Fast Allocations in Ruby 3.5
70 comments
·May 22, 2025hinkley
tenderlove
Thank you! Wish it had panned out for Class#new, but I don't feel bad about doing it. :)
hinkley
Giving new its own instruction makes sense.
N_A_T_E
Ruby keeps on getting better. I wouldn't hesitate to start new projects in ruby.
Alifatisk
Don't hesitate, I reach out for Ruby whenever I want to glue things together
alberth
Can someone explain, is YJIT being abandoned over the new ZJIT? [0]
And if so, will these YJIT features likes Fast Allocations be brought to ZJIT?
tenderlove
It's not being abandoned, we're just shifting focus to evaluate a new style of compiler. YJIT will still get bug fixes and performance improvements.
ZJIT is a method based JIT (the type of compiler traditionally taught in schools) where YJIT is a lazy basic block versioning (LBBV) compiler. We're using what we learned developing and deploying YJIT to build an even better JIT compiler. IOW we're going to fold some of YJIT's techniques in to ZJIT.
> And if so, will these YJIT features likes Fast Allocations be brought to ZJIT?
It may not have been clear from the post, but this fast allocation strategy is actually implemented in the byte code interpreter. You will get a speedup without using any JIT compiler. We've already ported this fast-path to YJIT and are in the midst of implementing it in ZJIT.
ysavir
Thanks for all the work you all are putting into Ruby! The improvements in the past few years have been incredible and I'm excited to see the continuous efforts in this area.
FooBarWidget
Why is a traditional method based JIT better than an LBBV JIT? I thought YJIT is LBBV because it's a better fit for Ruby, whereas traditional method based JIT is more suitable for static languages like Java.
tenderlove
One reason is that we think we can make better use of registers. Since LBBV doesn't "see" all blocks in a particular method all at once, it's much more challenging to optimize register use across basic blocks. We've added type profiling, so ZJIT can "learn" types from the runtime.
pjmlp
Usual caveat that while Java is mostly static, is has dynamic runtime semantics inherited from Smalltalk and Objective-C, with dynamic class loading, bytecode generation, proxy classes, reflection, hence why the research work on Smalltalk and Strongtalk ended up being so useful for Hotspot.
null
pusewicz
But Aaron, what do you actually do here?!
I’m so glad to see your work, and it’s always such a treat to read any of your new posts. Hope to see upcoming ones more often!
strzibny
Awesome, thanks for all the good work on Ruby!
whizzter
Yjit is based on Maxime's basic block vesioning PHD work (a JS JIT), the approach both has taken is very dynamic-type-focused, it's a clever way to basically propagate type-info as the code is built. The main benefit is that you get a sane JIT fairly quickly and it should behave well in most dynamic typing scenarios.
They're pivoting (successfully?) to a more traditional way, letting the interpreter first profile the code (to figure out the types) and THEN produce entire methods with heavier optimizations that can do better register allocation.
The BBV approach is sane out of the box but kinda unfamiliar for many compiler writers (problems hiring?) and probably has some performance ceilings without much complexity.
The major question as to what method will win out depends on this question, how "monomorphic" or "polymorphic" is Ruby code in real life?
Monomorphic basically means that only one "real type"(from the compilers point of view) will ever pass a codepath (and thus extra machinery to allow multiple types won't bring much benefit).
ksec
>For this reason, we will continue maintaining YJIT for now and Ruby 3.5 will ship with both YJIT and ZJIT. In parallel, we will improve ZJIT until it is on par (features and performance) with YJIT.
I guess YJIT will always be faster in warmup and minimal increase of memory usage. ZJIT being more traditional should bring more speedup than YJIT.
But most of the speedup right now is still coming from rewriting C into Ruby.
uticus
> But most of the speedup right now is still coming from rewriting C into Ruby.
