 0;
 			}
+			if (gil_drop_request) {
 #ifdef WITH_THREAD
-			if (interpreter_lock) {
 				/* Give another thread a chance */
-
 				if (PyThreadState_Swap(NULL) != tstate)
 					Py_FatalError("ceval: tstate mix-up");
-				PyThread_release_lock(interpreter_lock);
+				drop_gil(tstate);
 	
 				/* Other threads may run now */
 	
-				PyThread_acquire_lock(interpreter_lock, 1);
+				take_gil(tstate);
 				if (PyThreadState_Swap(tstate) != NULL)
 					Py_FatalError("ceval: orphan tstate");
-
+#endif
-				/* Check for thread interrupts */
+			}
-
+			/* Check for asynchronous exceptions. */
 			if (tstate->async_exc != NULL) {
 				x = tstate->async_exc;
 				tstate->async_exc = NULL;
+				UNSIGNAL_ASYNC_EXC();
 				PyErr_SetNone(x);
 				Py_DECREF(x);
 				why = WHY_EXCEPTION;
 				goto on_error;
 			}
 		}
-#endif
-		}
 
 	fast_next_opcode:
 		f->f_lasti = INSTR_OFFSET();

Python/ceval_gil.h

@@ -0,0 +1,335 @@
+/*
+ * Implementation of the Global Interpreter Lock (GIL).
+ */
+
+#include <stdlib.h>
+#include <errno.h>
+
+
+/* First some general settings */
+
+/* microseconds (the Python API uses seconds, though) */
+#define DEFAULT_INTERVAL 5000
+static unsigned long gil_interval = DEFAULT_INTERVAL;
+#define INTERVAL (gil_interval >= 1 ? gil_interval : 1)
+
+/* Enable if you want to force the switching of threads at least every `gil_interval` */
+#undef FORCE_SWITCHING
+#define FORCE_SWITCHING
+
+
+/*
+   Notes about the implementation:
+
+   - The GIL is just a boolean variable (gil_locked) whose access is protected
+     by a mutex (gil_mutex), and whose changes are signalled by a condition
+     variable (gil_cond). gil_mutex is taken for short periods of time,
+     and therefore mostly uncontended.
+
+   - In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
+     able to release the GIL on demand by another thread. A volatile boolean
+     variable (gil_drop_request) is used for that purpose, which is checked
+     at every turn of the eval loop. That variable is set after a wait of
+     `interval` microseconds on `gil_cond` has timed out.
+      
+      [Actually, another volatile boolean variable (eval_breaker) is used
+       which ORs several conditions into one. Volatile booleans are
+       sufficient as inter-thread signalling means since Python is run
+       on cache-coherent architectures only.]
+
+   - A thread wanting to take the GIL will first let pass a given amount of
+     time (`interval` microseconds) before setting gil_drop_request. This
+     encourages a defined switching period, but doesn't enforce it since
+     opcodes can take an arbitrary time to execute.
+ 
+     The `interval` value is available for the user to read and modify
+     using the Python API `sys.{get,set}switchinterval()`.
+
+   - When a thread releases the GIL and gil_drop_request is set, that thread
+     ensures that another GIL-awaiting thread gets scheduled.
+     It does so by waiting on a condition variable (switch_cond) until
+     the value of gil_last_holder is changed to something else than its
+     own thread state pointer, indicating that another thread was able to
+     take the GIL.
+ 
+     This is meant to prohibit the latency-adverse behaviour on multi-core
+     machines where one thread would speculatively release the GIL, but still
+     run and end up being the first to re-acquire it, making the "timeslices"
+     much longer than expected.
+     (Note: this mechanism is enabled with FORCE_SWITCHING above)
+*/
+
+#ifndef _POSIX_THREADS
+/* This means pthreads are not implemented in libc headers, hence the macro
+   not present in unistd.h. But they still can be implemented as an external
+   library (e.g. gnu pth in pthread emulation) */
+# ifdef HAVE_PTHREAD_H
+#  include <pthread.h> /* _POSIX_THREADS */
+# endif
+#endif
+
+
+#ifdef _POSIX_THREADS
+
+/*
+ * POSIX support
