Mold: A Modern Linker

by signa11 • 5 years ago

299 122 5 years ago

CyberRabbi3• 5 years ago

I won't avoid Unix-ism when writing code (e.g. I'll probably use fork(2)).

It’s probably fine to not avoid most Unix APIs but fork() is truly an exception here. Fork() is not friendly to any third party library you may use because their state may become invalidated after a fork but the library has no way to know if the process has been forked. This is especially bad if the library uses multi threading. The best way to avoid a random unintended bug later on is to only use fork() when you plan to execve() right after.

rui31416• 5 years ago

Author here. Good point. I ended up not using fork() without exec(), so that should be fine now, but here is my original plan to use fork():

I wanted to keep a linker process running as a daemon so that it doesn't read the same files over and over again. After loading input files, the linker becomes a daemon and calls fork() to create a worker process. Then the worker process does the rest of linking. In other word, a daemon is a "clean" copy of a linker process image, and each child is specialized for each actual linker invocation.

It turned out that the linker runs much slower with fork() because of the overhead of copy-on-write. You cannot keep a fresh copy of a process just by calling fork() for free. There's a tax associated with it. I tried to workaround, but in the end I had to give up with the fork()-based worker process design.

dataflow• 5 years ago

in the end I had to give up with the fork()-based worker process design.

Honestly that's a good thing, or your program just fundamentally couldn't possibly ever work on native Windows!

cout2• 5 years ago

I wonder if the tax could be reduced with huge pages. Much of the cost of COW is the large number of page faults, but with huge pages you could reduce the number of faults to 1.

bla32• 5 years ago

What about posix_spawn() with POSIX_SPAWN_USEVFORK? That saves some of the overhead. See eg https://github.com/rtomayko/posix-spawn#benchmarks

chubot3• 5 years ago

I can't think of any reason a linker needs to use many libraries, especially not one that start threads. A linker reads files and writes a file. It's a single-purpose tool, not a monolithic app with many dependencies.

Either way, threads should be controlled by the application and not libraries. Well-written libraries like sqlite and Lua are parameterized by I/O and concurrency. They don't read files and start threads behind your back.

_dh54• 5 years ago

I agree with you on both points but when building large complex applications (or tools!) this stuff tends to unintentionally creep up over time and it’s not fun to debug non-deterministic bugs due to an opaquely broken implicit contract between you and a poorly written library that got pulled in transitively. Sometimes those libraries are unavoidable closed source system libraries (https://bugs.python.org/issue33725) and the issue ends up being not easily fixable. It’s just risk minimization.

0823498723498722• 5 years ago

Forget libraries, a static linker (back in the day) only needed something like half-a-dozen different syscalls, not even a full userland or anything as bougie as libraries.

(if it's unclear, this is strong agreement with chubot. If you make the problem harder than it has to be, you have only yourself to blame...)

FeepingCreature• 5 years ago

Damn, the idea of string interning symbol names in a preload pass is tight. That's one of those sentences that you read and from then on it's just obviously the right way to write a linker.

cout2• 5 years ago

I was surprised to learn that this isn't how linkers work. Apparently one of the reasons c++ template code is so slow to link is long symbol names.

_a1_2• 5 years ago

As an implementation strategy, we do not care about memory leak because we really can't save that much memory by doing precise memory management. It is because most objects that are allocated during an execution of mold are needed until the very end of the program. I'm sure this is an odd memory management scheme (or the lack thereof), but this is what LLVM lld does too.

The fact that someone does it wrong doesn't mean that we should do it wrong as well ;(

I don't think I like this approach. It may work now, but will probably seriously limit the possibilities in the future.

klodolph• 5 years ago

If you think this approach is wrong, could you articulate the reasons why you think it is wrong? This is a classic memory management strategy... if a program is running as a batch program, all memory will be freed when the program exits. Any alternative memory management strategy would have to free and then reuse memory in order to show improvement. If it's a small amount of memory freed, or if the memory is unlikely to be reused, the benefits of freeing memory are smaller and it may actually slow the program down.

The fact that this program can successfully link Chrome means that we have fairly solid baseline performance metrics we can use for "big" programs. Chrome is just about the largest program you might ever need to link.

spijdar• 5 years ago

Yeah, this is also the strategy GCC and co use generally AFAIK. In a program like GCC where a single invocation will operate over a single file/unit, there's just not much benefit to trying to re-use data; if GCC or LLVM were closer to "build servers" with persistent state that compiled and linked objects on demand then it'd make sense, but in their current model, it's easier and safer to just keep data around.