Quick glance, this statement seems backwards - shouldn't C always be faster? or maybe i'm misunderstanding how the JIT truly works
molf
C itself is fast; it's calls to C from Ruby that are slow. [1]
Crossing the Ruby -> C boundary means that a JIT compiler cannot optimize the code as much; because it cannot alter or inline the C code methods. Counterintuitively this means that rewriting (certain?) built-in methods in Ruby leads to performance gains when using YJIT. [2]
[1]: https://railsatscale.com/2023-08-29-ruby-outperforms-c/ [2]: https://jpcamara.com/2024/12/01/speeding-up-ruby.html
vidarh
Unless your JIT can analyse the full code, a transition between byte code and native code is often costly because the JIT won't be able to optimize the full path. Once your JIT generates good enough code, it then becomes faster to avoid that transition even in cases when in isolation native code might still be faster.
EDIT: Note that this isn't an inherent limit. You could write a JIT that could analyze the compiled C code too. It's just that it's much harder to do.
nightpool
The sibling comments mention that C is used in a lot of places in Ruby that incur cross-language overheads, which is true, but it's also just true that in general, even ignoring this overhead, JIT'd functions are going to be faster then their comparable C functions, because 1) they have more profiling information to be able to work from, 2) they have more type information, and (as a consequence of 1&2) 3) they're more likely to be monomorphized, and the compiler is more able to inline specialized variants of them into different chunks of the code. Among other optimizations!
nightpool
It doesn't sound like YJIT is being abandoned at all. Reading between the lines, it sounds like they want to invest most of their new development right now into a less-experimental architecture that's closer to other JITs and is easier to develop, but that they consider this a somewhat risky endeavor and aren't sure whether this investment is going to pan out in the long run. So they're going to try ZJIT out, but YJIT and the ideas behind it are by no means abandoned. They're just taking a pause to see if a rewrite will make it easier to maintain or produce better results in the long term.
firemelt
after reading your source I'd say YJIT still there up until ZJIT is ready and on par with YJIT
and the features is there when its there
90s_dev
It seems to me like all languages are converging towards something like WASM. I wonder if in 20 years we will see WASM become the de facto platform that all apps can compile to and all operating systems can run near-natively with only a thin like WASI but more convenient.
berkes
Wasn't this the idea of the JVM?
hueho
Java bytecode was originally never intended to be used with anything other than Java - unlike WASM it's very much designed to describe programs using virtual dispatch and automatic memory management. Sun eventually added stuff like invokedynamic to make it easier to implement dynamic languages (at the time, stuff like Ruby and Python), but it was always a bit of round peg in square hole.
By comparison, WASM is really more like traditional assembly, only running inside a sandbox.
pjmlp
Just like CLR bytecode, IBM i TIMI bytecode and many others since 1958.
For some reason when people advocate for WASM outside of the browser, they only remember of the JVM.
90s_dev
I think so, but that was the 90s where we needed a lot more hindsight to get it right. Plus that was mostly just Sun, right? WASM is backed by all browsers and it looks like MS might be looking at bridging it with its own kernel or something?
bgwalter
I don't know. The integration of Java applets was way smoother than WASM.
Security wise, perhaps a different story, though let's wait until WASM is in wide use with filesystem access and bugs start to appear.
lloeki
> that was the 90s
In the meantime the CLR happened too.
And - to an extent - LLVM IR.
foldr
And of course the ill-fated Parrot VM associated with the Perl 6 project.
rhdjsjebshjffn
I think that was more of a language-oriented effort rather than runtime/abi oriented effort.
taf2
It’s not a bad idea. Lot of the same people who worked on JVM were around while the asm - wasm ideas emerged
pjmlp
This has been an idea since UNCOL was discussed as idea back in 1958.
https://en.wikipedia.org/wiki/UNCOL
There are countless bytecode based platforms since 1958, including all famous Xerox PARC systems (the CPUs were microcoded and loaded the related translation code on boot), yet WASM is doing it first keeps being brought up.
