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a1d23326b1
If it doesn't work (doesn't solve erratic freezes) we'll have to resort to tougher (Windows-only) measures.
348 lines
11 KiB
C
348 lines
11 KiB
C
/*
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* Implementation of the Global Interpreter Lock (GIL).
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*/
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#include <stdlib.h>
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#include <errno.h>
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/* First some general settings */
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/* microseconds (the Python API uses seconds, though) */
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#define DEFAULT_INTERVAL 5000
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static unsigned long gil_interval = DEFAULT_INTERVAL;
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#define INTERVAL (gil_interval >= 1 ? gil_interval : 1)
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/* Enable if you want to force the switching of threads at least every `gil_interval` */
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#undef FORCE_SWITCHING
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#define FORCE_SWITCHING
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/*
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Notes about the implementation:
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- The GIL is just a boolean variable (gil_locked) whose access is protected
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by a mutex (gil_mutex), and whose changes are signalled by a condition
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variable (gil_cond). gil_mutex is taken for short periods of time,
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and therefore mostly uncontended.
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- In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
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able to release the GIL on demand by another thread. A volatile boolean
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variable (gil_drop_request) is used for that purpose, which is checked
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at every turn of the eval loop. That variable is set after a wait of
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`interval` microseconds on `gil_cond` has timed out.
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[Actually, another volatile boolean variable (eval_breaker) is used
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which ORs several conditions into one. Volatile booleans are
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sufficient as inter-thread signalling means since Python is run
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on cache-coherent architectures only.]
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- A thread wanting to take the GIL will first let pass a given amount of
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time (`interval` microseconds) before setting gil_drop_request. This
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encourages a defined switching period, but doesn't enforce it since
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opcodes can take an arbitrary time to execute.
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The `interval` value is available for the user to read and modify
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using the Python API `sys.{get,set}switchinterval()`.
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- When a thread releases the GIL and gil_drop_request is set, that thread
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ensures that another GIL-awaiting thread gets scheduled.
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It does so by waiting on a condition variable (switch_cond) until
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the value of gil_last_holder is changed to something else than its
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own thread state pointer, indicating that another thread was able to
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take the GIL.
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This is meant to prohibit the latency-adverse behaviour on multi-core
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machines where one thread would speculatively release the GIL, but still
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run and end up being the first to re-acquire it, making the "timeslices"
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much longer than expected.
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(Note: this mechanism is enabled with FORCE_SWITCHING above)
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*/
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#ifndef _POSIX_THREADS
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/* This means pthreads are not implemented in libc headers, hence the macro
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not present in unistd.h. But they still can be implemented as an external
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library (e.g. gnu pth in pthread emulation) */
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# ifdef HAVE_PTHREAD_H
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# include <pthread.h> /* _POSIX_THREADS */
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# endif
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#endif
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#ifdef _POSIX_THREADS
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/*
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* POSIX support
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*/
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#include <pthread.h>
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#define ADD_MICROSECONDS(tv, interval) \
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do { \
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tv.tv_usec += (long) interval; \
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tv.tv_sec += tv.tv_usec / 1000000; \
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tv.tv_usec %= 1000000; \
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} while (0)
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/* We assume all modern POSIX systems have gettimeofday() */
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#ifdef GETTIMEOFDAY_NO_TZ
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#define GETTIMEOFDAY(ptv) gettimeofday(ptv)
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#else
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#define GETTIMEOFDAY(ptv) gettimeofday(ptv, (struct timezone *)NULL)
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#endif
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#define MUTEX_T pthread_mutex_t
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#define MUTEX_INIT(mut) \
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if (pthread_mutex_init(&mut, NULL)) { \
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Py_FatalError("pthread_mutex_init(" #mut ") failed"); };
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#define MUTEX_LOCK(mut) \
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if (pthread_mutex_lock(&mut)) { \
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Py_FatalError("pthread_mutex_lock(" #mut ") failed"); };
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#define MUTEX_UNLOCK(mut) \
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if (pthread_mutex_unlock(&mut)) { \
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Py_FatalError("pthread_mutex_unlock(" #mut ") failed"); };
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#define COND_T pthread_cond_t
