Re: Multithreaded C API Python questions
Svein Seldal wrote: A real beauty about the PyGILState_Ensure() is the fact that if it's called from a thread that is (yet) unknown to python, it will actually create the thread state object for you (with PyThreadState_New) and acquire the lock. Hence, I dont have to worry about the process of swapping thread states with PyEval_xxxThread() functions. I ONLY need to use PyGILState_Ensure() prior to any py-ops. Interesting - the PyGILState_Ensure was new to py2.3. I didn't notice so far, that it will even auto-create a fresh tread state (from default InterpreterState only). From the docs its not that clear. The confusion in usage is that one has to do Py_Initialize() PyEval_InitThreads() ts=PyEval_SaveThread() in order to start clean for further pure meta-high-level PyGILState_Ensure/Release. They obviously meant PyGILState_Ensure to make live easier, but the discussion shows, it adds additional worries probably forcing each user into investigating the Python C-code :-) There should probably a extra init function for this meta-high-level usage like .. PyGILState_PyInitThreads() // which would not pre-acquire the lock! - as anyone expects to enter then with PyGILState_Ensure . and these 3 Functions grouped together in the docs for an easy-to-use meta-high-level thread state API. The current thread state API doc, as you read it from top to bottom now, is in fact totally confusing for anyone who didn't develop Python himself :-) Robert -- http://mail.python.org/mailman/listinfo/python-list
Re: Multithreaded C API Python questions
Svein Seldal wrote:
You seem to use the functions in very rude manner - wanting to force the GIL
around at lowest level.
Better forget the word GIL and think about acquiring and releasing Threads.
For each thread wanting to execute Python stuff there has to be a thread state
(ts). And then you have to enter/leave for executing Python by using the thread
state.
PyEval_InitThreads enters Python (incl. the GIL) first time.
To let Python free in main thread do
ts=PyEval_SaveThread()// or more easy: Py_BEGIN_ALLOW_THREADS
to reenter Python do simply:
PyEval_RestoreThread(ts) // or: Py_END_ALLOW_THREADS
Forget all the low level PyGIL... functions.
when you really want another c thread to enter Python first time with no thread
state existing through Python itsel (better make the thread in Python with
thread.start_new ?), then you have to do once
ts_cthread = PyThreadState_New(interp)
then enter Python in this thread:
PyEval_AcquireThread( ts_cthread )
to leave again:
PyEval_ReleaseThread( ts_cthread )
If you just loop the call_py_function_send() mainly in this thread you
probably better create the thread at all in Python and make the loop there. You
probably stick too much to C-level at any price :-) Probably you just do a
PyRun_ in main thread and then everything else in Python, and expose
C-parts for the thread-loop to Python as function (in other_py_inits) - where
in the c-function you probably have the usual
Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS bracket during time
consuming/system/reenter-endangered message stuff.
Robert
I think I've found the bug, but I need to confirm this behavior.
My findings is that if you use PyEval_InitThreads(), it is crucial to
release the GIL with PyEval_ReleaseThread() afterwards.
The API docs states that you can release the GIL with
PyEval_ReleaseLock() or PyEval_ReleaseThread() AFAICS.
http://docs.python.org/api/threads.html under docs of void
PyEval_InitThreads().
However, if I do use PyEval_ReleaseLock() it will crash shortly after
with Fatal Python error: ceval: tstate mix-up in a multithreaded C
environment.
If I use PyEval_ReleaseThread() to release the GIL, my app seems stable.
I release the GIL like this:
PyThreadState *pts = PyGILState_GetThisThreadState();
PyEval_ReleaseThread(pts);
Now, is this a feature or expected behavior in python (i.e. unclear API
documentation), or is this a bug in python itself?
Regards,
Svein
PS:
For reference I did something like this in pseudo-code:
Py_Initialize();
PyEval_InitThreads();
other_py_inits(); // Load py modules, etc.
