gemini-code-assist[bot] commented on code in PR #327:
URL: https://github.com/apache/tvm-ffi/pull/327#discussion_r2611143073
##########
python/tvm_ffi/error.py:
##########
@@ -121,19 +121,66 @@ def append_traceback(
The new traceback with the appended frame.
"""
- frame = self._create_frame(filename, lineno, func)
- return types.TracebackType(tb, frame, frame.f_lasti, lineno)
+
+ # This approach avoid binding self._create_frame to a writable local
variable
+ # please see the reference cycle diagram in _with_append_backtrace and
pull request #327 for more details
+ #
+ # This hack prevent binding `self._create_frame` to a writable local
variable
+ # Thus, frame object is a temporary object that won't be held by the
locals of append_traceback
Review Comment:
The comments here are a bit repetitive and could be clearer. The phrase
"binding `self._create_frame`" is also slightly inaccurate, as it's the
*result* of the call that's being bound. Consider consolidating and clarifying
the comments to improve readability and maintainability, especially since this
is a subtle part of the code.
```suggestion
# This approach avoids binding the created frame object to a local
variable
# in `append_traceback`, which would create a reference cycle. By
using a
# nested function, the frame object is a temporary that is not held
by
# the locals of `append_traceback`. See the diagram in
`_with_append_backtrace`
# and PR #327 for more details.
```
##########
python/tvm_ffi/error.py:
##########
@@ -121,19 +121,66 @@ def append_traceback(
The new traceback with the appended frame.
"""
- frame = self._create_frame(filename, lineno, func)
- return types.TracebackType(tb, frame, frame.f_lasti, lineno)
+
+ # This approach avoid binding self._create_frame to a writable local
variable
+ # please see the reference cycle diagram in _with_append_backtrace and
pull request #327 for more details
+ #
+ # This hack prevent binding `self._create_frame` to a writable local
variable
+ # Thus, frame object is a temporary object that won't be held by the
locals of append_traceback
+ def create(
+ tb: types.TracebackType | None, frame: types.FrameType, lineno: int
+ ) -> types.TracebackType:
+ return types.TracebackType(tb, frame, frame.f_lasti, lineno)
+
+ return create(tb, self._create_frame(filename, lineno, func), lineno)
_TRACEBACK_MANAGER = TracebackManager()
def _with_append_backtrace(py_error: BaseException, backtrace: str) ->
BaseException:
"""Append the backtrace to the py_error and return it."""
+ # We manually delete py_error and tb to avoid reference cycle, making it
faster to gc the locals inside the frame
+ # please see pull request #327 for more details
+ #
+ # Memory Cycle Diagram:
+ #
+ # [Stack Frames] [Heap Objects]
+ # +-------------------+
+ # | outside functions | -----------------------> [ Tensor ]
+ # +-------------------+ (Held by cycle, slow to free)
+ # ^
+ # | f_back
+ # +-------------------+ locals py_error
+ # | py_error (this) | -----+--------------> [ BaseException ]
+ # +-------------------+ | |
+ # ^ | | (with_traceback)
+ # | f_back | v
+ # +-------------------+ +--------------> [ Traceback Obj ]
+ # | append_traceback | tb |
+ # +-------------------+ |
+ # ^ |
+ # | f_back |
+ # +-------------------+ |
+ # | _create_frame | |
+ # +-------------------+ |
+ # ^ |
+ # | f_back |
+ # +-------------------+ |
+ # | _get_frame | <----------------------------+
+ # +-------------------+ (Cycle closes here)
tb = py_error.__traceback__
- for filename, lineno, func in _parse_backtrace(backtrace):
- tb = _TRACEBACK_MANAGER.append_traceback(tb, filename, lineno, func)
- return py_error.with_traceback(tb)
+ try:
+ for filename, lineno, func in _parse_backtrace(backtrace):
+ tb = _TRACEBACK_MANAGER.append_traceback(tb, filename, lineno,
func)
+ return py_error.with_traceback(tb)
+ finally:
+ # We explicitly break reference cycle here
+ # when the try block has return statement, the finally body is executed
+ # **after** the function returns (which is a special feature of
try...finally)
+ # after deleting, the py_error and tb are not held by the locals of
this function
+ # and the reference cycle above is resolved
Review Comment:
The comment on line 180 stating that the `finally` block executes `**after**
the function returns` is incorrect. The `finally` block is executed *before*
the function returns to its caller, just after the `return` statement's
expression has been evaluated. This is a subtle but critical detail for
understanding why this code works. Correcting this will prevent future
confusion.
```suggestion
# We explicitly break the reference cycle here. The `finally` block
is
# executed just before the function returns, after the `return`
expression
# in the `try` block has been evaluated. Deleting `py_error` and `tb`
# here ensures they are not held by this function's frame's locals,
# which resolves the cycle.
```
--
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.
To unsubscribe, e-mail: [email protected]
For queries about this service, please contact Infrastructure at:
[email protected]
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