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#define COND_INIT(cond) \
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if (pthread_cond_init(&cond, NULL)) { \
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Py_FatalError("pthread_cond_init(" #cond ") failed"); };
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#define COND_RESET(cond)
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#define COND_SIGNAL(cond) \
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if (pthread_cond_signal(&cond)) { \
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Py_FatalError("pthread_cond_signal(" #cond ") failed"); };
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#define COND_WAIT(cond, mut) \
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if (pthread_cond_wait(&cond, &mut)) { \
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Py_FatalError("pthread_cond_wait(" #cond ") failed"); };
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#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
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{ \
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int r; \
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struct timespec ts; \
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struct timeval deadline; \
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\
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GETTIMEOFDAY(&deadline); \
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ADD_MICROSECONDS(deadline, microseconds); \
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ts.tv_sec = deadline.tv_sec; \
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ts.tv_nsec = deadline.tv_usec * 1000; \
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\
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r = pthread_cond_timedwait(&cond, &mut, &ts); \
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if (r == ETIMEDOUT) \
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timeout_result = 1; \
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else if (r) \
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Py_FatalError("pthread_cond_timedwait(" #cond ") failed"); \
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else \
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timeout_result = 0; \
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} \
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#elif defined(NT_THREADS)
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/*
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* Windows (2000 and later, as well as (hopefully) CE) support
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*/
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#include <windows.h>
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#define MUTEX_T HANDLE
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#define MUTEX_INIT(mut) \
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if (!(mut = CreateMutex(NULL, FALSE, NULL))) { \
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Py_FatalError("CreateMutex(" #mut ") failed"); };
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#define MUTEX_LOCK(mut) \
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if (WaitForSingleObject(mut, INFINITE) != WAIT_OBJECT_0) { \
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Py_FatalError("WaitForSingleObject(" #mut ") failed"); };
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#define MUTEX_UNLOCK(mut) \
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if (!ReleaseMutex(mut)) { \
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Py_FatalError("ReleaseMutex(" #mut ") failed"); };
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/* We emulate condition variables with events. It is sufficient here.
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WaitForMultipleObjects() allows the event to be caught and the mutex
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to be taken atomically.
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As for SignalObjectAndWait(), its semantics are unfortunately a bit
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more foggy. Many sources on the Web define it as atomically releasing
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the first object while starting to wait on the second, but MSDN states
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it is *not* atomic...
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In any case, the emulation here is tailored for our particular use case.
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For example, we don't care how many threads are woken up when a condition
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gets signalled. Generic emulations of the pthread_cond_* API using
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Win32 functions can be found on the Web.
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The following read can be edificating (or not):
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http://www.cse.wustl.edu/~schmidt/win32-cv-1.html
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*/
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#define COND_T HANDLE
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#define COND_INIT(cond) \
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/* auto-reset, non-signalled */ \
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if (!(cond = CreateEvent(NULL, FALSE, FALSE, NULL))) { \
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Py_FatalError("CreateMutex(" #cond ") failed"); };
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#define COND_RESET(cond) \
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if (!ResetEvent(cond)) { \
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Py_FatalError("ResetEvent(" #cond ") failed"); };
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#define COND_SIGNAL(cond) \
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if (!SetEvent(cond)) { \
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Py_FatalError("SetEvent(" #cond ") failed"); };
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#define COND_WAIT(cond, mut) \
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{ \
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if (SignalObjectAndWait(mut, cond, INFINITE, FALSE) != WAIT_OBJECT_0) \
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Py_FatalError("SignalObjectAndWait(" #mut ", " #cond") failed"); \
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MUTEX_LOCK(mut); \
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}
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#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
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{ \
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DWORD r; \
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HANDLE objects[2] = { cond, mut }; \
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MUTEX_UNLOCK(mut); \
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r = WaitForMultipleObjects(2, objects, TRUE, microseconds / 1000); \
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if (r == WAIT_TIMEOUT) { \
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MUTEX_LOCK(mut); \
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timeout_result = 1; \
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} \
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else if (r != WAIT_OBJECT_0) \
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Py_FatalError("WaitForSingleObject(" #cond ") failed"); \
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else \
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timeout_result = 0; \
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}
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#else
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#error You need either a POSIX-compatible or a Windows system!
