Jemalloc Postmortem
(jasone.github.io)789 points by jasone 4 days ago
789 points by jasone 4 days ago
This sounds interesting. Can you share the title of the book?
Stuff like this is what keeps me coming back here. Thanks for posting this!
What's hard about using TCMalloc if you're not using bazel? (Not asking to imply that it's not, but because I'm genuinely curious.)
It’s just a huge pain to build and link against. Before the bazel 7.4.0 change your options were basically:
1. Use it as a dynamically linked library. This is not great because you’re taking at a minimum the performance hit of going through the PLT for every call. The forfeited performance is even larger if you compare against statically linking with LTO (i.e. so that you can inline calls to malloc, get the benefit of FDO , etc.). Not to mention all the deployment headaches associated with shared libraries.
2. Painfully manually create a static library. I’ve done this, it’s awful; especially if you want to go the extra mile to capture as much performance as possible and at least get partial LTO (i.e. of TCMalloc independent of your application code, compiling all of TCMalloc’s compilation units together to create a single object file).
When I was at Meta I imported TCMalloc to benchmark against (to highlight areas where we could do better in Jemalloc) by pain-stakingly hand-translating its bazel BUILD files to buck2 because there was legitimately no better option.
As a consequence of being so hard to use outside of Google, TCMalloc has many more unexpected (sometimes problematic) behaviors than Jemalloc when used as a general purpose allocator in other environments (e.g. it basically assumes that you are using a certain set of Linux configuration options [1] and behaves rather poorly if you’re not)
[1] https://google.github.io/tcmalloc/tuning.html#system-level-o...
Thanks for sharing the insight!
As I observed when I was at Google: tcmalloc wasn't a dedicated team but a project driven by server performance optimization engineers aiming to improve performance of important internal servers. Extracting it to github.com/google/tcmalloc was complex due to intricate dependencies (https://abseil.io/blog/20200212-tcmalloc ). As internal performance priorities demanded more focus, less time was available for maintaining the CMake build system. Maintaining the repo could at best be described as a community contribution activity.
> Meta’s needs stopped aligning well with those of external uses some time ago, and they are better off doing their own thing.
I think Google's diverged from the external uses even long ago:) (For a long time google3 and gperftools's tcmalloc implementations were so different.)
Everything from Google is an absolute pain to work with unless you're in Google using their systems, FWIW. Anything from the Chromium project is deeply intangled with everything else from the Chromium project as part of one gigantic Chromium source tree with all dependencies and toolchains vendored. They do not care about ABI what so ever, to the point that a lot of Google libraries change their public ABI based on whether address sanitizer is enabled or not, meaning you can't enable ASAN for your code if you use pre-built (e.g package manager provided) versions of their code. Their libraries also tend to break if you link against them from a project with RTTI enabled, a compiler set to a slightly different compiler version, or any number of other minute differences that most other developers don't let affect their ABI.
And if you try to build their libraries from source, that involves downloading tens of gigabytes of sysroots and toolchains and vendored dependencies.
Oh and you probably don't want multiple versions of a library in your binary, so be prepared to use Google's (probably outdated) version of whatever libraries they vendor.
And they make no effort what so ever to distinguish between public header files and their source code, so if you wanna package up their libraries, be prepared to make scripts to extract the headers you need (including headers from vendored dependencies), you can't just copy all of some 'include/' folder.
And their public headers tend to do idiotic stuff like `#include "base/pc.h"`, where that `"base/pc.h"` path is not relative to the file doing the include. So you're gonna have to pollute the include namespace. Make sure not to step on their toes! There's a lot of them.
I have had the misfortune of working with Abseill, their WebRTC library, their gRPC library and their protobuf library, and it's all terrible. For personal projects where I don't have a very, very good reason to use Google code, I try to avoid it like the plague. For professional projects where I've had to use libwebrtc, the only reasonable approach is to silo off libwebrtc into its own binary which only deals with WebRTC, typically with a line-delimited JSON protocol on stdin/stdout. For things like protobuf/gRPC where that hasn't been possible, you just have to live with the suffering.
..This comment should probably have been a blog post.
I would love to see these changes - or even some sort of blog post or extended documentation explaining rational. As is the docs are somewhat barren. I feel that there’s a lot of knowledge that folks like you have right now from all of the work that was done internally at Meta that would be best shared now before it is lost.