PyEval_ReleaseLock();// -- MAKES THE APP UNSTABLE
create_c_thread();
PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
call_py_function_main(); // Py main() wont return
PyGILState_Release(gstate);
And the main of the C thread function looks like this:
while(1)
{
PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
call_py_function_send();
PyGILState_Release(gstate);
}
--
http://mail.python.org/mailman/listinfo/python-list
Re: Multithreaded C API Python questions
robert wrote: Forget all the low level PyGIL... functions.' Quite the contrary, I would say! From the Py C API doc, I interpret the PyGIL... functions to be higher leveled than eg. PyEval_SaveThread(). I've checked into the source of python to find out what's really going on, and I've learnt a couple of interesting facts. The PyGIL... are helper functions build over the PyEval... functions, not the other way around. A real beauty about the PyGILState_Ensure() is the fact that if it's called from a thread that is (yet) unknown to python, it will actually create the thread state object for you (with PyThreadState_New) and acquire the lock. Hence, I dont have to worry about the process of swapping thread states with PyEval_xxxThread() functions. I ONLY need to use PyGILState_Ensure() prior to any py-ops. From my previous post, I asked about the difference between PyEval_ReleaseLock() and PyEval_ReleaseThread() and I've found one major difference. Both release the GIL as the docs states, but the PyEval_ReleaseThread() saves the current thread state as well. When I used PyEval_ReleaseLock() this didnt happen, preventing proper saving of the current thread, causing py crash when control were handed back to this thread. By using: PyEval_InitThreads(); py_ops(); PyThreadState *pts = PyGILState_GetThisThreadState(); PyEval_ReleaseThread(pts); And for each py-op later on (from arbitrary thread): PyGILState_STATE gstate; gstate = PyGILState_Ensure(); py_ops(); PyGILState_Release(gstate); Then you're home free. Everything related to threading is handled by Py/API itself. Probably you just do a PyRun_ in main thread and then everything else in Python, and expose C-parts for the thread-loop to Python as function (in other_py_inits) - where in the c-function you probably have the usual Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS bracket during time consuming/system/reenter-endangered message stuff. The app i'm building is a plugin to a server system, and I cant control nor remove any threads that the server uses. I am given one main thread to run python forever, and messages that are to be delivered into python are called from another. I could do it like you propose: The data coming from my server will arrive into a c-function called by a server thread. The c-part of the thread-loop would then be run as another thread (started from python). Its fully feasible, yet the challenge is to make proper data sync-ing between these two threads. Well, I think I've achieved what I wanted. Python seems apparently stable, and reading from the python sources, I cant see any immediate reasons why it shouldn't. Thanks for letting me closer to a working solution! Regads, Svein -- http://mail.python.org/mailman/listinfo/python-list
Re: Multithreaded C API Python questions
Hi!
I think I've found the bug, but I need to confirm this behavior.
My findings is that if you use PyEval_InitThreads(), it is crucial to
release the GIL with PyEval_ReleaseThread() afterwards.
The API docs states that you can release the GIL with
PyEval_ReleaseLock() or PyEval_ReleaseThread() AFAICS.
http://docs.python.org/api/threads.html under docs of void
PyEval_InitThreads().
However, if I do use PyEval_ReleaseLock() it will crash shortly after
with Fatal Python error: ceval: tstate mix-up in a multithreaded C
environment.
If I use PyEval_ReleaseThread() to release the GIL, my app seems stable.
I release the GIL like this:
PyThreadState *pts = PyGILState_GetThisThreadState();
PyEval_ReleaseThread(pts);
Now, is this a feature or expected behavior in python (i.e. unclear API
documentation), or is this a bug in python itself?
Regards,
Svein
PS:
For reference I did something like this in pseudo-code:
Py_Initialize();
PyEval_InitThreads();
other_py_inits(); // Load py modules, etc.
PyEval_ReleaseLock();// -- MAKES THE APP UNSTABLE
create_c_thread();
PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
call_py_function_main(); // Py main() wont return
PyGILState_Release(gstate);
And the main of the C thread function looks like this:
while(1)
{
PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
call_py_function_send();
PyGILState_Release(gstate);
}
--
http://mail.python.org/mailman/listinfo/python-list
Multithreaded C API Python questions
Hi A couple of mulithreaded C API python questions: I) The PyGILState_Ensure() simply ensures python api call ability, it doesnt actually lock the GIL, right? PyGILState_STATE gstate; gstate = PyGILState_Ensure(); II) Am I required to lock the GIL prior to running any python functions (in a threaded app)? PyThreadState *pts = PyGILState_GetThisThreadState(); PyEval_AcquireThread(pts); PyObject_CallObject(...); III) Shouldn't the GIL be released after 100 bytecodes or so to enable other waiting threads to get the GIL? I'm unable to get access to python as long as another python call is executing. The PyEval_AcquireThread() call blocks until the first call returns. I was hoping that the python system itself would release the GIL after some execution, but it itsnt. I am dependent upon the ability to have to threads executing in python land at the same time. How can this be done? Regards, Svein Seldal -- http://mail.python.org/mailman/listinfo/python-list
Re: Multithreaded C API Python questions
