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| ▲ | masklinn 16 hours ago | parent | next [-] | | > it might completely upend the generational hypothesis The generational hypothesis is about object lifetime, and that doesn't change. It does change the relevance of the generational hypothesis to garbage collection. > Surely someone's done some studies on this? The go team has, and that's why go doesn't have a generational GC. The complexity of adding generational support, especially in a mutation-based language (so needing memory barriers and the like) was found not to benefit when a significant fraction of the newborn objects don't even reach the youngen. See https://github.com/golang/go/discussions/70257#discussioncom... from the current discussion of adding opt-in ad-hoc support for memory regions. | |
| ▲ | ithkuil 14 hours ago | parent | prev | next [-] | | If you flip the argument on its head you can frame it as: since most objects die young it's very likely they will stay on the stack and thus it makes sense to invest in an allocation-site optimizer that will put the object on the heap only if static escape analysis says it may escape the lexical scope. | |
| ▲ | uluyol 15 hours ago | parent | prev | next [-] | | You might be interested in this talk: https://go.dev/blog/ismmkeynote | |
| ▲ | DamonHD 17 hours ago | parent | prev | next [-] | | AFAIK not nearly enough stuff gets caught by escape analysis - and thus stack allocated - to make a difference. | | |
| ▲ | masklinn 16 hours ago | parent | next [-] | | Go has much more significant stack allocation capabilities, most notably it has no problem allocating entire structs on the stack so doesn't need scalar replacement, which falls over if you breathe on it (https://pkolaczk.github.io/overhead-of-optional/). According to https://github.com/golang/go/discussions/70257#discussioncom... > the weak generational hypothesis does not hold up well with respect to heap-allocated memory in many real-world Go programs (think 60-70% young object mortality vs. the 95% typically expected) | |
| ▲ | kibwen 17 hours ago | parent | prev | next [-] | | I'm open to believing that this is true, but some real numbers would be nice. Surely it wouldn't be a hugely invasive change to fork the Go compiler, change the stack allocation check to `return false`, and then measure the overhead of the garbage collector on real Go programs with stack allocation both enabled and disabled. | | |
| ▲ | DarkNova6 16 hours ago | parent | next [-] | | The reason escape analysis is not "good enough" is why we have project Valhalla trying to bring Value Types into the JVM. I don't have numbers at hand, but I remember the JDK Expert Group talking about this extensively in the past and why they deferred bringing Value Types for such a long time. They hoped complex enough EA can get rid of indirections and heap allocations but it just wasn't powerful enough, even with all advances throughout the years. | |
| ▲ | DamonHD 17 hours ago | parent | prev [-] | | I may have been answering past you - I am thinking of Java running on the JDK here. And indeed I may be out of date also. | | |
| ▲ | cempaka 15 hours ago | parent | next [-] | | Yeah in Java land specifically I think the question would become, "does the generational hypothesis still hold up once we have Valhalla and a much larger share of short-lived objects are stack allocated as value types?" but of course it may be years until the ecosystem reaches that point, if ever. | | |
| ▲ | neonsunset 15 hours ago | parent [-] | | As shown by C#, it will generally continue to be relevant since both primarily use JIT compilation with ability to modify code at runtime which can violate inter-procedural escape analysis assumptions leading to heap allocations of the objects that are passed down to the callees (there is work scheduled for .NET 10 to address this, at least for AOT compilation where interproc analysis conclusions will be impossible to violate). You can craft a workload which violates the hypothesis by only allocating objects that live for a long time but both JVM and .NET GC implementations are still much faster designs than Go's GC which prioritizes small memory footprint and consistent latency on low allocation traffic (though as of .NET 9, SRV GC puts much more priority on this, making similar tradeoffs). | | |
| ▲ | cempaka 15 hours ago | parent [-] | | > ability to modify code at runtime Would Java's moves towards "integrity by default" mean that this could be ruled out in more cases? | | |
