> but it’s the first language I’ve experimented with that has made them a first-class feature
It is a feature of ALGOL68, Pascal, Ada and quite some newer languages:
It is a feature of ALGOL68, Pascal, Ada and quite some newer languages:
Rust has just managed to take all these features and put them together well, and strive to be more than a research language by working on things that would make others actually use the language.
(Of course, this particular feature is common in many, many languages)
Ha! I didn't know that. Thanks :)
I would probably restate it though. What is research vs. a product. I just read about 1nm transistors, but we're probably looking at 10 years before Intel, et al, have built all the infrastructure to reliably deliver a CPU based on it.
In the case of a language, I would say it's similar, do you have the support infrastructure in place? Rust is amazing in this regard: Cargo, crates.io, docs.rs, rustup, etc. on top of that you have at least one large company and many others pushing the language in a large and distributed product.
I would classify Rust as a production ready coding platform.
It seems very obvious now. I was thinking the purpose of the language was for performing research, but that didn't make much sense. I see now that it's the language itself that is the subject of research, not the tool. English is fun.
If you hear the term used disparagingly it's because they tend to have issues you generally wouldn't want to put up with when you're on the clock: compiler bugs, lack of error messages, large missing pieces in the standard lib, spectacularly bad performance, etc because those weren't the problems the language was meant to explore.
Writing proper and safe concurrent code in Rust still looks horrible compared to languages which supports it natively in the type system, e.g. ponylang type capabilities. You still have to manually maintain locks, and it's also much slower.
There's a safety model, but it's only best practice, not enforced by the language nor the compiler. So calling it "safe" and "truly unique" is way off. Even parrot has a better, safe and lockless threading model, which guarantees safety.
And of course the means to get rid of any abstraction and safety when required.
edit: accidentally a word
LISP-strength macros give you most of this capability.
It works fine in Ada.
Edit: Why am I downvoted for this? A language-defined concurrency model is indispensable for having safe and deadlock-free concurrency in the language (rather than in arbitrary utility libraries). That's why concurrency was explicitly included into Ada and into the additional Ravenscar profile. The rationale for this has always convinced me and it also works fine in Ada, so anybody care to elaborate what's wrong with it?
http://brinch-hansen.net/papers/
I agree it's better to have mechanisms that can be turned into a proper model. A good default, though, greatly improves consistency and reliability in real-world systems.
You ... don't. That's just the concurrency model that the abstractions in the stdlib expose, but Rust's safety is generic enough that you can use different abstractions (e.g. lockfree ones). See crossbeam for some alternative models. There are also transactional memeory impls in Rust. They still use Send and Sync for safety though. Manually maintaining locks is a feature of the concurrency library used, not the safety model.
Pony's system is actually pretty close to that of Rust. Sync is "immutable", Send is "isolated" (sort of). Of course, capabilities are different from auto traits, but the idea behind using these two capabilities for concurrency safety is similar.
(So I was wrong that Rust's concurrency safety system is unique, since Pony has something based on the same concepts)
> There's a safety model, but it's only best practice, not enforced by the language nor the compiler.
Yeah, Send and Sync are technically a part of the stdlib (and can be reimplemented outside of it, aside from a small interaction with statics -- Send/Sync are treated as special by the compiler when it comes to statics). However, it is enforced by the compiler in the sense that if you avoid best practice (using Send and Sync), you can't write parallel code without dropping to `unsafe`. If you do that you can design safe abstractions around that and use whatever concurrency enforcement you wish, though pretty much everyone sticks to Send and Sync since it works well with the rest of the language.
(Because of the statics thing, "not part of the language" is debatable, anyway)
What? How is it not enforced?
> You still have to manually maintain locks, and it's also much slower.
Slower? I don't think it can be any faster even in theory!
Smalltalk has had that since the early 1980s
how would it statically enforce the equivalent of Send and Sync constraints?
In some Smalltalks, normal execution is synchronous. Many of them also use read and write boundaries for various purposes. The former gives you Send and Sync constraints for free. The latter can be used in difficult edge cases. (Like when you're calling out to the OS.)
Uh, no, it gives you thread safety for free.
When my comment was talking about Send and Sync I was talking about the specific way Rust's typesystem enforces thread safety. I'm not saying that other languages don't, I'm saying that Rust's method of enforcing it might be one of the few unique things it does
It would be great to see how other languages have achieved a similar balance, and my impression is that Smalltalk (and like most languages) does not put nearly as much effort into controlling mutability, but maybe I'm wrong. Could you post some links/a description that explores how Smalltalk achieves a similar level of safety?
http://brinch-hansen.net/papers/1975a.pdf
http://brinch-hansen.net/papers/
His first, concurrent OS was RC 4000 in 1969. It had many mechanisms in place. He got the key parts of the safety problem figured out by 1972. His language to handle much of it statically at compile-time was done in 1975. His Boss 2 system same year used coroutines + similar concepts to run 100 activities at a time with a proof of deadlock freedom. Finally, he used Concurrent Pascal to implement Solo system which ran its processes safely without using physical, memory protection. These summaries came from "Evolution of Operating Systems" on 2nd link.