> filed an issue because our test suite didn’t pass on Itanium lol
For the non low-level programmers in the bowels of memory allocators among us, why is this a "lol"?
The Itanium ISA was an infamous failure, never seeing widespread usage, hence people often referring to it as “The Itanic” (i.e. the much-touted ship that immediately sunk). The fact that anyone would be using it today at all is sort of hilariously niche, and is illustrative of how wide-ranging and obscure the issues filed to the GitHub repo could be. On a similar token I recall seeing an issue (or maybe it was a PR?) to fix our build on GNU Herd.
> we (i.e. the Jemalloc team) weren’t really in a great place to respond to all the random GitHub issues people would file
Why not? I mean this is complete drive-by comment, so please correct me, but there was a fully staffed team at Meta that maintained it, but was not in the best place to manage the issues?
Because you need to build it to use it, and you likely already have significant build related infrastructure, and you are going to need to integrate any new dependencies into that. I'm increasingly convinced that the various build systems are elaborate and wildly successful ploys intended only to sap developer time and energy.
Because you have to build it. If they don't use the same build system as you, you either want to invoke their system, or import it into yours. The former is unappealing if it's 'heavy' or doesn't play well as a subprocess; the latter can take a lot of time if the build process you're replicating is complex.
I've done both before, and seen libraries at various levels of complexity; there is definitely a point where you just want to give up and not use the thing when it's very complex.
This. When step one is "install our weird build system," I'll immediately look for something else that meets my needs. All build systems suck, so everyone thinks they can write a better one, and too many people try. Pretty soon you end up having to learn a majority of this (https://en.wikipedia.org/wiki/List_of_build_automation_softw...) to get your code to compile.
Jason, here is a story about how much your work impacts us. We run a decently sized company that processes hundreds of millions of images/videos per day. When we first started about 5 years ago, we spent countless hours debugging issues related to memory fragmentation.
One fine day, we discovered Jemalloc and put it in our service, which was causing a lot of memory fragmentation. We did not think that those 2 lines of changes in Dockerfile were going to fix all of our woes, but we were pleasantly surprised. Every single issue went away.
Today, our multi-million dollar revenue company is using your memory allocator on every single service and on every single Dockerfile.
Thank you! From the bottom of our hearts!
indeed! most image processing golang services suggest/use jemalloc
the top 3 from https://github.com/topics/resize-images (as of 2025-06-13)
imaginary: https://github.com/h2non/imaginary/blob/1d4e251cfcd58ea66f83...
imgproxy: https://web.archive.org/web/20210412004544/https://docs.imgp... (linked from a discussion in the imaginary repo)
imagor: https://github.com/cshum/imagor/blob/f6673fa6656ee8ef17728f2...
Yep, imgproxy seems to use libvips, that recommends jemalloc. I was checking and this is a funny (not) bug report:
Hello, libvips author here. This is probably the canonical thread about libvips and memory fragmentation, and the funniest graph:
https://github.com/lovell/sharp/issues/955#issuecomment-5458...
(that specific graph is for switching from glib to the musl memory allocator, but jemalloc gives a very similar result)
We use libvips as well sir. We can't overstate how much we benefit from your work!
I really don't mean to be snarky, but honest question: Did you donate? Nothing says thank you like some $$$...
We regularly donate to project via open collective. We frankly did not see here due to FB involvement I think.
> jemalloc was probably booted from Rust binaries sooner than the natural course of development might have otherwise dictated.
FWIW while it was a factor it was just one of a number: https://github.com/rust-lang/rust/issues/36963#issuecomment-...
And jemalloc was only removed two years after that issue was opened: https://github.com/rust-lang/rust/pull/55238
Interesting that one of the factor listed in there, the hardcoded page-size on arm64, is still is an unsolved issue upstream, and that forces app developers to either ship multiple arm64 linux binaries, or drop support for some platforms.
I wonder if some kind of dynamic page-size (with dynamic ftrace-style binary patching for performance?) would have been that much slower.
I've used jemalloc in every game engine I've written for years. It's just the thing to do. WAY faster on win32 than the default allocator. It's also nice to have the same allocator across all platforms.
