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Fiber in C++: Understanding the Basics

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Re: Fiber in C++: Understanding the Basics

#31

Post by gpderetta »

geertj wrote: post #25
munch117 wrote: post #24

Earlier quoted context omitted.

Advantage: No function colouring. Just like with threads, you don't need to declare which functions might potentially yield. There's none of that noisy async/await syntax. Advantage: Fast switching. Switching between fibers is done by swapping in new values for the program counter and stack pointer. Compare with async, where yielding only takes you up one level of stack, so your code ends up walking a stack of corout…

A disadvantage of the ‘no function coloring’ in fibers is that it makes lockless programming harder. A nested function call can switch from under you without your knowledge, making it hard to know where the preemption points are and whether to take locks when making updates to shared state. With function coloring you immediately see whether a function might switch as there’s a different calling syntax. I’ve programme…

[I assume you are using lock free in a loose way, as lacking explicit mutual exclusion].

In practice in most real world applications in C++, absence of async calls is not enough to guard against re-entrancy. Shared state might still be mutated by callbacks, by recursively re-entering the event loop or by other threads. So you need to either document explicitly all the assumptions you are making and vet every single function you are calling in your implicit critical section, or you need some other mechanism that is going to look like a critical section anyway.

Re: Fiber in C++: Understanding the Basics

#32

Post by snaury »

indigoabstract wrote: post #18

Since coroutines have been in the standard since C++20, I wish he would have explained better what are the advantages of using a Fiber over a Coroutine.

Unfortunately in 2023 c++20 coroutines are just barely starting to become usable. Compilers have bugs, especially with symmetric transfer, e.g. msvc doesn't suspend until await_suspend returns (which causes races and double-frees), gcc stops using tail calls in some modes (which can cause stack overflow), I ended up emulating symmetric transfer because you can't rely on it, and emulating it is of course slower. Compilers spill local variables from await_suspend to coroutine frames, which again causes races, which necessitates workarounds like marking await_suspend noinline (in a non-portable way). Having noinline anything in the code path disables heap allocation elision, so all coroutines start to allocate from the heap. When comparing a function call to a coroutine call there's a vast difference in performance. And you need to use coroutines all the way down, which really adds up.

Fibers have their own problems: you need to allocate stacks (and that may be expensive), you can't switch fibers between system threads (compilers cache thread local addresses, unfortunately with x86 linux tls often uses fs: segment addressing, so it may appear to work until it doesn't, and weirdly this problem existed in c++20 coroutines too until recent clang versions), widely used open source implementations (e.g. boost context / boost coroutine / boost fiber) are not even exception safe, because you need to save/restore exception globals when switching fibers, and nobody seems to care. :-/

One huge upside to fibers is that a function call is just a normal function call, and it's fast as a result. I wish c++ taken fibers more seriously and added steps for making them safe to use (portable way to handle global state, portable way to mark switching functions as invalidating tls addresses, etc.), we could have something similar to java virtual threads or go goroutines then.

Re: Fiber in C++: Understanding the Basics

#33

Post by bicarbonato »

hackyhacky wrote: post #3

I'm confused in how this article defines "fiber." At the beginning of the article, fibers are described as a lightweight userspace thread, similar to what is provided in the runtimes of Haskell and Go, and has been recently revived in Java as "virtual threads." However, later in the article (after a long and apparently irrelevant digression about stack management), they describe Windows API functions that are require…

Setting aside certain implementation details, here's the basics:

Kernel Threads (aka threads): OS-Level and preemptive.

Green Threads (aka lightweight threads, virtual threads or goroutines): User-level and preemptive.

Fibers: User-level (sometimes OS-level or offered as a OS library, like in Windows) and cooperative.

Coroutines: User-level and cooperative.

Re: Fiber in C++: Understanding the Basics

#34

Post by dfawcus »

geertj wrote: post #25
munch117 wrote: post #24

Earlier quoted context omitted.

Advantage: No function colouring. Just like with threads, you don't need to declare which functions might potentially yield. There's none of that noisy async/await syntax. Advantage: Fast switching. Switching between fibers is done by swapping in new values for the program counter and stack pointer. Compare with async, where yielding only takes you up one level of stack, so your code ends up walking a stack of corout…

A disadvantage of the ‘no function coloring’ in fibers is that it makes lockless programming harder. A nested function call can switch from under you without your knowledge, making it hard to know where the preemption points are and whether to take locks when making updates to shared state. With function coloring you immediately see whether a function might switch as there’s a different calling syntax. I’ve programme…

That is where an Actor or CSP style comes in useful, one then avoids the locks by having that state mutated in the context of one coroutine alone.

