New submission from Daniel Colascione <[email protected]>:
Right now, the main loop in semlock_acquire looks like this:
do {
Py_BEGIN_ALLOW_THREADS
if (blocking && timeout_obj == Py_None)
res = sem_wait(self->handle);
else if (!blocking)
res = sem_trywait(self->handle);
else
res = sem_timedwait(self->handle, &deadline);
Py_END_ALLOW_THREADS
err = errno;
if (res == MP_EXCEPTION_HAS_BEEN_SET)
break;
} while (res < 0 && errno == EINTR && !PyErr_CheckSignals());
Here, we unconditionally release the GIL even we could acquire the mutex
without blocking! As a result, we could end up switching to another thread in
the process and greatly increasing the latency of operations that lock and
release multiple shared data structures.
Instead, we should unconditionally try sem_trywait, and only then, if we want
to block, release the GIL and try sem_wait or sem_timedwait. This way, we'll
churn only when we need to.
Note that threading.Lock works the way I propose.
----------
components: Library (Lib)
messages: 303441
nosy: Daniel Colascione
priority: normal
severity: normal
status: open
title: Don't release the GIL if we can acquire a multiprocessing semaphore
immediately
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8
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<https://bugs.python.org/issue31653>
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