+ */
+
+#include <pthread.h>
+
+#define ADD_MICROSECONDS(tv, interval) \
+do { \
+    tv.tv_usec += (long) interval; \
+    tv.tv_sec += tv.tv_usec / 1000000; \
+    tv.tv_usec %= 1000000; \
+} while (0)
+
+/* We assume all modern POSIX systems have gettimeofday() */
+#ifdef GETTIMEOFDAY_NO_TZ
+#define GETTIMEOFDAY(ptv) gettimeofday(ptv)
+#else
+#define GETTIMEOFDAY(ptv) gettimeofday(ptv, (struct timezone *)NULL)
+#endif
+
+#define MUTEX_T pthread_mutex_t
+#define MUTEX_INIT(mut) \
+    if (pthread_mutex_init(&mut, NULL)) { \
+        Py_FatalError("pthread_mutex_init(" #mut ") failed"); };
+#define MUTEX_LOCK(mut) \
+    if (pthread_mutex_lock(&mut)) { \
+        Py_FatalError("pthread_mutex_lock(" #mut ") failed"); };
+#define MUTEX_UNLOCK(mut) \
+    if (pthread_mutex_unlock(&mut)) { \
+        Py_FatalError("pthread_mutex_unlock(" #mut ") failed"); };
+
+#define COND_T pthread_cond_t
+#define COND_INIT(cond) \
+    if (pthread_cond_init(&cond, NULL)) { \
+        Py_FatalError("pthread_cond_init(" #cond ") failed"); };
+#define COND_PREPARE(cond)
+#define COND_SIGNAL(cond) \
+    if (pthread_cond_signal(&cond)) { \
+        Py_FatalError("pthread_cond_signal(" #cond ") failed"); };
+#define COND_WAIT(cond, mut) \
+    if (pthread_cond_wait(&cond, &mut)) { \
+        Py_FatalError("pthread_cond_wait(" #cond ") failed"); };
+#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
+    { \
+        int r; \
+        struct timespec ts; \
+        struct timeval deadline; \
+        \
+        GETTIMEOFDAY(&deadline); \
+        ADD_MICROSECONDS(deadline, microseconds); \
+        ts.tv_sec = deadline.tv_sec; \
+        ts.tv_nsec = deadline.tv_usec * 1000; \
+        \
+        r = pthread_cond_timedwait(&cond, &mut, &ts); \
+        if (r == ETIMEDOUT) \
+            timeout_result = 1; \
+        else if (r) \
+            Py_FatalError("pthread_cond_timedwait(" #cond ") failed"); \
+        else \
+            timeout_result = 0; \
+    } \
+
+#elif defined(NT_THREADS)
+
+/*
+ * Windows (2000 and later, as well as (hopefully) CE) support
+ */
+
+#include <windows.h>
+
+#define MUTEX_T HANDLE
+#define MUTEX_INIT(mut) \
+    if (!(mut = CreateMutex(NULL, FALSE, NULL))) { \
+        Py_FatalError("CreateMutex(" #mut ") failed"); };
+#define MUTEX_LOCK(mut) \
+    if (WaitForSingleObject(mut, INFINITE) != WAIT_OBJECT_0) { \
+        Py_FatalError("WaitForSingleObject(" #mut ") failed"); };
+#define MUTEX_UNLOCK(mut) \
+    if (!ReleaseMutex(mut)) { \
+        Py_FatalError("ReleaseMutex(" #mut ") failed"); };
+
+/* We emulate condition variables with events. It is sufficient here.
+   (WaitForMultipleObjects() allows the event to be caught and the mutex
+   to be taken atomically) */
+#define COND_T HANDLE
+#define COND_INIT(cond) \
+    /* auto-reset, non-signalled */ \
+    if (!(cond = CreateEvent(NULL, FALSE, FALSE, NULL))) { \
+        Py_FatalError("CreateMutex(" #cond ") failed"); };
+#define COND_PREPARE(cond) \
+    if (!ResetEvent(cond)) { \
+        Py_FatalError("ResetEvent(" #cond ") failed"); };
+#define COND_SIGNAL(cond) \
+    if (!SetEvent(cond)) { \
+        Py_FatalError("SetEvent(" #cond ") failed"); };
+#define COND_WAIT(cond, mut) \
+    { \
+        DWORD r; \
+        HANDLE objects[2] = { cond, mut }; \
+        MUTEX_UNLOCK(mut); \
+        r = WaitForMultipleObjects(2, objects, TRUE, INFINITE); \
+        if (r != WAIT_OBJECT_0) \
+            Py_FatalError("WaitForSingleObject(" #cond ") failed"); \
+    }
+#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
+    { \
+        DWORD r; \
+        HANDLE objects[2] = { cond, mut }; \
+        MUTEX_UNLOCK(mut); \
+        r = WaitForMultipleObjects(2, objects, TRUE, microseconds / 1000); \
+        if (r == WAIT_TIMEOUT) { \
+            MUTEX_LOCK(mut); \
+            timeout_result = 1; \
+        } \
