expose usable_size

[ordian]

https://microsoft.github.io/mimalloc/group__extended.html#ga089c859d9eddc5f9b4bd946cd53cebee

[dvdplm]

Not sure I understand this, can you elaborate? Does it mean that the result of calling usable_size is overestimated by at most 16.7%?

[ordian]

Yes, this is copied from the mimalloc docs (see the link above) and is an implementation detail. Other allocators can also overestimate usable_size.

[gnzlbg]

This means that usable_size might return a size that's 16.7% less than the actual allocation size. But always larger than the size used to request the allocation, right ?

[dvdplm]

@gnzlbg mmmh, no that's not how I read it. "at least equal to the allocated size", so it never returns less than the actual size, but may overestimate it. If it does overestimate, the error is at most 16.7%. Or am I reading it wrong?

[ordian]

I understand this sentence the same way @dvdplm does, the more part at the end refers to overestimation.

[gnzlbg]

That doesn't mean that the size of the allocation is overestimated. What this means is that the memory block where the allocation lives can be greater than the allocation, which is standard practice.

[gnzlbg]

Jemallocator provides the same exact same semantics that malloc_usable_size has in all platforms, e.g., glibc says exactly that as well: http://man7.org/linux/man-pages/man3/malloc_usable_size.3.html

[gnzlbg]

The recommendation for using this also for debugging purposes is in glibc as well, because malloc_usable_size is super slow compared with other functions to query the size of an allocation.

Because of its type signature, it needs to find the allocation that a pointer points to in the internal data-structures, which might require jumping around memory around.

Other APIs like mi_good_size are just pure functions that give you the size of the allocation block as a function of its arguments, no memory traversal necessary.

[ordian]

I see, thanks for the clarification. The confusion comes from using the allocated size of 'ptr' wording instead requested size of allocation for 'ptr'.

[gnzlbg]

Feel free to send a PR to this crate and/or jemallocator improving the docs to make things clearer. What's clear to each of us in isolation is not necessarily clear to everybody 😆

[gnzlbg]

@ordian @dvdplm FYI #6 removes usable_size from mimallocator - you can use it directly via mimalloc-sys, no need to repeat it there.