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#endif /* _POSIX_THREADS, NT_THREADS */
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/* Whether the GIL is already taken (-1 if uninitialized). This is volatile
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because it can be read without any lock taken in ceval.c. */
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static volatile int gil_locked = -1;
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/* Number of GIL switches since the beginning. */
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static unsigned long gil_switch_number = 0;
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/* Last thread holding / having held the GIL. This helps us know whether
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anyone else was scheduled after we dropped the GIL. */
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static PyThreadState *gil_last_holder = NULL;
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/* This condition variable allows one or several threads to wait until
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the GIL is released. In addition, the mutex also protects the above
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variables. */
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static COND_T gil_cond;
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static MUTEX_T gil_mutex;
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#ifdef FORCE_SWITCHING
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/* This condition variable helps the GIL-releasing thread wait for
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a GIL-awaiting thread to be scheduled and take the GIL. */
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static COND_T switch_cond;
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static MUTEX_T switch_mutex;
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#endif
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static int gil_created(void)
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{
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return gil_locked >= 0;
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}
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static void create_gil(void)
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{
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MUTEX_INIT(gil_mutex);
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#ifdef FORCE_SWITCHING
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MUTEX_INIT(switch_mutex);
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#endif
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COND_INIT(gil_cond);
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#ifdef FORCE_SWITCHING
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COND_INIT(switch_cond);
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#endif
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gil_locked = 0;
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gil_last_holder = NULL;
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}
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static void recreate_gil(void)
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{
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create_gil();
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}
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static void drop_gil(PyThreadState *tstate)
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{
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/* NOTE: tstate is allowed to be NULL. */
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if (!gil_locked)
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Py_FatalError("drop_gil: GIL is not locked");
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if (tstate != NULL && tstate != gil_last_holder)
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Py_FatalError("drop_gil: wrong thread state");
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MUTEX_LOCK(gil_mutex);
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gil_locked = 0;
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COND_SIGNAL(gil_cond);
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MUTEX_UNLOCK(gil_mutex);
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#ifdef FORCE_SWITCHING
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if (gil_drop_request && tstate != NULL) {
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MUTEX_LOCK(switch_mutex);
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/* Not switched yet => wait */
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if (gil_last_holder == tstate) {
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RESET_GIL_DROP_REQUEST();
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/* NOTE: if COND_WAIT does not atomically start waiting when
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releasing the mutex, another thread can run through, take
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the GIL and drop it again, and reset the condition
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before we even had a chance to wait for it. */
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COND_WAIT(switch_cond, switch_mutex);
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COND_RESET(switch_cond);
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}
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MUTEX_UNLOCK(switch_mutex);
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}
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#endif
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}
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static void take_gil(PyThreadState *tstate)
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{
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int err;
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if (tstate == NULL)
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Py_FatalError("take_gil: NULL tstate");
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err = errno;
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MUTEX_LOCK(gil_mutex);
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if (!gil_locked)
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goto _ready;
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COND_RESET(gil_cond);
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while (gil_locked) {
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int timed_out = 0;
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unsigned long saved_switchnum;
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saved_switchnum = gil_switch_number;
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COND_TIMED_WAIT(gil_cond, gil_mutex, INTERVAL, timed_out);
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/* If we timed out and no switch occurred in the meantime, it is time
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to ask the GIL-holding thread to drop it. */
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if (timed_out && gil_locked && gil_switch_number == saved_switchnum) {
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SET_GIL_DROP_REQUEST();
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}
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}
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_ready:
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#ifdef FORCE_SWITCHING
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/* This mutex must be taken before modifying gil_last_holder (see drop_gil()). */
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MUTEX_LOCK(switch_mutex);
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#endif
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/* We now hold the GIL */
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gil_locked = 1;
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if (tstate != gil_last_holder) {
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gil_last_holder = tstate;
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++gil_switch_number;
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}
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#ifdef FORCE_SWITCHING
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COND_SIGNAL(switch_cond);
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MUTEX_UNLOCK(switch_mutex);
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#endif
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if (gil_drop_request) {
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RESET_GIL_DROP_REQUEST();
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}
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if (tstate->async_exc != NULL) {
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_PyEval_SignalAsyncExc();
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}
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MUTEX_UNLOCK(gil_mutex);
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errno = err;
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}
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void _PyEval_SetSwitchInterval(unsigned long microseconds)
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{
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gil_interval = microseconds;
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}
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unsigned long _PyEval_GetSwitchInterval()
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{
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return gil_interval;
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}
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