I learned of it from it's integration in FreeBSD and never looked back.
jemalloc has help entertained a lot of people :)
Nice post -- so does Facebook no longer use jemalloc at all? Or is it maintenance mode?
Or I wonder if they could simply use tcmalloc or another allocator these days?
Facebook infrastructure engineering reduced investment in core technology, instead emphasizing return on investment.
As of when I left Meta nearly two years ago (although I would be absolutely shocked if this isn’t still the case) Jemalloc is the allocator, and is statically linked into every single binary running at the company.
> Or I wonder if they could simply use tcmalloc or another allocator these days?
Jemalloc is very deeply integrated there, so this is a lot harder than it sounds. From the telemetry being plumbed through in Strobelight, to applications using every highly Jemalloc-specific extension under the sun (e.g. manually created arenas with custom extent hooks), to the convergent evolution of applications being written in ways such that they perform optimally with respect to Jemalloc’s exact behavior.
Meta has a fork that they still are working on, where development is continuing.
The point of the blog post is that repo is over-focused on Facebook's needs instead of "general utility":
> as a result of recent changes within Meta we no longer have anyone shepherding long-term jemalloc development with an eye toward general utility
> we reached a sad end for jemalloc in the hands of Facebook/Meta
> Meta’s needs stopped aligning well with those of external uses some time ago, and they are better off doing their own thing.
They take everything FLOSS and ruin it with bureaucracy, churn, breakage, and inconsideration to external use. They may claim FOSS broadly but it's mostly FOSS-washed, unusable garbage except for a few popular things.
React, PyTorch, and RocksDB are all extremely significant. Not to mention them being one of the biggest contributors to the Linux kernel.
The big recent change is that jemalloc no longer has any of its previous long-term maintainers. But it is receiving more attention from Facebook than it has in a long time, and I am somewhat optimistic that after some recent drama where some of that attention was aimed in a counterproductive direction that the company can aim the rest of it in directions that Qi and Jason would agree with, and that are well aligned with the needs of external users.
Suppose this is as good a place to pile-on as any.
Though this was not the post I was expecting to show up today, it was super awesome for me to get to have played my tiny part in this big journey. Thanks for everything @je (and qi + david -- and all the contributors before and after my time!).
I’ve wondered about this before but never when around people who might know. From my outsider view, jemalloc looked like a strict improvement over glibc’s malloc, according to all the benchmarks I’d seen when the subject came up. So, why isn’t it the default allocator?
It is on FreeBSD. :P Change your malloc, change your life? May as well change your libc while you're there and use FreeBSD libc too, and that'll be easier if you also adopt the FreeBSD kernel.
I will say, the Facebook people were very excited to share jemalloc with us when they acquired my employer, but we were using FreeBSD so we already had it and thought it was normal. :)
Disclaimer: I'm not an allocator engineer, this is just an anecdote.
A while back, I had a conversation with an engineer who maintained an OS allocator, and their claim was that custom allocators tend to make one process's memory allocation faster at the expense of the rest of the system. System allocators are less able to make allocation fair holistically, because one process isn't following the same patterns as the rest.
Which is why you see it recommended so frequently with services, where there is generally one process that you want to get preferential treatment over everything else.
The only way I can see that this would be true is if a custom allocator is worse about unmapping unused memory than the system allocator. After all, processes aren't sharing one heap, it's not like fragmentation in one process's address space is visible outside of that process... The only aspects of one process's memory allocation that's visible to other processes is, "that process uses N pages worth of resident memory so there's less available for me". But one of the common criticisms against glibc is that it's often really bad at unmapping its pages, so I'd think that most custom allocators are nicer to the system?
It would be interested in hearing their thoughts directly, I'm also not an allocator engineer and someone who maintains an OS allocator probably knows wayyy more about this stuff than me. I'm sure there's some missing nuance or context or which would've made it make sense.
I don't think that's really a position that can be defended. Both jemalloc and tcmalloc evolved and were refined in antagonistic multitenant environments without one overwhelming application. They are optimal for that exact thing.
> Both jemalloc and tcmalloc evolved and were refined in antagonistic multitenant environments without one overwhelming application. They are optimal for that exact thing.
They were mostly optimised on Facebook/Google server-side systems, which were likely one application per VM, no? (Unlike desktop usage where users want several applications to run cooperatively). Firefox is a different case but apparently mainline jemalloc never matched Firefox jemalloc, and even then it's entirely plausible that Firefox benefitted from a "selfish" allocator.