_a1_2• 5 years ago

If you think this approach is wrong, could you articulate the reasons why you think it is wrong?

Because later if you want to reuse parts of the code in a continuous environment (e.g. a daemon), then you will be surprised that you have memory leaks all over the place (or worse, someone else will discover it by accident).

I don't have a problem with the end-of-process-releases-all-memory optimization. But I had the impression that the author uses let's-worry-about-leaks-later-because-OS-takes-care-of-it-for-free-(in-my-use-case).

Best approach to take would be to create a memory pool with fast allocation (e.g. TLAB allocation in Java, or how computer games do it), in order to have control over how the memory is freed or when.

gravypod• 5 years ago

This page is extremely correct about how extremely under-documented linker scripts are. I wish I could learn more about them since you can do a lot of C/C++ magic.

dbrgn• 5 years ago

This might be of interest to you: https://twitter.com/theavalkyrie/status/1349458442734469123

brokencode• 5 years ago

Is it possible to use alternate linkers with Rust? Anything that helps Rust compile faster is pretty cool, especially if it allows better multithreaded scaling.

rui31416• 5 years ago

I believe the Rust compiler can use an alternative linker, but I'm pretty sure that mold can't link Rust programs because it lacks lots of features. It's so experimental that I didn't even think about trying to link a Rust program with it.

k_bx• 5 years ago

Yes, put this in your ~/.cargo/config.toml:

[target.x86_64-unknown-linux]

linker = "ld.lld"

zamalek2• 5 years ago

And it does help compilation times quite a bit.

lankalanka• 5 years ago

Sure you can. Please refer to the doc of zld on how to use zld as an alternative linker for Rust as an example:

https://github.com/michaeleisel/zld#if-using-rust

jdblair• 5 years ago

From a marketing perspective, "mold" meaning "a form used to cast an object from liquid" is a lot more appealing than "green fungus growing on bread." A mold for casting objects is also a lot closer, metaphorically speaking, to what a linker does.

I honestly thought that was the meaning the author was trying to evoke before I saw the picture on the github page.

nothasan• 5 years ago

Would emulsifier roll off the tongue better?

curlypaul9242• 5 years ago

It would likely get shortened to emu, and then we would all be wondering if it's an emulator or a large bird.

rui31416• 5 years ago

Author here. Haha, that's perhaps true. But at the same time, it seems like a tradition to give a silly name (e.g. "git") to a tool, and I actually like that name and the image to show that I'm not too serious. This is a fun project but not ready for production use.

hobofan• 5 years ago

that name and the image to show that I'm not too serious. This is a fun project but not ready for production use.

I'm not sure if that image is the best way to communicate that status, given that the sudo sandwich logo exists (which coincidentally bears some resemblance to your moldy bread). A big bold "not ready for production" at the top of the README is probably a better way to achieve that.

cb321• 5 years ago

Sounds like you just need a multi-bump cake/jello mold like [1] with multiple "input spigots" pouring in with a "fast harden" aspect to have the perfect logo/name combo. Not sure how to convey rapid hardening with simple art, though... :-)

EDIT: You may just have to settle for speed/parallelism being conveyed by 2..3 spigots pouring in. :-) It's perfect - you can stay with moldy bread while it is a major work in progress and evolve to the more finished logo when your own work is "hardened" -- all without changing the name. ;-)

[1] https://www.foodandwine.com/cooking-techniques/baking/best-b...

XorNot2• 5 years ago

Given the C was named because it was derived from B, I think there's an effective tradition in compsci of naming things by just playing with the letters.

M comes after G so the name tracks perfectly while also being weird and distinctive.

felixr• 5 years ago

I was going to say: but the casting template is spelled 'mould' not 'mold'.

Looked it up and realised that the US spelling is actually also 'mold' https://www.oxfordlearnersdictionaries.com/definition/englis...

