Exercise 2 - Clinging Onto Memory
=================================

Intro
-----

Unlike some low level languages like C, Python will manage our memory for us and will free up memory
that's no longer needed. Python's automatic memory management makes our lives easier, but sometimes,
it may not work the way you would expect it to...

Working Through an Example
^^^^^^^^^^^^^^^^^^^^^^^^^^

Let's go through an example where memory management in Python doesn't work how a person coming from
other programming languages might expect.

Exercise
""""""""

Take a look at the example in ``holding_onto_memory.py``: how many MB of memory will it use at peak?
Take a guess, and then confirm by running ``memray`` and generating a ``flamegraph``.

Expectations vs Reality
"""""""""""""""""""""""

Let's presume that we can't mutate the original data, the best we can do is peak memory of 200MB:
for a brief moment in time both the original 100MB of data and the modified copy of the data will
need to be present. In practice, however, the actual peak usage will be 400MB as demonstrated by the
``flamegraph``:

.. image:: ../_static/images/exercise2_flamegraph.png

Examining our flame graph further, we can see that we peak at 400MB of allocated memory in
``add_scalar`` due to four 100MB allocations that are all alive simultaneously:

1. The return value from ``subtract_scalar``, held by the ``data`` variable in ``process_data``
2. The return value from ``raise_to_power``, held by the ``data_pow`` variable in ``process_data``
3. The return value from ``duplicate_data``, held by the ``data`` argument in ``add_scalar``
4. The return value from ``add_scalar``, which is created and populated before the function returns
   and the ``data`` argument goes out of scope

Challenge
"""""""""

Experiment with the code in ``holding_onto_memory.py`` and try to get the peak memory usage down to
200MB. Test your solutions by running the unit test in ``tests/test_exercise_2.py`` and examine them
with the help of ``memray`` reports.


Solutions
"""""""""

.. raw:: html

   <details>
   <summary><i>Toggle to see the sample solutions</i></summary>

After examining the flame graph, we can see that the problem is caused by local variables which are
no longer needed, but continue to use memory until ``process_data()`` has finished running.
Therefore, we need to refactor the method in a way that does not use unnecessary variables to store
data that will not be read afterwards. There are two main approaches we can use to solve our issue
here:

1. Avoiding local variables in ``process_data()`` all together and instead returning the result
   of nested function calls::

    def process_data():
        # no extra reference to the original array
        return add_scalar(
            duplicate_data(
                raise_to_power(
                    subtract_scalar(
                        load_xMb_of_data(SIZE_OF_DATA_IN_MB),
                        SUBTRACT_AMOUNT
                    ),
                    POWER_AMOUNT
                )
            ),
            ADD_AMOUNT
        )

2. Reassigning one variable: we can create a single variable, and re-use it multiple times to store
   the new value of the manipulated array. This way, we will only hold one array in memory at a time,
   instead of holding on to older versions of the mutated array unnecessarily::

    def process_data():
        # reusing the local variable instead of allocating more space
        # this approach is called 'hidden mutability'
        data = load_xMb_of_data(SIZE_OF_DATA_IN_MB)
        data = subtract_scalar(data, SUBTRACT_AMOUNT)
        data = raise_to_power(data, POWER_AMOUNT)
        data = duplicate_data(data)
        data = add_scalar(data, ADD_AMOUNT)
        return data

Full code solution `here
<https://github.com/bloomberg/memray/blob/main/docs/tutorials/solutions/exercise_2/holding_onto_memory.py>`_.

.. raw:: html

   </details>

Conclusion
^^^^^^^^^^

Typically, holding onto data in memory a little longer than needed is not a big issue. However, when
we are working with large objects, we should be particularly careful. Over-allocating unnecessary
memory can lead to running out of memory on the machine (especially for Linux VMs which are
typically smaller than physical machines).

Memray can be a helpful tool when trying to debug where we are over-allocating memory unnecessarily.

More resources:

- A more detailed walkthrough with a `similar example
  <https://pythonspeed.com/articles/function-calls-prevent-garbage-collection/>`_
- Another `related article <https://pythonspeed.com/articles/minimizing-copying/>`_ about
  unnecessary memory allocation
