Date: Fri, 29 Nov 2019 08:55:23 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: New malloc - first preview On Fri, Nov 29, 2019 at 09:37:15AM +0300, Vasya Boytsov wrote: > Is your malloc implementation any better than > https://github.com/microsoft/mimalloc? > AFAIK one can use mimalloc in their projects as it has compatible license. > the only thing that bothers is that it is done by Microsoft research. "Better" is not meaningful without specifying criteria for evaluation. The information on mimalloc is sparse, not covering the basic design it uses AFAICT, rather mainly the new properties it has which are mostly for the purpose of its specialized sharding API (not useful/usable for malloc API). So without reading into it much more, it's hard to say. >From the summary, their idea of "small" is on a completely different order of magnitude from musl's. 6kLOC is gigantic, far larger than any single component of musl. Of course it might be really sparse code with lots of whitespace and comments, but still doesn't bode well. Regarding "fast", everyone aiming for "fast" claims fast with their own benchmarks. I generally don't trust malloc benchmarks after N times of digging into them and finding they were tuned just above/below a threshold to make a competitor look bad. That's not to say the same happened here, but "fast at what?" is a real question here, and the only meaningful test seems to be measurement under real-world workloads. The security measures mentioned seem weaker than WIP new malloc and likely not much better than current one. But it's hard to evaluate security/hardening measures without a model of proposed attacks and which ones they mitigate. This is one thing I've focused on in the new malloc, classifying and addressing them such that we can rule out entire classes. Roughly, without already having complex capability to follow multiple pointers and clobber resulting memory, only the application data stored in the heap, not the allocator's heap state, is subject to corruption by common methods (linear overflow, UAF, DF). Not all of this is implemented yet in the draft. Those things aside, ultimately the answer is that the question is not what is the best malloc for some arbitrary definition of best, but what's the best that fits into the constraints of musl, including: - low absolute space cost at low malloc utilization - low relative space cost at moderate to high utilization - compatibility with 32-bit address space - compatibility with nommu - compatibility with RLIMIT_AS, no-overcommit configurations and also provides hardening improvements over the current allocator, which gets by with only minimal measures. Nobody should be expecting magical performance improvements out of this. The likely user-facing changes will be: - elimination of heap size explosion in multithreaded apps (known bug) - better return of freed memory to system - greatly reduced exploitability of bugs related to malloc usage Speed could go up or down. Hopefully it goes up for lots of things. It will be a "win" in terms of speed if it merely avoids going down by more than fixing the "heap size explosion" bug would make it go down with the current dlmalloc-type design -- see: https://www.openwall.com/lists/musl/2019/04/12/4 since fixing this bug is absolutely necessary; it keeps impacting users. Rich > On 11/29/19, Rich Felker <dalias@...c.org> wrote: > > On Thu, Nov 28, 2019 at 04:56:42PM -0500, Rich Felker wrote: > >> Work on the new malloc is well underway, and I have a draft version > >> now public at: > >> > >> https://github.com/richfelker/mallocng-draft > >> > >> Some highlights: > >> > >> - Smallest size-class now has 12 of 16 bytes usable for storage, > >> compared to 8 of 16 (32-bit archs) or 16 of 32 (64-bit archs) with > >> the old malloc, plus 1.5 bytes (32-bit) or 2.5 bytes (64-bit) of > >> out-of-band metadata. This overhead (5.5 or 6.5 bytes per > >> allocation) is uniform across all sizes. > > > > Make that 6 or 7 since there's also a 16-byte (counting alignment, > > which is most of it) group header that imposes 0.5 bytes of overhead > > per slot for a full-length 32-slot group. > > > > Rich
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