It's possible that they were referring to something specific about their platform and its system allocator, but like I said it was an anecdote about one engineer's statement. I just remember thinking it sounded fair at the time.
For a long time, one of the major problems with alternate allocators is that they would never return free memory back to the OS, just keep the dirty pages in the process. This did eventually change, but it remains a strong indicator of different priorities.
There's also the fact that ... a lot of processes only ever have a single thread, or at most have a few background threads that do very little of interest. So all these "multi-threading-first allocators" aren't actually buying anything of value, and they do have a lot of overhead.
Semi-related: one thing that most people never think about: it is exactly the same amount of work for the kernel to zero a page of memory (in preparation for a future mmap) as for a userland process to zero it out (for its own internal reuse)
> Semi-related: one thing that most people never think about: it is exactly the same amount of work for the kernel to zero a page of memory (in preparation for a future mmap) as for a userland process to zero it out (for its own internal reuse)
Possibly more work since the kernel can't use SIMD
That’s actually particular try to alternate allocators and not true for glibc if I recall correctly (it’s much worse at returning memory).
As far as I know there is no technical reason why jemalloc shouldn't be the default allocator. In fact, as pointed out in the article, it IS the default allocator on FreeBSD. My understanding is it is largely political.
jemalloc’s been battle tested in prod at scale, its license is permissive, and performance wins are known. so what exactly are we protecting by clinging to glibc malloc? ideological purity? legacy inertia? who’s actually benefiting from this status quo, and why do we still pretend it’s about “compatibility”?
I believe there’s no other allocator besides jemalloc that can seamlessly override macOS malloc/free like people do with LD_PRELOAD on Linux (at least as of ~2020). jemalloc has a very nice zone-based way of making itself the default, and manages to accommodate Apple’s odd requirements for an allocator that have tripped other third-party allocators up when trying to override malloc/free.
Sounds like every workplace I've 'enjoyed' since ~2008
There’s something to that but it is victim blaming if you’re not acknowledging the larger trends. There are a lot of places whose MBAs are attending the same conferences, getting the same recommendations from consultants, and hearing the same demands from investors. The push against remote work, for example, was all driven by ideology against most of the available data but it affected a huge number of jobs.
> The push against remote work, for example, was all driven by ideology against most of the available data but it affected a huge number of jobs.
And before that, open office plans.
You're saving on rent: great. But what is it doing to productivity?
* https://business.adobe.com/blog/perspectives/what-science-sa...
Of course productivity doesn't show up on a spreadsheet, but rent does, so it's what about "the numbers" say.
Oh that's interesting. jemalloc is the memory allocator used by redis, among other projects. Wonder what the performance impact will be if they have to change allocators.
Why would they have to change? Sometimes software development is largely "done" and there isn't much more you need to do to a library.
Memory allocators are something I expect to rapidly degrade in the absence of continuous updates as the world changes underneath you. Changing page sizes, new ucode latencies, new security features etc. all introduce either outright breakage or at least changing the optimum allocation strategy and making your old profiling obsolete. Not to mention the article already pointed out one instance where a software stack (KDE, in that case) used allocation profiles that broke an earlier version completely. Even though that's fixed now, any language runtime update or new feature could introduce a new allocation style that grinds you down.
As much as it's nice to think software can be done, I think something so closely tied to the kernel and hardware and the application layer, which all change constantly, never can be.
While I certainly wish that more software would reach a "done" stage, I don't think jemalloc is necessarily there yet. Unfortunately I'm aware of there being bugs in the current version of jemalloc, when run in certain environment configurations, including memory leaks. I know the folks that found it were looking to report it, but I guess that won't happen now.
Even from a quick look at the open issues, I can see https://github.com/jemalloc/jemalloc/issues/2838, and https://github.com/jemalloc/jemalloc/issues/2815 as two examples, but there's a fair number of issues still open against the repository.
So that'll leave projects like redis & valkey with some decisions to make.
1) Keep jemalloc and accept things like memory leak bugs
2) Fork and maintain their own version of jemalloc.
3) Spend time replacing it entirely.
4) Hope someone else picks it up?
jemalloc is used enough at Amazon that it would make sense for them to maintain it, but that's not really their style.