| ▲ | neonsunset 13 hours ago | parent [-] | | Reading through the JEP again it does not seem to be related - it is about deprecating unsafe APIs that the executed code itself uses. OpenJDK also has "partial escape analysis" where the object that only conditionally escapes can still be placed on the stack/scalar replaced. I'm not privy to the exact APIs that OpenJDK exposes but in .NET the main limitation around escape analysis that spans multiple methods is the fact that CoreCLR has re-JIT API which allows to perform a multitude of actions like attaching a profiler or a debugger to a live application and forcing the runtime to deoptimize a particular method for debugging, or modifying the implementation and re-JITting the result. Debug codegen is very different especially around GC liveness tracking and escape analysis that builds on top of it - it means that even debug code would have to uphold stack-allocated nature of such object in some way, complicating the design significantly. In addition to that, debuggers and other instrumentation may observe object references that would have otherwise not escaped local scope. This creates an unfortunate situation where the above almost never happens in production, but ignoring it and "just stack-allocating anyway" would lead to disastrous breakage for all sorts of existing instrumentation. Because Go does not have to deal with this constraint, it can perform interproc escape analysis without risk - whether a pointer escapes or not can be statically proven. For NativeAOT, .NET could approach this problem in the same way, but paraphrasing compiler team: "We would like to avoid optimizations only available for one of the target types be it JIT or AOT, and only supporting AOT would not benefit the majority of the .NET users". There is, however, recognition that more comprehensive interproc analysis could be very beneficial, including the EA which is why it is planned to work on it in .NET 10: - https://github.com/dotnet/runtime/issues/108931 IPA framework - https://github.com/dotnet/runtime/issues/104936 Stack allocation enhancements | | |
| ▲ | pjmlp 3 hours ago | parent | next [-] | | Integrity by default is what the OpenJDK folks are pushing for so that any API that can break runtime assumptions, has to be explicitly allowed, so that they can actually make use of performance optimizations that would otherwise be too risky if anyone at any time could violate them. | |
| ▲ | cempaka 12 hours ago | parent | prev [-] | | Yeah there's a JEP around deprecating access to sun.misc.Unsafe, but that's part of a larger effort including Jigsaw to push the Java ecosystem in the direction of modular builds, where more invariants are assumed to hold (e.g. " 'final' fields are actually final") unless explicitly opted out for each module. I would assume the lack of such guarantees in the status quo wreaks a lot of havoc with EA. Profiling and debugging would be separate considerations -- I'm really not sure what limitations those impose on the JVM JIT. |
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| ▲ | DamonHD 14 hours ago | parent | prev [-] | | Ahem --- JDK => JVM! |
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| ▲ | fweimer 16 hours ago | parent | prev | next [-] | | Historically, Hotspot's escape analysis only resulted in avoided heap allocations (via scalar replacement) if all uses were inlined. I don't think this has changed. | |
| ▲ | eikenberry 16 hours ago | parent | prev [-] | | Is there a language that makes this explicit, allocates the variables on the stack via compiler enforced notation? | | |
| ▲ | fanf2 16 hours ago | parent | next [-] | | C, C++, Rust, Zig, … | | | |
| ▲ | neonsunset 16 hours ago | parent | prev [-] | | C# (.NET in general) :) Well, variables cannot be forced to stack specifically. They are placed in the "local scope". And that would usually be either stack or CPU registers - thinking in stack only is a somewhat flawed mental model. Both C# and F# complicate this by supporting closures, iterator and async methods which capture variables placing them in a state machine box / display class instead which would be located on the heap, unless stack-allocated by escape analysis (unlikely because these usually cross method boundaries). However, .NET has `ref structs` (or, in F#, [<Struct; IsByRefLike>] types) which are subject to lifetime analysis and can never be placed on the heap directly or otherwise. |
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| ▲ | DarkNova6 16 hours ago | parent | prev [-] | | But stack allocated objects are not part of the heap and therefore not even part of Garbage Collection? And afaik stack allocation is already done for objects which don't escape a method. | | |
| ▲ | masklinn 5 hours ago | parent [-] | | Yes, but that’s the point: objects which don’t escape are pretty much all young objects. So by this process the stack captures a significant fraction of the young generation, that young generation never reaches the heap and this is never under consideration by the GC. Essentially the stack is a form of younggen. It is not as complete (as there are things which must be heap allocated) but because it is, it reduces the benefits of a generational GC… without having much impact on its costs and complexity. Depending on work load, that competition can be sufficient to make a generational GC net negative. | | |
| ▲ | DarkNova6 an hour ago | parent [-] | | Thanks for the answer. But is this actual behaviour for the GCs of the JDK? I was certain that at the very least Hotspot makes use of stack allocation as much as possible. But perhaps the JDK GCs don't care so much about the stack because that is already dealt by the JVM a step prior? In any case, there will likely still be young objects allocated in the heap and this new algorithm might prove useful. But you can tell I am far from an expert here. |
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