Poireau: A Sampling Allocation Debugger

One key question in these sort of things is how free() works: it is given a pointer, and it has to decide whether this was sampled or not, with _minimum_ effort.

Poireau does this, IIRC, by putting the pointers it sampled in a different memory address.

Sciagraph (https://sciagraph.com), a profiler I created, uses allocation size. If an allocation is chosen for sampling the profiler makes sure its size is at least 16KiB. Then free() will assume that any allocation 16KiB or larger is sampled. This may not be true, it might be false positive, but it means you don't have to do anything beyond malloc_usable_size() if you have free() on lots and lots of small allocations. A previous iteration used alignment as a heuristic, so that's another option.

Recently saw this through https://discuss.systems/@pkhuong/114532017716618023 which summaries why this would be useful:

> Flagging old live (i.e., potentially immortal) allocations is easier (https://github.com/backtrace-labs/poireau) and more closely matches the goal of identifying why the heap grows instead of entering a steady state.

Ej. Why is Sidekiq memory consumption growing over time.