It's been 14 years since THP got added to the kernel[1], surely we're past calling that "recent" :)
Another example is rseq (which was originally implemented for tcmalloc).
Some people believe everything must always be constantly tweaked, redone, broken and fixed, and churned for no reason. The only things that need to be fixed in mature, working software are bugs and security issues. It doesn't magically stop working or get "stale" unless dependencies, the OS, or build tools break.
> Sometimes software development is largely "done"
Lol absolutely not
Back in 2008-2009 I remember the Varnish project struggled with what looked very much like a memory leak. Because of the somewhat complex way memory was used, replacing the Glibc malloc with jemalloc was an immediate improvement and removed the leak-like behavior.
The article mentioned the influence of large-scale profiling on both jemalloc and tcmalloc, but doesn't mention mimalloc. I consider mimalloc to be on par with these others, and now I am wondering whether Microsoft also used large scale profiling to develop theirs, or if they just did it by dead reckoning.
All the allocators have the same issue. They largely work against a shared set of allocation APIs. Many of their users mostly engage via malloc and free.
So the flow is like this: user has an allocation looking issue. Picks up $allocator. If they have an $allocator type problem then they keep using it, otherwise they use something else.
There are tons of users if these allocators but many rarely engage with the developers. Many wouldn’t even notice improvements or regressions on upgrades because after the initial choice they stop looking.
I’m not sure how to fix that, but this is not healthy for such projects.
This is true, but the unfortunate thing with how C and C++ were developed is that pretty much everything just assumes the existence of malloc/free. So if you’re using third-party libraries then it’s out of your control mostly. Linking a new allocator is a very easy and pretty much free way to improve performance.
That’s because sane allocators that aren’t glibc will return unused memory periodically to the OS while glibc prefers to permanently retain said memory.
glibc will return memory to the OS just fine, the problem is that its arena design is extremely prone to fragmentation, so you end up with a bunch of arenas which are almost but not quite empty and can't be released, but can’t really be used either.
In fact, Jason himself (the author of jemalloc and TFA) posted an article on glibc malloc fragmentation 15 years ago: https://web.archive.org/web/20160417080412/http://www.canonw...
And it's an issue to this day: https://blog.arkey.fr/drafts/2021/01/22/native-memory-fragme...
glibc does NOT return memory to the OS just fine.
In my experience it delays it way too much, causing memory overuse and OOMs.
I have a Python program that allocates 100 GB for some work, free()s it, and then calls a subprocess that takes 100 GB as well. Because the memory use is serial, it should fit in 128 GB just fine. But it gets OOM-killed, because glibc does not turn the free() into an munmap() before the subprocess is launched, so it needs 200 GB total, with 100 GB sitting around pointlessly unused in the Python process.
This means if you use glibc, you have no idea how much memory your system will use and whether they will OOM-crash, even if your applications are carefully designed to avoid it.
Similar experience: https://news.ycombinator.com/item?id=24242571
I commented there 4 years ago the glibc settings MALLOC_MMAP_THRESHOLD_ and MALLOC_TRIM_THRESHOLD_ should fix that, but I was wrong: MALLOC_TRIM_THRESHOLD_ is apparently bugged and has no effect in some situations.
A bug I think might be involved: "free() doesn't honor M_TRIM_THRESHOLD" https://sourceware.org/bugzilla/show_bug.cgi?id=14827
Open since 13 years ago. This stuff doesn't seem to get fixed.
The fix in general is to use jemalloc with
MALLOC_CONF="retain:false,muzzy_decay_ms:0,dirty_decay_ms:0"
So in jemalloc, the settings to control this behaviour seem to actually work, in contrast to glibc malloc.
(I'm happy to be proven wrong here, but so far no combination of settings seem to actually make glibc return memory as written in their docs.)
From this perspective, it is frightening to see the jemalloc repo being archived, because that was my way to make sure stuff doesn't OOM in production all the time.
You could do the memory heavy python part in a separate process as well. That removes the need to depend on quirks of the allocator
Can you elaborate on this? I don't know much about allocators.
How would the allocator know that some block is unused, short of `free` being called? Does glibc not return all memory after a `free`? Do other allocators do something clever to automatically release things? Is there just a lot of bookkeeping overhead that some allocators are better at handling?