+        else if (r != WAIT_OBJECT_0) \
+            Py_FatalError("WaitForSingleObject(" #cond ") failed"); \
+        else \
+            timeout_result = 0; \
+    }
+
+#else
+
+#error You need either a POSIX-compatible or a Windows system!
+
+#endif /* _POSIX_THREADS, NT_THREADS */
+
+
+/* Whether the GIL is already taken (-1 if uninitialized). This is volatile
+   because it can be read without any lock taken in ceval.c. */
+static volatile int gil_locked = -1;
+/* Number of GIL switches since the beginning. */
+static unsigned long gil_switch_number = 0;
+/* Last thread holding / having held the GIL. This helps us know whether
+   anyone else was scheduled after we dropped the GIL. */
+static PyThreadState *gil_last_holder = NULL;
+
+/* This condition variable allows one or several threads to wait until
+   the GIL is released. In addition, the mutex also protects the above
+   variables. */
+static COND_T gil_cond;
+static MUTEX_T gil_mutex;
+
+#ifdef FORCE_SWITCHING
+/* This condition variable helps the GIL-releasing thread wait for
+   a GIL-awaiting thread to be scheduled and take the GIL. */
+static COND_T switch_cond;
+static MUTEX_T switch_mutex;
+#endif
+
+
+static int gil_created(void)
+{
+    return gil_locked >= 0;
+}
+
+static void create_gil(void)
+{
+    MUTEX_INIT(gil_mutex);
+#ifdef FORCE_SWITCHING
+    MUTEX_INIT(switch_mutex);
+#endif
+    COND_INIT(gil_cond);
+#ifdef FORCE_SWITCHING
+    COND_INIT(switch_cond);
+#endif
+    gil_locked = 0;
+    gil_last_holder = NULL;
+}
+
+static void recreate_gil(void)
+{
+    create_gil();
+}
+
+static void drop_gil(PyThreadState *tstate)
+{
+    /* NOTE: tstate is allowed to be NULL. */
+    if (!gil_locked)
+        Py_FatalError("drop_gil: GIL is not locked");
+    if (tstate != NULL && tstate != gil_last_holder)
+        Py_FatalError("drop_gil: wrong thread state");
+
+    MUTEX_LOCK(gil_mutex);
+    gil_locked = 0;
+    COND_SIGNAL(gil_cond);
+#ifdef FORCE_SWITCHING
+    COND_PREPARE(switch_cond);
+#endif
+    MUTEX_UNLOCK(gil_mutex);
+    
+#ifdef FORCE_SWITCHING
+    if (gil_drop_request) {
+        MUTEX_LOCK(switch_mutex);
+        /* Not switched yet => wait */
+        if (gil_last_holder == tstate)
+            COND_WAIT(switch_cond, switch_mutex);
+        MUTEX_UNLOCK(switch_mutex);
+    }
+#endif
+}
+
+static void take_gil(PyThreadState *tstate)
+{
+    int err;
+    if (tstate == NULL)
+        Py_FatalError("take_gil: NULL tstate");
+
+    err = errno;
+    MUTEX_LOCK(gil_mutex);
+
+    if (!gil_locked)
+        goto _ready;
+    
+    COND_PREPARE(gil_cond);
+    while (gil_locked) {
+        int timed_out = 0;
+        unsigned long saved_switchnum;
+
+        saved_switchnum = gil_switch_number;
+        COND_TIMED_WAIT(gil_cond, gil_mutex, INTERVAL, timed_out);
+        /* If we timed out and no switch occurred in the meantime, it is time
+           to ask the GIL-holding thread to drop it. */
+        if (timed_out && gil_locked && gil_switch_number == saved_switchnum) {
+            SET_GIL_DROP_REQUEST();
+        }
+    }
+_ready:
+#ifdef FORCE_SWITCHING
+    /* This mutex must be taken before modifying gil_last_holder (see drop_gil()). */
+    MUTEX_LOCK(switch_mutex);
+#endif
+    /* We now hold the GIL */
+    gil_locked = 1;
+
+    if (tstate != gil_last_holder) {
+        gil_last_holder = tstate;
+        ++gil_switch_number;
+    }
+#ifdef FORCE_SWITCHING
+    COND_SIGNAL(switch_cond);
+    MUTEX_UNLOCK(switch_mutex);
+#endif
+    if (gil_drop_request) {
+        RESET_GIL_DROP_REQUEST();
+    }
+    if (tstate->async_exc != NULL) {
+        _PyEval_SignalAsyncExc();
+    }
+    
+    MUTEX_UNLOCK(gil_mutex);
+    errno = err;
+}
+
+void _PyEval_SetSwitchInterval(unsigned long microseconds)
+{
+    gil_interval = microseconds;
+}
+
+unsigned long _PyEval_GetSwitchInterval()
+{
+    return gil_interval;
+}