Most problems can be recast in to that form, only a small subset are better handled by mutable shared state (and hence explicit locks. In those cases one should probably be modelling the updates as 'Compare and Swap' / 'Compare and Set' types of mutable operation, hence avoiding the situation where unexpected preemption can occur while holding a lock.

From my perspective, having use both forms (manually and with language support), I generally find the coroutine (plus Actor/CSP) scheme to be easier to work with and reason about.

Re: Fiber in C++: Understanding the Basics

#36

Post by couchand »

flohofwoe wrote: post #29
hackyhacky wrote: post #3

I'm confused in how this article defines "fiber." At the beginning of the article, fibers are described as a lightweight userspace thread, similar to what is provided in the runtimes of Haskell and Go, and has been recently revived in Java as "virtual threads." However, later in the article (after a long and apparently irrelevant digression about stack management), they describe Windows API functions that are require…

"Fibers", "green threads", "stack switching", "cooperative multitasking" are essentially all the same thing, they all rely on being able to switch execution context by switching to a different stack within the same OS thread. As such they can be implemented either in (CPU/OS-specific) user code or in "OS APIs" (like the Windows Fiber functions). Only downside of the technique is that it cannot be implemented in WASM…

Having a native stack switch for WASM would be nice but it's not necessary. Map all memory segments as shared into several instances of a single module -- each instance is a fiber.

Implementation is left as an exercise for the reader.

Re: Fiber in C++: Understanding the Basics

#37

Post by gpderetta »

snaury wrote: post #32
indigoabstract wrote: post #18

Since coroutines have been in the standard since C++20, I wish he would have explained better what are the advantages of using a Fiber over a Coroutine.

Unfortunately in 2023 c++20 coroutines are just barely starting to become usable. Compilers have bugs, especially with symmetric transfer, e.g. msvc doesn't suspend until await_suspend returns (which causes races and double-frees), gcc stops using tail calls in some modes (which can cause stack overflow), I ended up emulating symmetric transfer because you can't rely on it, and emulating it is of course slower. Compi…

> exception safe, because you need to save/restore exception globals when switching fibers,

Do you have more details about this issue? What exception globals? And for which ABI? You mean some sort of current pending exception when switching during unwinding?

[I'm a huge fan of stackful coroutines and I wish were blessed by the standard]

Re: Fiber in C++: Understanding the Basics

#38

Post by munch117 »

geertj wrote: post #25
munch117 wrote: post #24

Earlier quoted context omitted.

Advantage: No function colouring. Just like with threads, you don't need to declare which functions might potentially yield. There's none of that noisy async/await syntax. Advantage: Fast switching. Switching between fibers is done by swapping in new values for the program counter and stack pointer. Compare with async, where yielding only takes you up one level of stack, so your code ends up walking a stack of corout…

A disadvantage of the ‘no function coloring’ in fibers is that it makes lockless programming harder. A nested function call can switch from under you without your knowledge, making it hard to know where the preemption points are and whether to take locks when making updates to shared state. With function coloring you immediately see whether a function might switch as there’s a different calling syntax. I’ve programme…

> A disadvantage of the ‘no function coloring’ in fibers is that it makes lockless programming harder.

But if you treat fiber-style coroutines like threads, and use locks and message queues exactly the same as you would with threads (although differently implemented, of course), then coroutines are basically better-behaved, and nicely deterministic, threads. It's a wonderful programming model, with the one big drawback that you can't take advantage of multiple cores.

Re: Fiber in C++: Understanding the Basics

#39

Post by munch117 »

pjmlp wrote: post #27
munch117 wrote: post #24

Earlier quoted context omitted.

Advantage: No function colouring. Just like with threads, you don't need to declare which functions might potentially yield. There's none of that noisy async/await syntax. Advantage: Fast switching. Switching between fibers is done by swapping in new values for the program counter and stack pointer. Compare with async, where yielding only takes you up one level of stack, so your code ends up walking a stack of corout…

C++ coroutines are based on C# async model, as they were originally proposed by Microsoft, they even use similar concepts in regards to compiler magic of Awaitable types with special methods being reckognised by the compiler, as means to create an awaitable type for any kind of data structure.

That's what I thought, thanks for confirming.

Re: Fiber in C++: Understanding the Basics

#40

Post by Diggsey »

hackyhacky wrote: post #3

I'm confused in how this article defines "fiber." At the beginning of the article, fibers are described as a lightweight userspace thread, similar to what is provided in the runtimes of Haskell and Go, and has been recently revived in Java as "virtual threads." However, later in the article (after a long and apparently irrelevant digression about stack management), they describe Windows API functions that are require…

Userspace != Application-managed

On Windows, many things exist in user-space that are managed by the OS. This is in contrast to linux where the Linux kernel is its own thing with its own stable API.

Fibers are an OS-level concept that exists in user-space.

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