__alexs• 5 years ago

I wonder if this another Buffalo situation. Mold is also a place name...

kleiba• 5 years ago

Moreover, the fungus is spelled 'mould' in British English: https://dictionary.cambridge.org/dictionary/english/mould

Twisol2• 5 years ago

On the other hand, Rust was apparently named after the fungus, not iron oxide. There seems to be a new category of low-level tools named after life forms that grow in ecological niches :D

curlypaul9242• 5 years ago

How interesting. Doesn't the fungus get its name from the oxidized metal though and not the other way around? Mold the form and mold the fungus seem etymologically unrelated, however.

rui31416• 5 years ago

For the record, a candidate for another name was "weld" as it joins pieces of data into a single binary. That's I think a good name, but I couldn't come up with a backronym.

formerly_proven• 5 years ago

wildly exhilarated link editor

pwdisswordfish6• 5 years ago

That's fine, because it's not exactly clear what "ld" stands for to begin with. If anyone asks, you could just say that the W and E stand for "Weally Efficient".

siithy• 5 years ago

How about "wildly experimental linking device"?

Edit: Had fun thinking up a couple others:

"whimsically eclectic logical design"

"willfully egregious lackadaisical decision"

"wise element location director"

"world exploring layout detector"

"wrong-headed exasperating liability defender"

"workaday execution layer developer"

"wonderfully elegant logistical delegator"

Supersaiyan_IV• 5 years ago

That's funny because BFD linker's original meaning was Big F**ing Deal. Linkers sure are marketable oddballs.

saagarjha• 5 years ago

It’s interesting to see where the bottlenecks are. Some of these are clearly algorithmic and many are solved with simplifying the work or using concurrency, but a lot are platform-specific and nonobvious.

KingOfCoders2• 5 years ago

Can this already be used with Rust? I currently use LDD which already brought some speed up.

rjsw• 5 years ago

Did you mean lld ?

KingOfCoders2• 5 years ago

Yes, thx!

kasabali2• 5 years ago

...

thethirdone• 5 years ago

Not caring amount memory leaks is not the same as not caring about memory consumption. The next line shows exactly why the choice is made.

It is because most objects that are allocated during an execution of mold are needed until the very end of the program.

Using free would not affect that memory consumption much because most consumption would be freed at the end of the program.

kasabali2• 5 years ago

...

zamalek2• 5 years ago

Raymond Chen has an amusing story about this[1]:

Since the missile will explode when it hits its target or at the end of its flight, the ultimate in garbage collection is performed without programmer intervention.

[1] https://devblogs.microsoft.com/oldnewthing/20180228-00/?p=98...

hyperman1• 5 years ago

There might be a 'best of both worlds' idea in there: If mold mmaps the input files instead of reading them, the linker would not trash all cached files, and be faster itself as it reuses already cached files.

Now the author seems a smart fellow, so maybe he did just that already, I didn't check the source.

rui31416• 5 years ago

mold uses only mmap for file IO. Not only input files but also an output file are mapped to memory using mmap(2).

teddyh2• 5 years ago

I am not sure that multi-threaded is the way to optimize file writes; I would have thought that leaving as much as possible up to the kernel, by using writev(2), splice(2), etc. would be best.

rui31416• 5 years ago

I chose mmap based on benchmarking. Writing a 2 GiB memory buffer to a file using write(2) was slower than directly constructing file contents to mmap'ed memory region.

What is more interesting is the real bottleneck was not about mmap vs write(v) but in the filesystem. If you create a fresh file and write 2 GiB of data to that file, it takes like 700 milliseconds on my machine (ext4 fs), but if you write the same amount of data to an existing 2 GiB file, the IO speed doubles. So, the filesystem's performance to allocate new disk blocks seems to limit the performance of my linker. That reminded me of the axiom: don't guess about performance but measure.

teddyh2• 5 years ago

So, the filesystem's performance to allocate new disk blocks seems to limit the performance of my linker.

If you know the total size ahead of time, you might try using fallocate(2).

pjmlp3• 5 years ago

That is only a solution if you want to hardcore it to a specific OS.

enriquto3• 5 years ago

So happy to see progress in linkers!

Can this one statically link shared libraries? That would be a great feature.

EDIT: I realize that such a feature would necessarily involve a fair amount of black magic. But it does not seem an impossible endeavor.

rui31416• 5 years ago

Are you asking if mold can link an executable file and its depending .so files into a single binary? If so, neither mold nor other major linkers can't do that.

enriquto3• 5 years ago

Exactly. That would be a pretty big deal and definitely a killer feature. Equivalently, a tool for converting .a to .so

PoignardAzur• 5 years ago

I wonder how rui314's assertion that incremental linking is a poor tradeoff squares with Zig's decision[1] to build its own linker with "in-place binary patching".