Do you know what I call WASI containers running on a Kubernetes cluster, or serverless cloud vendors?
Application Server, https://en.wikipedia.org/wiki/Application_server
writebetterc
Multi-tenancy on the JVM makes me shudder, though. The general point you're making, thumbs up to that.
zerd
Like predicted in 2014 here: https://www.destroyallsoftware.com/talks/the-birth-and-death...
hinkley
> It’s very rare for code to allocate exactly the same type of object many times in a row, so the class of the instance local variable will change quite frequently.
That’s dangerous thinking because constructors will be a bimodal distribution.
Either a graph of calls or objects will contain a large number of unique objects, layers of alternating objects, or a lot of one type of object. Any map function for instance will tend to return a bunch of the same object. When the median and the mean diverge like this your thinking about perf gets muddy. An inline cache will make bulk allocations in list comprehensions faster. It won’t make creating DAGs faster. One is better than none.
munificent
> Any map function for instance will tend to return a bunch of the same object.
Yes, but if it ends up creating any ephemeral objects in the process of determining those returned objects, then the allocation sequence is still not homogeneous. In Ruby, according to the article, even calling a constructor with named arguments allocates, so it's very easy to still end up cycling through allocating different types.
At the same time, the callsite for any given `.new()` invocation will almost always be creating an instance of the exact same class. The target expression is nearly always just a constant name. That makes it a prime candidate for good inline caching at those callsites.
tenderlove
> Yes, but if it ends up creating any ephemeral objects in the process of determining those returned objects, then the allocation sequence is still not homogeneous.
Yes! People might do `map` transformations, but it's very common to do other stuff at the same time. Any other allocations during that transformation would ruin cache hit rate.
> At the same time, the callsite for any given `.new()` invocation will almost always be creating an instance of the exact same class. The target expression is nearly always just a constant name. That makes it a prime candidate for good inline caching at those callsites.
Yes again!
titzer
This is why it's imperative that inline caches learn and adapt to the observed behavior. As long as learning is cheap, identifies profitable cases effectively, and backs off for polymorphic and megamorphic scenarios, it's a win.
VM implementer intuition only goes so far, and as the internet is the greatest fuzzer invented, you're definitely going to encounter programs that break your best laid plans.
munificent
> This is why it's imperative that inline caches learn and adapt to the observed behavior.
True, but if you only have a single bottleneck cache site for all constructor invocations across the program, the only reasonable thing that callsite can learn is "wow, every single constructed class goes through here".
That's why it makes sense to have a separate cache at every `.new()` location.
masklinn
> One is better than none.
Not necessarily. An inline cache is cheap but it's not free, even less so when it also comes with the expense of moving Class#new from C to Ruby. It's probably not worth speeding up the 1% at the expense of the 99%.
> An inline cache will make bulk allocations in list comprehensions faster.
Only if such comprehensions create exactly one type of object, if they create two it's going to slow them down, and if they create zero (just do data extraction) it won't do anything.
hinkley
> Only if such comprehensions create exactly one type of object,
We just had this conversation maybe a month ago. If it’s 50-50 then you are correct. However if it’s skewed then it depends. I can’t recall what ratio was discovered to be workable, it was more than 50% and less than or equal to 90%.
firemelt
did it means more speeds to all rails/active records collections?
ksec
I know I may be jumping the gun a little here but I wonder what percentage speedup could we expect on typical rails applications. Especially with Active Record.
GGO
so far no diff here (https://speed.yjit.org/). But the build is from May 14 so maybe it will show up in new build?
null
> I’ve been interested in speeding up allocations for quite some time. We know that calling a C function from Ruby incurs some overhead, and that the overhead depends on the type of parameters we pass.
> it seemed quite natural to use the triple-dot forwarding syntax (...).
> Unfortunately I found that using ... was quite expensive
> This lead me to implement an optimization for ... .
That’s some excellent yak shaving. And speaking up … in any language is good news even if allocation is not faster.