[Python-checkins] r76110 - sandbox/trunk/newgil/Python/ceval_gil.h (original) (raw)

antoine.pitrou python-checkins at python.org
Wed Nov 4 22:04:05 CET 2009

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Author: antoine.pitrou Date: Wed Nov 4 22:04:05 2009 New Revision: 76110

Log: Document the implementation

Modified: sandbox/trunk/newgil/Python/ceval_gil.h

Modified: sandbox/trunk/newgil/Python/ceval_gil.h

--- sandbox/trunk/newgil/Python/ceval_gil.h (original) +++ sandbox/trunk/newgil/Python/ceval_gil.h Wed Nov 4 22:04:05 2009 @@ -25,6 +25,40 @@ /* #define TRACE_PRIO */

+/*

• Notes about the implementation:
• • The GIL is just a boolean variable (gil_locked) whose access is protected

• by a mutex, and whose changes are signalled by a condition variable. The

• mutex itself is rarely taken 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.

•  [Actually, another volatile boolean variable (eval_breaker) is used

•   which aggregates several conditions into one. Volatile booleans are

•   ok as signalling means since Python is run on cache-coherent

•   architectures only.]

• • A thread wanting to take the GIL will first wait for a given amount of

• time before setting gil_drop_request. This amount of time defines the

• ideal thread switching period. It is available for the user to read

• and modify using `sys.{get,set}switchinterval()`.

• • Forced thread switching when releasing the GIL is implemented. 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 variable (gil_last_holder) controlled through another

• {mutex, condition} pair.

• • An optional policy mechanism, priority requests, is currently disabled.

• The intent was to further improve the latency of some types of GIL-taking

• activities, such as being woken up on a socket. If it is confirmed that

• this feature is unnecessary, support code should be removed.

+/ + #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

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