When `free()` is called, the allocator internally marks that specific memory area as unused, but it doesn't necessarily return that area back to the OS, for two main reasons:
1. `malloc()` is usually called with sizes smaller than the sizes by which the allocator requests memory from the OS, which are at least page-sized (4096 bytes on x86/x86-64) and often much larger. After a `free()`, the freed memory can't be returned to the OS because it's only a small chunk in a larger OS allocation. Only after all memory within a page has been `free()`d, the allocator may, but doesn't have to, return that page back to the OS.
2. After a `free()`, the allocator wants to hang on to that memory area because the next `malloc()` is sure to follow soon.
This is a very simplified overview, and different allocators have different strategies for gathering new `malloc()`s in various areas and for returning areas back to the OS (or not).
They're not really correct, glibc will return stuff back to the OS. It just has some quirks about how and when it does it.
First, some background: no allocator will return memory back to the kernel for every `free`. That's for performance and memory consumption reasons: the smallest unit of memory you can request from and return to the kernel is a page (typically 4kiB or 16kiB), and requesting and returning memory (typically called "mapping" and "unmapping" memory in the UNIX world) has some performance overhead.
So if you allocate space for one 32-byte object for example, your `malloc` implementation won't map a whole new 4k or 16k page to store 32 bytes. The allocator probably has some pages from earlier allocations, and it will make space for your 32-byte allocation in pages it has already mapped. Or it can't fit your allocation, so it will map more pages, and then set aside 32 bytes for your allocation.
This all means that when you call `free()` on a pointer, the allocator can't just unmap a page immediately, because there may be other allocations on the same page which haven't been freed yet. Only when all of the allocations which happen to be on a specific page are freed, can the page be unmapped. In a worst-case situation, you could in theory allocate and free memory in such a way that you end up with 100 1-byte allocations allocated across 100 pages, none of which can be unmapped; you'd be using 400kiB or 1600kiB of memory to store 100 bytes. (But that's not necessarily a huge problem, because it just means that future allocations would probably end up in the existing pages and not increase your memory consumption.)
Now, the glibc-specific quirk: glibc will only ever unmap the last page, from what I understand. So you can allocate megabytes upon megabytes of data, which causes glibc to map a bunch of pages, then free() every allocation except for the last one, and you'd end up still consuming many megabytes of memory. Glibc won't unmap those megabytes of unused pages until you free the allocation that sits in the last page that glibc mapped.
This typically isn't a huge deal; yes, you're keeping more memory mapped than you strictly need, but if the application needs more memory in the future, it'll just re-use the free space in all the pages it has already mapped. So it's not like those pages are "leaked", they're just kept around for future use.
It can sometimes be a real problem though. For example, a program could do a bunch of memory-intensive computation on launch requiring gigabytes of memory at once, then all that computation culminates in one relatively small allocated object, then the program calls free() on all the allocations it did as part of that computation. The application could potentially keep around gigabytes worth of pages which serve no purpose but can't be unmapped due to that last small allocation.
If any of this is wrong, I would love to be corrected. This is my current impression of the issue but I'm not an authoritative source.
Looking at all the comments and lightly browsing the source code, I'm amazed. Both at how much impact a memory allocator can make, but also how much code is involved.
I'm not really sure what I expected, but somehow I expect a memory allocator to be ... smaller, simpler perhaps?
Memory allocators can be simple. In fact it was an assignment for a course in the 2nd year of my CS degree to make an (almost) complete allocator.
However it is typically always more complex to make production quality software, especially in a performance sensitive domain.
Naive allocators are very easy: just subdivide RAM and defragment only when absolutely necessary (if virtual memory is unavailable). Performant allocators are hard.
I think we lost a great deal of potential when ORCA was too tied to Pony and not extracted to a framework, tool, and/or library useful outside of it such as integrated or working with LLVM.
It’s the same way with garbage collectors.
You can write a naive mark-and-sweep in an afternoon. You can write a reference counter in even less time. And for some runtimes this is fine.
But writing a generational, concurrent, moving GC takes a lot of time. But if you can achieve it, you can get amazing performance gains. Just look at recent versions of Java.
You can write a simple size-class allocator (even lock-free) in just a couple dozen lines of code. (I've done it both for interviews and for a work presentation.) But an allocator that is fast, scalable, and performs well over diverse workloads--that is HARD.