Python/pystate.c

@@ -434,6 +434,7 @@ PyThreadState_SetAsyncExc(long id, PyObject *exc) {
 			p->async_exc = exc;
 			HEAD_UNLOCK();
 			Py_XDECREF(old_exc);
+			_PyEval_SignalAsyncExc();
 			return 1;
 		}
 	}

Python/sysmodule.c

@@ -448,10 +448,18 @@ Return the profiling function set with sys.setprofile.\n\
 See the profiler chapter in the library manual."
 );
 
+/* TODO: deprecate */
+static int _check_interval = 100;
+
 static PyObject *
 sys_setcheckinterval(PyObject *self, PyObject *args)
 {
-	if (!PyArg_ParseTuple(args, "i:setcheckinterval", &_Py_CheckInterval))
+	if (PyErr_WarnEx(PyExc_DeprecationWarning,
+			 "sys.getcheckinterval() and sys.setcheckinterval() "
+			 "are deprecated.  Use sys.setswitchinterval() "
+			 "instead.", 1) < 0)
+		return NULL;
+	if (!PyArg_ParseTuple(args, "i:setcheckinterval", &_check_interval))
 		return NULL;
 	Py_INCREF(Py_None);
 	return Py_None;
@@ -467,13 +475,59 @@ n instructions.  This also affects how often thread switches occur."
 static PyObject *
 sys_getcheckinterval(PyObject *self, PyObject *args)
 {
-	return PyLong_FromLong(_Py_CheckInterval);
+	if (PyErr_WarnEx(PyExc_DeprecationWarning,
+			 "sys.getcheckinterval() and sys.setcheckinterval() "
+			 "are deprecated.  Use sys.getswitchinterval() "
+			 "instead.", 1) < 0)
+		return NULL;
+	return PyLong_FromLong(_check_interval);
 }
 
 PyDoc_STRVAR(getcheckinterval_doc,
 "getcheckinterval() -> current check interval; see setcheckinterval()."
 );
 
+#ifdef WITH_THREAD
+static PyObject *
+sys_setswitchinterval(PyObject *self, PyObject *args)
+{
+	double d;
+	if (!PyArg_ParseTuple(args, "d:setswitchinterval", &d))
+		return NULL;
+	if (d <= 0.0) {
+		PyErr_SetString(PyExc_ValueError,
+				"switch interval must be strictly positive");
+		return NULL;
+	}
+	_PyEval_SetSwitchInterval((unsigned long) (1e6 * d));
+	Py_INCREF(Py_None);
+	return Py_None;
+}
+
+PyDoc_STRVAR(setswitchinterval_doc,
+"setswitchinterval(n)\n\
+\n\
+Set the ideal thread switching delay inside the Python interpreter\n\
+The actual frequency of switching threads can be lower if the\n\
+interpreter executes long sequences of uninterruptible code\n\
+(this is implementation-specific and workload-dependent).\n\
+\n\
+The parameter must represent the desired switching delay in seconds\n\
+A typical value is 0.005 (5 milliseconds)."
+);
+
+static PyObject *
+sys_getswitchinterval(PyObject *self, PyObject *args)
+{
+	return PyFloat_FromDouble(1e-6 * _PyEval_GetSwitchInterval());
+}
+
+PyDoc_STRVAR(getswitchinterval_doc,
+"getswitchinterval() -> current thread switch interval; see setswitchinterval()."
+);
+
+#endif /* WITH_THREAD */
+
 #ifdef WITH_TSC
 static PyObject *
 sys_settscdump(PyObject *self, PyObject *args)
@@ -895,6 +949,12 @@ static PyMethodDef sys_methods[] = {
 	 setcheckinterval_doc},
 	{"getcheckinterval",	sys_getcheckinterval, METH_NOARGS,
 	 getcheckinterval_doc},
+#ifdef WITH_THREAD
+	{"setswitchinterval",	sys_setswitchinterval, METH_VARARGS,
+	 setswitchinterval_doc},
+	{"getswitchinterval",	sys_getswitchinterval, METH_NOARGS,
+	 getswitchinterval_doc},
+#endif
 #ifdef HAVE_DLOPEN
 	{"setdlopenflags", sys_setdlopenflags, METH_VARARGS,
 	 setdlopenflags_doc},