I assume part of the difference is that Zig has complete control over its environment, whereas Mold is trying to be a general-purpose linker, but still, I wonder if there's some insight to be gained from crosspollination there.

(like, maybe Mold could have an "--incremental" option with associate documentation explaining that this option is only beneficial if the input object files follow some specific guidelines)

[1] https://kristoff.it/blog/zig-new-relationship-llvm/

rui31416• 5 years ago

I got an impression that (so take it with a grain of salt), for incremental linking, Zig emits code that always uses PLT and GOT to access global functions and variables. Later, in order to replace functions or variables, Zig rewrites only PLT or GOT entries.

On the other hand, by default, gcc or clang emits code that does not use GOT or PLT, which makes the situation much more complicated.

In addition to that, maybe you don't have to support all ELF fancy features if you know that you are linking Zig programs? I'm not familiar with Zig, but I can say that some kind of minor feature, such as weak symbol, can make incremental linking a lot harder.

AndyKelley• 5 years ago

Your impression is correct.

When the Zig compiler is asked to produce an executable, using only .zig source code, it is in full control of the entire compilation process, frontend to backend, so it can make decisions that ease the requirements of its linker, in order to facilitate incremental compilation and linking. For example, when linking pure Zig projects, there are no relocations; the code is generated directly in place in the final executable, almost as if there is no linker step. However, when asked to link against third party objects or static libraries, Zig must compromise some of these advantages. Currently, in this situation, Zig will fall back to doing the incremental compilation of .zig code into an object file, and then invoke LLD (via the zig executable invoking itself as a child process) to link the objects together. As the Zig self-hosted linker gains functionality, this fallback will happen less often; instead the compromise will be in the code paths taken in the linker, based on what assumptions it can make about the linking requirements that are required for a given job. The long term plan is to eliminate the dependency on LLD altogether.

Side note - mold is a brilliant project! Thank you for making it and pushing the state of the art forward! Also I love the logo.

lhorie• 5 years ago

Zig's compiler isn't optimizing for linking speed over everything else though AFAIK. Also, Andrew says in the thread[0] that the linker doesn't (yet) address the types of complex scenarios that rui talks about.

[0] https://github.com/ziglang/zig/issues/1535#issuecomment-6784...

jeffrallen• 5 years ago

Yes!!!! This is excellent tool engineering, choosing the right target and setting a high but attainable target which fundamentally drives the design. I love the idea that `mold` "competes" with `cat`. That right there is genius framing of the problem.

Ecco• 5 years ago

I’m surprised by the statement that ‘cat’ is slow because it’s not multithreaded.

Isn’t ‘cat’ io-bound?

IshKebab• 5 years ago

I think his point is that you can match `cat`s speed because it is IO bound and you can make a linker IO bound by using lots of threads.

0823498723498722• 5 years ago

It used to be lore in the industry that any decent linker is IO bound, including the single-threaded ones.

baruch2• 5 years ago

One way to speed up IO-bound code is to have multiple threads-of-execution (threads or async-io) in order to have multiple requests in the air. Specifically SSDs benefit a lot from multiple requests in parallel as that utilizes their internal parallelism. HDDs would benefit only a little bit but RAIDed HDDs can also benefit from parallelism.

fanf2• 5 years ago

Assuming that the linker script(s) used by a program don’t change very often, it might be interesting to compile them, in effect building a custom linker specialized to the script(s). Most programs don’t specify a linker script so maybe the linker can be specialized just to the usual scripts, and odd cases can use post-link binary editing.

fwsgonzo2• 5 years ago

I would use this in a heartbeat if you make execute-only a first-class feature. That means segments with E only, no reading.

My experience with every linker so far is that to have XO I will need to specify the memory layout in a linker script manually. It's not as nice as simple linker argument, if that is possible.

rui31416• 5 years ago

By execute-only segment, you mean a segment which is not readable but executable, right?

If so, that's a relatively new CPU security feature. I think some ARM processors support it, but AFAIK x86 doesn't support it at the moment. On x86, if you make a page executable, it automatically makes the page readable. R and X bits are not separated in the page table. I bet Intel and AMD will ad NR bit (no read bit - analogous to NX bit) soon, though.

fwsgonzo2• 5 years ago

Intel slide deck here: https://linuxplumbersconf.org/event/4/contributions/283/atta...

