[stcredzero]

This document describes how the GC works and how to tune it for (soft) realtime systems.

The basic algorithm is Deferred Reference Counting with cycle detection.

I'm sitting here feeling very impressed. Deferred reference counting has very good semantics for games. Even better, you can control the cycle detection part separately and run that part at an advantageous time. (Though it's probably better to just let GC do its thing, unless you really know what you're doing.)

I am currently writing a multiplayer game server in golang, by making sure almost everything is allocated on the stack, and heap sizes are small. This gives me an efficient, nearly pauseless server. However, something like Nim could give me even more flexibility.

[pcwalton]

> I'm sitting here feeling very impressed. Deferred reference counting has very good semantics for games.

Not if you need to be thread-safe.

> Even better, you can control the cycle detection part separately and run that part at an advantageous time.

How would that work with multithreading? (Assuming you had a thread-safe GC, which Nim's isn't.)

[stcredzero]

Not if you need to be thread-safe.

Everything that has to be thread-safe uses channels. I use channels to sanitize everything for a purely synchronous game loop. As an optimization, in cases where there are atomic operations available, there are places where concurrent code can mutate values visible to the game loop, but this is strictly an optimization technique, to be used judiciously. (So only values like "speed" can use this technique. Anything that's a reference is verboten.)

How would that work with multithreading? (Assuming you had a thread-safe GC, which Nim's isn't.)

I didn't realize Nim's GC wasn't thread safe. In my current architecture, you'd only have to worry about the part using channels to sanitize things for the synchronous game loop. If everything outside of the game loop was written such that most everything was allocated on the stack, the GC would never have to collect anything outside of the game loop. So maybe it could work as a port. I couldn't say for sure, though.

[Pharohbot]

A more general approach to shared memory via lockable heaps is a feature Nim seems will implement soon.

[pcwalton]

Sounds interesting. Do you have any links to documentation?

I'm interested in reading more about it, because Nim is doing a lot of experimental stuff and it's always interesting to look at its designs.

(Edited to remove speculation about how well it will perform before reading about it.)

[rbehrends]

Well, this is currently highly speculative. What I proposed to Andreas was essentially a model based on Eiffel's SCOOP (with some additional influence from Erlang). Whether it's a practical design remains to be seen.

Note that shared, lockable heaps need not be heavyweight structures. It is entirely possible to imagine a shared hash table with one heap per bucket and fine-grained locking, for example. Collections for such small heaps can be fast because the number of roots is limited, and (depending on what invariants you guarantee), you can even forgo stack scanning for most collections or limit the number of stack frames that need to be traversed.

[pcwalton]

Neat, I'll read more on SCOOP. Thanks!

[rbehrends]

SCOOP is not a horribly complicated idea (well, other than the using preconditions as wait conditions, which has been critiqued in the past and is not a critical ingredient). It's basically an extension of the basic idea of monitors. It is based on the idea of having a unified approach for shared memory and distributed system and accomplishes that by assuming that objects can be partitioned into disjoint ("separate") data spaces, access to which is regulated to ensure mutual exclusion; this is why it translates nicely to a model involving thread-local heaps.

At the programming language level, this then mostly involves maintaining mutual exclusion (in Eiffel, the necessary semantics are attached to how "separate" types are handled) and having the optimizer get rid of unnecessary copies.

[vvanders]

If you're crossing threads in games stuff you're doing it wrong. There's a good chance you're running into false-sharing and other pitfalls.

Most games use worker-queues(in which case you can use non-GC objects) to deal with architectures like the CELL and for better cache coherency. In that case Nim is a pretty good fir.

[pcwalton]

> If you're crossing threads in games stuff you're doing it wrong. There's a good chance you're running into false-sharing and other pitfalls.

Of course, you shouldn't use shared memory unless you need it. But often you need it. Look at how game developers have demanded shared memory in JavaScript, for example. Modern multicore CPUs do a lot of work to make shared memory work, and work well.

> Most games use worker-queues(in which case you can use non-GC objects) to deal with architectures like the CELL and for better cache coherency.

I agree with you that GC is often not the best solution for shared memory concurrency. But I think you really need to design the language around "no GC" in order to make that really ergonomic relative to C++. The entire C++ language and library ecosystem is based around making manual memory management (relatively) easy to use; going back to malloc and free is a hard sell.

[vvanders]

> Look at how game developers have demanded shared memory in JavaScript

Have a source for that? I find it pretty dubious.

If you're looking to multicore for performance with javascript then you're using the wrong language. Correct memory layout and access patterns will give you a real-world 50-100x win.

[pcwalton]

Source: I work directly with people who interact with game developers who are asking for it.

Look for asm.js threads on HN: virtually every time it shows up someone brings up shared memory multithreading.

SharedArrayBuffer is the direct result of this popular demand: https://blog.mozilla.org/javascript/2015/02/26/the-path-to-p...

[fjehcjriendje]

So, I've noticed that over the past six months that every time something about Nim gets posted to HN, you make an effort to discredit the language.

Care to offer an explanation why?

[pcwalton]

I think Nim is an impressive language that does a lot of things really well. I don't think the memory management is one of them. I believe that memory management and compilation to C are the only two major things I've ever talked about in regards to Nim, because I'm abstractly interested in those topics. If an article about thread-local deferred reference counting in Ruby hit the top of HN and the comments were talking about how that's good for games, I'd probably comment there too.

