Deferred reference counting is not fully implemented yet. As a temporary
measure, we immortalize objects that would use deferred reference
counting to avoid multi-threaded scaling bottlenecks.
This is only performed in the free-threaded build once the first
non-main thread is started. Additionally, some tests, including refleak
tests, suppress this behavior.
This keeps track of the per-thread total reference count operations in
PyThreadState in the free-threaded builds. The count is merged into the
interpreter's total when the thread exits.
The free-threaded GC sometimes sees objects with zero refcount. This can
happen due to the delay in merging biased reference counting fields,
and, in the future, due to deferred reference counting. We should not
untrack these objects or they will never be collected.
This fixes the refleaks in the free-threaded build.
This isn't strictly necessary because the implementation of `gc_should_collect`
already checks `gcstate->enabled` in the free-threaded build, but it seems
like a good idea until the common pieces of gc.c and gc_free_threading.c are
refactored out.
<pycore_time.h> include is no longer needed to get the PyTime_t type
in internal header files. This type is now provided by <Python.h>
include. Add <pycore_time.h> includes to C files instead.
This change adds an `eval_breaker` field to `PyThreadState`. The primary
motivation is for performance in free-threaded builds: with thread-local eval
breakers, we can stop a specific thread (e.g., for an async exception) without
interrupting other threads.
The source of truth for the global instrumentation version is stored in the
`instrumentation_version` field in PyInterpreterState. Threads usually read the
version from their local `eval_breaker`, where it continues to be colocated
with the eval breaker bits.
The GC keeps track of the number of allocations (less deallocations)
since the last GC. This buffers the count in thread-local state and uses
atomic operations to modify the per-interpreter count. The thread-local
buffering avoids contention on shared state.
A consequence is that the GC scheduling is not as precise, so
"test_sneaky_frame_object" is skipped because it requires that the GC be
run exactly after allocating a frame object.
Biased reference counting maintains two refcount fields in each object:
`ob_ref_local` and `ob_ref_shared`. The true refcount is the sum of these two
fields. In some cases, when refcounting operations are split across threads,
the ob_ref_shared field can be negative (although the total refcount must be
at least zero). In this case, the thread that decremented the refcount
requests that the owning thread give up ownership and merge the refcount
fields.
* gh-112529: Remove PyGC_Head from object pre-header in free-threaded build
This avoids allocating space for PyGC_Head in the free-threaded build.
The GC implementation for free-threaded CPython does not use the
PyGC_Head structure.
* The trashcan mechanism uses the `ob_tid` field instead of `_gc_prev`
in the free-threaded build.
* The GDB libpython.py file now determines the offset of the managed
dict field based on whether the running process is a free-threaded
build. Those are identified by the `ob_ref_local` field in PyObject.
* Fixes `_PySys_GetSizeOf()` which incorrectly incorrectly included the
size of `PyGC_Head` in the size of static `PyTypeObject`.
The free-threaded build's GC implementation is non-generational, but was
scheduled as if it were collecting a young generation leading to
quadratic behavior. This increases the minimum threshold and scales it
to the number of live objects as we do for the old generation in the
default build.
Note that the scheduling is still not thread-safe without the GIL. Those
changes will come in later PRs.
A few tests, like "test_sneaky_frame_object" rely on prompt scheduling
of the GC. For now, to keep that test passing, we disable the scaled
threshold after calls like `gc.set_threshold(1, 0, 0)`.
* gh-112529: Implement GC for free-threaded builds
This implements a mark and sweep GC for the free-threaded builds of
CPython. The implementation relies on mimalloc to find GC tracked
objects (i.e., "containers").
Alex Turner
Sam Gross
Mark Shannon
Dino Viehland
Donghee Na
Victor Stinner
Brett Simmers