If they really support it, I didn't pay too close attention, but at least it's on their mind.

joosters• 5 years ago

If we mmap a lot of files, _exit(2) is not instantaneous but takes a few hundred milliseconds because the kernel has to clean up a lot of resources. As a workaround, we should organize the linker command as two processes; the first process forks the second process, and the second process does the actual work. As soon as the second process writes a result file to a filesystem, it notifies the first process, and the first process exits. The second process can take time to exit, because it is not an interactive process.

Is this safe? Are you sure that the _exit() delays are not part of the kernel committing all the pending mmap I/O to the buffer caches? If a build script links a binary using this, and then immediately executes it, the second (background process) might still not be finished. Is it guaranteed that all of the mmap I/O will be visible - or will the binary appear incomplete?

I don't know the answer to this myself - it would be clear cut if the linker was using write() calls, because UNIX guarantees that future read() calls will see the results, but the ordering guarantees with mmap I/O are far more loose, I believe.

rui31416• 5 years ago

It is safe because the child process calls munmap before telling its parent process to exit. munmap is guaranteed to act as a commit operation. Alternatively, you can call msync (https://man7.org/linux/man-pages/man2/msync.2.html) if you want to keep it mmapped.

amluto2• 5 years ago

Linux gives much stronger guarantees than POSIX here. I wonder if you save measurable time by skipping munmap.

hctaw• 5 years ago

Overwriting executables during a build rather than copying them at the end can be problematic if a build artifact is used to bootstrap itself (like a compiler). If the build fails you can't try again.

XorNot2• 5 years ago

I just want to say that this is one of the most interesting README.md's I've ever read in a project.

Those performance improvements are crazy as well: here's hoping this becomes the standard.

malkia4• 5 years ago

Since perf is at utmost importance for this project, and intern has been found to be used a lot, maybe a pinch of small optimization is to move the static ConcurrentMap out of the function, hence avoid atomic<bool> check on whether it's initialized -

  static Symbol *intern(std::string_view name) {
    static ConcurrentMap<Symbol> map;
    return map.insert(name, {name});
  }

  static ConcurrentMap<Symbol> map;

  static Symbol *intern(std::string_view name) {
    return map.insert(name, {name});
  }

probably though it won't bring much, but for the sake of squeezing every bit out there (and it was the easiest I could find - lol)

malkia4• 5 years ago

Wait this code can't work - as you holding only a string_view in the Symbol...

  // __start_ and __stop_ symbols
  for (OutputChunk *chunk : chunks) {
    if (is_c_identifier(chunk->name)) {
      start(Symbol::intern("__start_" + std::string(chunk->name)), chunk);
      stop(Symbol::intern("__stop_" + std::string(chunk->name)), chunk);
    }
  }

malkia4• 5 years ago

Or this:

for (std::string_view arg : config.version_script) parse_version_script(std::string(arg));

ArchOversight2• 5 years ago

This does seem broken, since the temporary string that is created by the concatenation is short lived, and the std::string_view inside the Symbol won't hold on to it, just the pointer to the data.

rui31416• 5 years ago

Ah, that's a bug. Thank you for finding it!

ncmncm2• 5 years ago

Standard C++ has parallel algorithm primitives so there no need to depend on TBB anymore.

You can give each thread its own malloc that just mmaps what it needs, if you are leaking everything anyway.

This is a case where using a raw new() and raw pointers is much better than using a smart pointer, because touching things to run destructors unnecessarily is itself expensive.

volta83• 5 years ago

the most important thing is to fix the layout of an output file as quickly as possible, so that we can start copying actual data from input object files to an output file as soon as possible.

Why do we need to copy data at all ?

Can't we just have an object file with pointers to other files ?

Like sure, if I ever want to ship my binary somewhere else, I'd like to do this. But for local interactive development, all the files are already in my local machine, so I don't know why we would need to create a second copy of them within some other file.

Tyr42• 5 years ago

Hm, well what happens when you execv a executable then? Everything needs to get copied into ram and then jumped to, so you would be putting a linker implementation into a syscall right? And if it's just as easy as concattening the files, them just do that in userspace.

Mold: A Modern Linker

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