[copx]

And how do you do "memory management well"? Like Rust? You pay a high price in complexity and inflexibility for that juicy GC-less yet safe memory management.

Ref counting is not superior or inferior to explicit, restrictive ownership semantics. Those are simply different trade-offs.

Nim might be strictly inferior for writing a heavily multi-threaded web browser because of its memory management approach but that does not mean the approach is generally inferior.

Seems to me that Nim aims to be a "casual", rapid development / friendly (Python-like) language. Ownership semantics like in Rust do not fit there.

[pcwalton]

I'm personally a fan of regular old Java/C#-like concurrent garbage collection for most "scripting" languages (perhaps surprisingly, given my work on Rust). It's a lot of work to get there, but I think there's no substitute for doing the work—apps really end up needing the flexibility of shared memory. Shortcuts that seem simple like DRC end up tending to run into walls in practice, which is something that the other languages discovered—history keeps pointing to the HotSpot-style garbage collector as the one that systems that need to offer a simple, general-purpose garbage-collected programming model migrate to.

For different use cases Rust-style ownership semantics (when the performance of GC and runtime interoperability become an issue), or Azul-style pauseless GC (when you're willing to trade throughput for latency), or shared-nothing architectures (when you need them for legacy reasons like JavaScript, or want a simple programming model like Erlang) can work great.

[tatterdemalion]

"apps really end up needing the flexibility of shared memory"

Why? Just performance or is there a design reason also?

[jibalt]

"You pay a high price in complexity and inflexibility for that juicy GC-less yet safe memory management."

It's a price that a lot of people are willing to pay, because of how badly they want what they're paying for.

Nim and Rust have different goals, different tradeoffs, and overlapping target audiences. Having both is good. Making them fight is bad.

[MichaelGG]

I'm guessing because Nim gets portrayed as safe, or offers some safe features, but overall is not memory safe. Rust has worked very hard and gotten through the problem of having memory safety, with zero runtime cost and reasonably good language features.

Since it's 2015, it seems fair to point out when a new language offers something neat, but in a way that isn't safe.

[Pharohbot]

Nim is as safe as any other language. Perhaps it's not as safe as Rust but that brings specific trade offs most people dont wan't to deal with. I don't understand why people think that Nim is "terribly unsafe" when in reality it's like any other language

[steveklabnik]

> Nim is as safe as any other language.

With regards to memory safety, it is not. https://news.ycombinator.com/item?id=9050999 is an old comment from Patrick, but in today's Nim, it segfaults in both release and development modes for me. Rust's guaranteed memory safety means that Rust code (without explicit unsafe, the vast vast majority of code) cannot segfault.

> I don't understand why people think that Nim is "terribly unsafe" when in reality it's like any other language

For example, unless I write a bad cext, I cannot get Ruby to segfault.

None of this makes Nim a bad language. All languages have tradeoffs.

[Pharohbot]

Yes Rust is more safe than Nim, I'm not arguing that. I'm also not arguing that Nim is as safe as languages with automatic memory management.

EDIT: Also, Nim is planning on turning those segfaults into runtime NilErrors and a nilChecks flag that will check for them at compile time, you can also avoid this by annotating Pointers with `not nil`

[heinrich5991]

It has a feel of a scripting language, but as far as I can tell, it rather has the safety of C/C++, which I personally wouldn't call "safe like any other language".

[Pharohbot]

Why not? I'm interested to know because in my opinion I don't see it any less safe than languages that don't have automatic memory management and/or languages like Rust.

[jibalt]

"Nim is as safe as any other language."

That is factually false.

"Perhaps it's not as safe as Rust"

And there even you have contradicted yourself.

"Perhaps it's not as safe as Rust but that brings specific trade offs most people dont wan't to deal with."

That much is true ... and can be said without telling falsehoods, like your first statement.

"I don't understand why people think that Nim is "terribly unsafe" when in reality it's like any other language"

You are confused by your own strawman.

[Pharohbot]

when --nilChecks:On become a thing, dereferencing null pointers will be like Java, a NilError (NullPointerException in Java). This is why I said it's as safe mainstream languages that dont have AMM but languages like Rust are safer than those mainstream languages. any others to point out?

[kibwen]

You yourself know that this claim is entirely unsubstantiated, as evidenced by the fact that you felt the need to create a throwaway account. pcwalton is an active commentator throughout HN in general, and garbage collection and parallelism are two of his areas of expertise. If his opinion is somehow uninformed, then tell him so and explain how. If he's not uninformed, then the only thing your comment is doing is trying to shut down legitimate criticism.

All languages have faults. Engage with your critics, own your faults, and either correct them or justify them based on your principles.

EDIT: To give an example, pcwalton also initially criticized Go for not being memory-safe for values of GOMAXPROCS greater than 1. However, the Go team later implemented the dynamic race detector, which, if you've followed pcwalton's comments at all, you know that he is actually quite impressed with.

[fjehcjriendje]

I always post using throwaways since I don't like karma influencing the content of my posts and it also makes it significantly more difficult for third parties to profile me.

[kibwen]

This is some glass-houses logic right here, given that you're attempting to leverage trivially-falsifiable assertions in order to profile pcwalton as a Nim hater with a personal vendetta. In the meantime you have yet to actually address his criticisms, which, to reiterate, indicates that you're trying to shut down critics via deflection.

(I suppose, in the future, pcwalton should just generate a throwaway before commenting on Nim.)

[jibalt]

I've noticed that a) that's an ad hominem attack and b) it isn't accurate.