FWIW, that synthetic benchmark was reflective of some real world code we were deploying. Using malloc/free for one function led to something like a 2x performance improvement of the whole program.

I think it's important to differentiate between malloc implementations/algorithms, just like it's important to differentiate between GCs. E.g., mimalloc "shards" size classes into pages, with separate free lists per page. This way, subsequent allocations are all from the same page. Freeing does not free eagerly; only if the entire page is freed, or if we hit a new allocation and the page is empty, then it can hit a periodic slow path to do deferred work. https://www.microsoft.com/en-us/research/uploads/prod/2019/0...

Good malloc implementations can also employ techniques to avoid fragmentation. It's unfortunate that the defaults are bad.

But I confess, compacting GCs and profiling the effects of heap fragmentation (especially over time in long running programs) are both things I lack experience in. Microbenchmarks are unlikely to capture that accurately.