So, the stuff was well-established by the mid-70's with operating systems using it in production. Just ignored by mainstream like a lot of good stuff for various reasons. ;)
"A disk buÆer is a data structure shared by two concurrent processes. The details of how such a buÆer is constructed are irrelevant to its users. All the processes need to know is that they can send and receive data through it. If they try to operate on the buÆer in any other way it is probably either a programming mistake or an example of tricky programming. In both cases, one would like a compiler to detect such misuse of a shared data structure."
"To make this possible, we must introduce a language construct that will enable a programmer to tell a compiler how a shared data structure can be used by processes. This kind of system component is called a monitor. A monitor can synchronize concurrent processes and transmit data between them. It can also control the order in which competing processes use shared, physical resources."
Sounds like he understands the problem is concurrent processes stepping on each others toes using a shared, data structure. He has nice drawings and charts to go with it. Plus, an early model to solve it statically at compile time. Jweb_Guru noted some limitations of his method but it understands and solves the fundamental problem. Hansen's own work improved on it later plus it was obsoleted by things like Ravenscar, SCOOP and now Rust's method. Nice that he had a whole OS protected from concurrency errors at compile time in the 70's, though. :)
Hansens work formalized what I just described in terms of English, diagrams, and compiler checks. He started with sequential operations on private data in modules. He says if two or more share thd same private data they might not execute in desired order. The monitor pattern enforced user-specified order on function calls to shared data. Built-in to language & compiler.
If my description of race conditions is inaccurate or insufficient, I'd appreciate a link to one that you think is more accurate that I could use as a comparison point against the Hansen paper. Otherwise, his description of problems implementing concurrency sounds exactly like what I learned in books on multithreading, supercomputing, etc. Shared resource used in incorrect order due to concurrency.
Note: Also, his colleagues Dijkstra and Hoare were still inventing and developing formal verification at the time. Tooling sucked. Standard practice, like he did with Algol and COBOL compilers, was writing things like this in precise English with code examples or diagrams. Not sure if you were expecting a HOL model or something when you said "modern" but I figured Id mention stuff was primitive then.
A data race is when
- two threads access a single memory location,
- at least one of which is a write, and
- at least one of which has no synchronisation^
(^ synchronisation in the sense of things like atomic instructions, not necessarily full locks.)
Knowing that, Hansen starts on problem with RC4000 (1969) describing protecting communication between "processes" via message buffers, message queues, and checks performed on them to ensure validity. That and Dijkstra's critical regions are where the foundations were laid. His next step was the formalization the problem in 1972:
http://brinch-hansen.net/papers/1972a.pdf
Section's 1 and 2 cover basics of concurrent operation + race conditions. It was clear to me by the abstract but should be extra clear he's talking about them by this statement about Algorithm 1:
"The copying, output, and input of a record can now be executed concurrently. To simplify the argument, we will only consider cases in which these processes are arbitrarily interleaved but not overlapped in time. The erroneous concurrent statement can then be executed in six different ways with three possible results."
That's definitely a concurrency error. He then talks about mutual exclusion and synchronization with an await primitive. Closer to modern language but the focus on operating systems in the early 1970's means he says processes instead of threads and things like disk buffers or message queues instead of shared memory. Although his examples in 1972 paper are clearly shared memory since it's at algorithm level.
So, they discovered the problem around late 60's, implemented an early solution for sharing resources among processes in 1969, fully described race conditions + some other stuff by 1972, had a safer-by-design language to catch it at compile time by 1975, and applied that to implement a concurrent, production OS (Solo) for day-to-day use by academics by 1976.
So, race conditions were both formalized and initially solved in early to mid 1970's. I don't know how much more you need given they had an OS running 100 jobs/users at once interacting on shared resources without visible failures using their concurrency model. Mainstream OS's and apps are still having race conditions pop up on occasion. Clearly, what they were doing was addressing root cause if no race conditions occurred after a successful compile of "multiprogrammed" apps. :)
A data race is when a program may give undefined results (in the sense of undefined behaviour in C) because you've violated core language semantics. Solving "all" concurrency problems is a much broader and far more restrictive paradigm than just ensuring that a program has defined behaviour, and not having restrictions is key to systems programming. (In fact, it's not even clear to me how one can "solve" race conditions at a language level without tying into some sort of machine-readable product specification or without just removing concurrency entirely: a single piece of non-determinism may be fine in one program but bad, i.e. a race condition, in another.)
