DAdalias·May 6, 2020
The justifications are partly the same as what Daniel Micay has written extensively on in the rational for hardened_malloc (https://github.com/GrapheneOS/hardened_malloc) - unsynchronized per-thread state inherently sacrifices global consistency for performance and makes it impossible to detect a lot of types of memory usage errors (DF/UAF, etc) that could otherwise be caught.
However musl has the additional constraint of being compatible with small/very-low-memory environments. Lack of global consistency inherently means you will end up using memory less efficiently and requesting significantly more from the system. The new malloc about to go upstream in musl is, to my knowledge, the first/only advanced hardened allocator using slab-type design rather than traditional dlmalloc type split/merge, but also designed for extremely low overhea
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d/waste at low to moderate usage rather than extreme performance. And in the vast majority of applications, this is perfectly reasonable. Even Firefox for example does very well with it.
With that said, new malloc is expected to be somewhat faster than old on lots of workloads (and considerably faster than old would be if we fixed the flaws in old that motivated it), but it's not a performance-oriented allocator. If you really want/need that you should probably link jemalloc or similar (and accept all the tradeoffs that come with that). In Rust programs without "unsafe", it may make sense to do that by default.