Maybe add a link to the post on the github repo. I feel like this is important context for people visiting the repo in the future
Thank you. Jemalloc was recently recommended to me on some presentation about Java optimization.
I wonder if you did get everything you should from the companies that use it. I mean sometimes I feel that big tech firms only use free software, never giving anything to it, so I hope you were the exception here.
Lesson: Don't let one megacorp dominate or take over your FOSS project. Push back somewhat and say "no" to too much help from one source.
I think the author was happy to be employed by a megacorp, along with a team to push jemalloc forward.
He and the other previous contributors are free to find new employers to continue such an arrangement, if any are willing to make that investment. Alternatively they could cobble together funding from a variety of smaller vendors. I think the author is happy to move on to other projects, after spending a long time in this problem space.
I don’t think that “don’t let one megacorp hire a team of contributors for your FOSS project” is the lesson here. I’d say it’s a lesson in working upstream - the contributions made during their Facebook / Meta investment are available for the community to build upon. They could’ve just as easily been made in a closed source fork inside Facebook, without violating the terms of the license.
Also Mozilla were unable to switch from their fork to the upstream version, and didn’t easily benefit from the Facebook / Meta investment as a result.
I very recently used jemalloc to resolve a memory fragmentation issue that caused a service to OOM every few days. While jemalloc as it is will continue to work, same as it does today, I wonder what allocator I should reach for in the future. Does anyone have any experiences to share regarding tcmalloc or other allocators that aim to perform better than stock glibc?
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.
been using jemalloc unknowingly for a long time. only after reading this post it hit how much of it was under the hood in things I’ve built. didn’t know the gc-style decay mechanism was that involved, or that it handled fragmentation with time-based heuristics. surprising how much tuning was exposed through env vars. solid closure
Why are those two mutually exclusive? I'd think that a high performance allocator would be especially crucial in the implementation of a fast garbage collected language. For example, in Python you can't alloc(n * sizeof(obj)) to reserve that much contiguous space for n objects. Instead, you use the builtins which isolate you from that low-level bookkeeping. Those builtins have to be pretty fast or performance would be terrible.
The maintainers are probably all making personally reasonable choices that we should support.
But it’s still sad that there’s probably no world where someone will still focus on jemalloc with professional support from their employer. It means that an important piece of technology will not continue improving.
Forking is possible, but it doesn’t look like the kind of project that many people could fork and improve, it requires a lot of focus by people with specific domain knowledge.
I don't understand why you don't understand that you can be sad about this.
Parent stated that he's sad the project is no longer maintained. That's a perfectly reasonable and human response. Parent does not have to defend having an emotion, even less provide objective truth for why he feels sad.
If you don't agree, fine. But I don't see why one would write a paragraph long statement, I validating someone's emotional response.
> That's a perfectly reasonable and human response.
It's hardly a reasonable response. By casually saying that it's "sad" when a maintainer gives up on a project, GP is also including a very real and heavy implication that this is somehow wrong on their part and that they should continue to shoulder that burden, as a demand from the community. This is a really unhealthy attitude and we should all do away with it. Instead, just acknowledge that the project is now there for the taking by anyone who may be interested. There's nothing "sad!" about this whatsoever, and we should not pretend that there is.
If my kid is sad that she cannot have an extra granola bar, that does not imply that I am wrong to deny her, or even that she thinks I am wrong, it just means she wishes she could have one.
You don't really get to decide whether someone's entirely in-passing mention of an emotional response to developments in something they worked on is 'reasonable' or not, and more importantly, it's not at all a topic of interesting conversation, just pedantic nitpicking.
A bad choice of title, as "postmortem" made me think there was some severe outage caused by jemalloc.
I think this implies your understanding of the term “post-mortem” is incorrect, rather than the title.
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."
postmortem is looking back after an event. That can be a security event/outage, it can also be the completion of a project (see: game studios often do postmortems once their game is out to look back on what went wrong and right between preproduction, production, and post launch).
It's weird that we use "postmortem" in those cases since the word literally means "after death"; kind of implying something bad happened. I get that most of these postmortems are done after major development ceases, so it kind of is "dead" but still.
Surely a "retrospective" would be a better word for a look back. It even means "look back.
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.