cpython/Python/ceval_gil.h at 3.3 · python/cpython · GitHub

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* 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)

#include "condvar.h"

#ifndef Py_HAVE_CONDVAR

#error You need either a POSIX-compatible or a Windows system!

#endif

#define MUTEX_T PyMUTEX_T

#define MUTEX_INIT(mut) \

if (PyMUTEX_INIT(&(mut))) { \

Py_FatalError("PyMUTEX_INIT(" #mut ") failed"); };

#define MUTEX_FINI(mut) \

if (PyMUTEX_FINI(&(mut))) { \

Py_FatalError("PyMUTEX_FINI(" #mut ") failed"); };

#define MUTEX_LOCK(mut) \

if (PyMUTEX_LOCK(&(mut))) { \

Py_FatalError("PyMUTEX_LOCK(" #mut ") failed"); };

#define MUTEX_UNLOCK(mut) \

if (PyMUTEX_UNLOCK(&(mut))) { \

Py_FatalError("PyMUTEX_UNLOCK(" #mut ") failed"); };

#define COND_T PyCOND_T

#define COND_INIT(cond) \

if (PyCOND_INIT(&(cond))) { \

Py_FatalError("PyCOND_INIT(" #cond ") failed"); };

#define COND_FINI(cond) \

if (PyCOND_FINI(&(cond))) { \

Py_FatalError("PyCOND_FINI(" #cond ") failed"); };

#define COND_SIGNAL(cond) \

if (PyCOND_SIGNAL(&(cond))) { \

Py_FatalError("PyCOND_SIGNAL(" #cond ") failed"); };

#define COND_WAIT(cond, mut) \

if (PyCOND_WAIT(&(cond), &(mut))) { \

Py_FatalError("PyCOND_WAIT(" #cond ") failed"); };

#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \

{ \

int r = PyCOND_TIMEDWAIT(&(cond), &(mut), (microseconds)); \

if (r < 0) \

Py_FatalError("PyCOND_WAIT(" #cond ") failed"); \

if (r) /* 1 == timeout, 2 == impl. can't say, so assume timeout */ \

timeout_result = 1; \

else \

timeout_result = 0; \

} \

/* Whether the GIL is already taken (-1 if uninitialized). This is atomic

because it can be read without any lock taken in ceval.c. */

static _Py_atomic_int gil_locked = {-1};

/* Number of GIL switches since the beginning. */

static unsigned long gil_switch_number = 0;

/* Last PyThreadState holding / having held the GIL. This helps us know

whether anyone else was scheduled after we dropped the GIL. */

static _Py_atomic_address 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 _Py_atomic_load_explicit(&gil_locked, _Py_memory_order_acquire) >= 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

_Py_atomic_store_relaxed(&gil_last_holder, NULL);

_Py_ANNOTATE_RWLOCK_CREATE(&gil_locked);

_Py_atomic_store_explicit(&gil_locked, 0, _Py_memory_order_release);

}

static void destroy_gil(void)

{

/* some pthread-like implementations tie the mutex to the cond

* and must have the cond destroyed first.

COND_FINI(gil_cond);

MUTEX_FINI(gil_mutex);

#ifdef FORCE_SWITCHING

COND_FINI(switch_cond);

MUTEX_FINI(switch_mutex);

#endif

_Py_atomic_store_explicit(&gil_locked, -1, _Py_memory_order_release);

_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);

}

static void recreate_gil(void)

{

_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);

/* XXX should we destroy the old OS resources here? */

create_gil();

}

static void drop_gil(PyThreadState *tstate)

{

if (!_Py_atomic_load_relaxed(&gil_locked))

Py_FatalError("drop_gil: GIL is not locked");

/* tstate is allowed to be NULL (early interpreter init) */

if (tstate != NULL) {

/* Sub-interpreter support: threads might have been switched

under our feet using PyThreadState_Swap(). Fix the GIL last

holder variable so that our heuristics work. */

_Py_atomic_store_relaxed(&gil_last_holder, tstate);

}

MUTEX_LOCK(gil_mutex);

_Py_ANNOTATE_RWLOCK_RELEASED(&gil_locked, /*is_write=*/1);

_Py_atomic_store_relaxed(&gil_locked, 0);

COND_SIGNAL(gil_cond);

MUTEX_UNLOCK(gil_mutex);

#ifdef FORCE_SWITCHING

if (_Py_atomic_load_relaxed(&gil_drop_request) && tstate != NULL) {

MUTEX_LOCK(switch_mutex);

/* Not switched yet => wait */

if (_Py_atomic_load_relaxed(&gil_last_holder) == tstate) {

RESET_GIL_DROP_REQUEST();

/* NOTE: if COND_WAIT does not atomically start waiting when

releasing the mutex, another thread can run through, take

the GIL and drop it again, and reset the condition

before we even had a chance to wait for it. */

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 (!_Py_atomic_load_relaxed(&gil_locked))

goto _ready;

while (_Py_atomic_load_relaxed(&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 &&

_Py_atomic_load_relaxed(&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 */

_Py_atomic_store_relaxed(&gil_locked, 1);

_Py_ANNOTATE_RWLOCK_ACQUIRED(&gil_locked, /*is_write=*/1);

if (tstate != _Py_atomic_load_relaxed(&gil_last_holder)) {

_Py_atomic_store_relaxed(&gil_last_holder, tstate);

++gil_switch_number;

}

#ifdef FORCE_SWITCHING

COND_SIGNAL(switch_cond);

MUTEX_UNLOCK(switch_mutex);

#endif

if (_Py_atomic_load_relaxed(&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;

}

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