> The problem you're having is you've constrained the definition of concurrency or race conditions to only be about the language common among application programmers, esp threading
No, I'm not having any problem here: you said yourself that you're not an expert. A data race (not arbitrary race condition) only makes sense if there's shared memory and thus thread is a perfectly reasonable description (although "thread" is also often used to just mean thread of execution, not literal pthread thread). It's becoming clear that we're talking about different things since you're not in-tune with the jargon/subtle terminology.
Hmm. Ill have to look into the old stuff further to see about whether any of it, including Hansen, covers the accepted definition of data races. There's potential that Hansen's does but Im holding off until I think on it more.
Some relevant lines:
> A Concurrent Pascal compiler will check that the private data of a process only are accessed by that process. It will also check that the data structure of a class or monitor only is accessed by its procedures
is significantly less than what Rust provides, because:
> Processes cannot operate directly on shared data. They can only call monitor procedures that have access to shared data. A monitor procedure is executed as part of a calling process (just like any other procedure).
> If concurrent processes simultaneously call monitor procedures that operate on the same shared data these procedures will be executed strictly one at a time. Otherwise, the results of monitor calls would be unpredictable. This means that the machine must be able to delay processes for short periods of time until it is their turn to execute monitor procedures. We will not be concerned with how this is done, but will just notice that a monitor procedure has exclusive access to shared data while it is being executed.
Rust allows shared access to immutable data and does not require either serialization or the invocation of a virtual machine; it also allows nondeterministic operation to be used as long as it cannot cause memory unsafety.
Additionally, there are other, more basic things that the system is not capable of. For instance:
> (Strictly speaking, a compiler can only check that single monitor calls are made correctly; it cannot check sequences of monitor calls, for example whether a resource is always reserved before it is released. So one can only hope for compile time assurance of partial correctness.)
A major part of what Rust brings to the table is the ability to statically avoid problems like use after free and double free even in a concurrent setting. How would I avoid resource leaks in a system like this without a per-thread allocator (which would likely prevent sending across threads)?
Of course, it's not clear to me that this matters anyway, since I can't destroy a thread or shared data structure!
> Dynamic process deletion will certainly complicate the semantics and implementation of a programming language considerably. And since it appears to be unnecessary for a large class of real-time applications, it seems wise to exclude it altogether. So an operating system written in Concurrent Pascal will consist of a fixed number of processes, monitors, and classes. These components and their data structures will exist forever after system initialization.
I agree that people have a strong tendency to not know what past work has been done, and Concurrent Pascal is admirable, but what Rust does is not the same thing. Rust specifically tackles a lot of hard problems in concurrency that simply do not exist as long as you (1) assume away deallocation for shared data structures, and (2) serialize all access to those data structures.
(There are lots of other specific points I could go into; for instance, Concurrent Pascal apparently lacks facilities for generic programming, so I probably couldn't combine two correct concurrent data structures and expect a third correct concurrent data structure to come out. But that stuff isn't directly related to concurrency).
I'm not saying it does. The claim I replied to said preventing things like data races was done till well into the 80's. I showed systematic investigations of the problem plus a production solution had been done by mid-70's. You've nicely showed how far ahead methods like Rust got. ;)
No, it definitely didn't: it said data races as a concept seemed to not be formalised until the late 80's.
This paper introduced Monitors. It was of course hugely influential.
Mutex and condition variables, the building blocks of monitors made their way into posix threads and from there into C++11.
I believe Java is a much closer realization of Hansen model as every java object is a Monitor.
VisualWorks got the ability to make certain objects immutable sometime in the early 2000's. The VM engineers were particularly proud of that one.
That really depends on which Smalltalk you're talking about, I think. Various implementations really had different concurrency models. Some implementations used native OS threads. Others ran synchronously "inside the image" but spawned threads to interact with the OS.
Just look at the dominance of Javascript in the programming landscape, you're seeing decades of high quality executions of the language(I include community in the "execution" definition) despite it being far from the best technical language.
Eh, that's underselling Rust's contributions. Rust is more flexible than anything I know of when it comes to enforcing aliasing-xor-mutability. Cyclone for example was much more restrictive in disallowing aliasing (see Grossman's "Existential Types for Imperative Languages").
The key feature that Rust has is flow-sensitive permissions on unique loan paths, which is actually pretty novel as far as I'm aware.
> The key feature that Rust has is flow-sensitive permissions on unique loan paths, which is actually pretty novel as far as I'm aware.
Huh, right.
Algebraic datatypes with (even just basic) pattern matching (preferably with compile-time exhaustiveness checking) are things I've wanted in C++ ten years ago. This gets me part of the way there, at least. :)
It is however a fine way to encourage sensible use of unions. But it's mostly just a suggestion.
Of course I implemented them in Virgil, too. (shameless plug: https://github.com/titzer/virgil)
> I’m aware the idea for type-safe unions isn’t unique or original to Rust, but it’s the first language I’ve experimented with that has made them a first-class feature.