This difference between "malloc() succeeded and physical/swap memory is actually available" also has somewhat corresponding impact on how you measure memory usage.

One approach is RSS, the memory in physical RAM... but what if you're swapping? Then again, maybe you swapped out memory you don't actually need and ignoring swap is fine.

The other approach is "how much memory you allocated", and then you hit fun issues mentioned in this article, like "the OS doesn't actually _really_ allocate until you touch the page".

(Longer version: https://pythonspeed.com/articles/measuring-memory-python/)