Svein Seldal wrote: Hi A couple of mulithreaded C API python questions: I) The PyGILState_Ensure() simply ensures python api call ability, it doesnt actually lock the GIL, right? PyGILState_STATE gstate; gstate = PyGILState_Ensure(); II) Am I required to lock the GIL prior to running any python functions (in a threaded app)? PyThreadState *pts = PyGILState_GetThisThreadState(); PyEval_AcquireThread(pts); PyObject_CallObject(...); III) Shouldn't the GIL be released after 100 bytecodes or so to enable other waiting threads to get the GIL? I'm unable to get access to python as long as another python call is executing. The PyEval_AcquireThread() call blocks until the first call returns. I was hoping that the python system itself would release the GIL after some execution, but it itsnt. I am dependent upon the ability to have to threads executing in python land at the same time. How can this be done? PyGILState_Ensure/Release guarantees to establish the GIL - even if it was already held (useful if you deal with complex call ordering/dependencies) PyObject_CallObject(...) calls Python code. Thus the interpreter will switch and do as usuall during that. In your/C/system... code you are responsible to release the GIL or not to enable other Python threads (and prevent from deadlocks) Usually you'd do this * if you do time consuming C/system stuff * or if the code can possibly renter Python through the system (e.g. when you call a Windows function which itself can create Windows messages to be routed back into Python message handlers) -robert -- http://mail.python.org/mailman/listinfo/python-list
Re: Multithreaded C API Python questions
robert wrote: PyGILState_Ensure/Release guarantees to establish the GIL - even if it was already held (useful if you deal with complex call ordering/dependencies) I understand this to mean that I dont need to explicitly lock the GIL (with PyEval_AcquireLock() or PyEval_AcquireThread()). Well I cant figure out the PyGILState_Ensure() because if I only use this function to establish the GIL, when calling python, python will shortly after crash with Fatal Python error: ceval: tstate mix-up. This happens consistently when the main app and the extra thread has called python and both are busy executing python code. PyObject_CallObject(...) calls Python code. Thus the interpreter will switch and do as usuall during that. BTW What do you mean by switch? In your/C/system... code you are responsible to release the GIL or not to enable other Python threads (and prevent from deadlocks) Usually you'd do this * if you do time consuming C/system stuff * or if the code can possibly renter Python through the system (e.g. when you call a Windows function which itself can create Windows messages to be routed back into Python message handlers) The main problem is that not done this way, it's the other way around. My main C app will call a python function which will be a lengthy time consuming process. The main C app will call python and it will never return from PyObject_CallObject(...), while the extra thread should call a py function to deliver occational messages into C. Svein -- http://mail.python.org/mailman/listinfo/python-list
Re: Multithreaded C API Python questions
Svein Seldal wrote: I'm unable to get access to python as long as another python call is executing. The PyEval_AcquireThread() call blocks until the first call returns. I was hoping that the python system itself would release the GIL after some execution, but it itsnt. I am dependent upon the ability to have to threads executing in python land at the same time. How can this be done? Are you creating your threads through python or through C code? If you are only creating it through C code, then make sure you initialize the GIL with the following call at the beginning of your application: PyEval_InitThreads() From my experience, calling PyGILState_Ensure() was enough. I didn't need to call any extra threading functions. But make sure that every call to PyGILState_Ensure() is matched with a call to PyGILState_Release() -Farshid -- http://mail.python.org/mailman/listinfo/python-list
Re: Multithreaded C API Python questions
Svein Seldal wrote: robert wrote: PyGILState_Ensure/Release guarantees to establish the GIL - even if it was already held (useful if you deal with complex call ordering/dependencies) I understand this to mean that I dont need to explicitly lock the GIL (with PyEval_AcquireLock() or PyEval_AcquireThread()). Well I cant figure out the PyGILState_Ensure() because if I only use this function to establish the GIL, when calling python, python will shortly after crash with Fatal Python error: ceval: tstate mix-up. This happens consistently when the main app and the extra thread has called python and both are busy executing python code. Do did't tell enough to see what you want to do on the big picture. usually you create a thread through Python. If you boot Python at all from C you have to do PyEval_InitThreads or maybe more simple use the high level layer PyRun_... If you boot a new Python thread manually or a separated interpreter (with separated module state) from outside the you have to also do: (PyInterpreterState* PyInterpreterState_New() ) PyThreadState* PyThreadState_New( PyInterpreterState *interp) void PyEval_AcquireThread( PyThreadState *tstate) If you are already in a Python thread but don't for some reason not know the current thread state: PyThreadState* PyThreadState_Get( ) but usually you release with PyThreadState* PyEval_SaveThread( ) / Py_BEGIN_ALLOW_THREADS PyObject_CallObject(...) calls Python code. Thus the interpreter will switch and do as usuall during that. BTW What do you mean by switch? the automatic scheduling every sys.getcheckinterval() etc.. you mentioned You'd only have to take care for that, if your C-code does things (e.g. calling python basic funcs) for a long time and doesn't release the lock with Py_BEGIN_ALLOW_THREADS ... The main problem is that not done this way, it's the other way around. My main C app will call a python function which will be a lengthy time consuming process. The main C app will call python and it will never return from PyObject_CallObject(...), while the extra thread should call a py function to deliver occational messages into C. Maybe simply boot Python in the main thread (Py_Initialize( )) and run off Python possibly as simple as PyRun_String and let Python do thread.start_new(...) You'd not have to worry much about states. ( and later you'd probably even more simple do python myapp.py and expose your C app as extension module :-) ) -robert -- http://mail.python.org/mailman/listinfo/python-list
Re: Multithreaded C API Python questions
Svein Seldal svein at seldal dot comwrote: 8--- I am dependent upon the ability to have to threads executing in python land at the same time. How can this be done? call time.sleep(0.001) in each, as well as the main thread, to politely give the rest a chance Fiddle with the sleep times in your setup to get the best performance - and don't be misled into thinking that faster is better - its not always true - there comes a point where the overhead of swapping a thread consumes the total resource of the process, and no work gets done. - HTH - Hendrik -- http://mail.python.org/mailman/listinfo/python-list
