Jemalloc Postmortem

A bad choice of title, as "postmortem" made me think there was some severe outage caused by jemalloc.

Well, that's not the only meaning of "postmortem". The fine article does open with,

"The jemalloc memory allocator was first conceived in early 2004, and has been in public use for about 20 years now. Thanks to the nature of open source software licensing, jemalloc will remain publicly available indefinitely. But active upstream development has come to an end. This post briefly describes jemalloc’s development phases, each with some success/failure highlights, followed by some retrospective commentary."

I understand the decision to archive the upstream repo; as of when I left Meta, we (i.e. the Jemalloc team) weren’t really in a great place to respond to all the random GitHub issues people would file (my favorite was the time someone filed an issue because our test suite didn’t pass on Itanium lol). Still, it makes me sad to see. Jemalloc is still IMO the best-performing general-purpose malloc implementation that’s easily usable; TCMalloc is great, but is an absolute nightmare to use if you’re not using bazel (this has become slightly less true now that bazel 7.4.0 added cc_static_library so at least you can somewhat easily export a static library, but broadly speaking the point still stands).

I’ve been meaning to ask Qi if he’d be open to cutting a final 6.0 release on the repo before re-archiving.

At the same time it’d be nice to modernize the default settings for the final release. Disabling the (somewhat confusingly backwardly-named) “cache oblivious” setting by default so that the 16 KiB size-class isn’t bloated to 20 KiB would be a major improvement. This isn’t to disparage your (i.e. Jason’s) original choice here; IIRC when I last talked to Qi and David about this they made the point that at the time you chose this default, typical TLB associativity was much lower than it is now. On a similar note, increasing the default “page size” from 4 KiB to something larger (probably 16 KiB), which would correspondingly increase the large size-class cutoff (i.e. the point at which the allocator switches from placing multiple allocations onto a slab, to backing individual allocations with their own extent directly) from 16 KiB up to 64 KiB would be pretty impactful. One of the last things I looked at before leaving Meta was making this change internally for major services, as it was worth a several percent CPU improvement (at the cost of a minor increase in RAM usage due to increased fragmentation). There’s a few other things I’d tweak (e.g. switching the default setting of metadata_thp from “disabled” to “auto”, changing the extent-sizing for slabs from using the nearest exact multiple of the page size that fits the size-class to instead allowing ~1% guaranteed wasted space in exchange for reducing fragmentation), but the aforementioned settings are the biggest ones.

Try mimalloc. I have prototyped a feature on top of mimalloc and while effort was a dead end, the code (this was around 2020) was nicely written and well maintained and it was fun to hack on it. When I swapped jemalloc in our system with mimalloc, it was on par if not better when it comes to fragmentation growth control and heap usage perspective.

Oh, interesting. If that's the case, I can see why that'd be a bummer for short-lived command line tools. "Makes ls run 10x slower" would not be well received. OTOH, FreeBSD uses it by default, and it's not known for being a sluggish OS.