this post was submitted on 15 Jun 2023
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Programming
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Meta: Hmmm... replying to kbin.social users appears to be bugged from my instance (lemmy.world).
I'm replying to you instead. It doesn't change the meaning of my post at least, but we're definitely experiencing some bugs / growing pains with regards to Lemmy (and particularly lemmy.world).
GC overhead is mostly memory-based too, not CPU-based.
Because modern C++ (and Rust) is almost entirely based around refcount++ and refcount-- (and if refcount==0 then call destructor), the CPU-usage of such calls is surprisingly high in a multithreaded environment. That refcount++ and refcount-- needs to be synchronized between threads (atomics + memory barriers, or lock/unlock), which is slower than people expect.
Even then, C malloc/free isn't really cheap either. Its just that in C we can do tricks like struct Foo{ char endOfStructTrick[0]; } and store malloc((sizeof(struct Foo)) + 255); or whatever the size of the end-of-struct string is, to collate malloc / frees together and otherwise abuse memory-layouts for faster code.
If you don't use such tricks, I don't think that C's malloc/free is much faster than GC.
Furthermore, Fragmentation is worse in C's malloc/free land (many GCs can compact and fix fragmentation issues). Once we take into account fragmentation issues, the memory advantage diminishes.
Still, C and C++ almost always seems to use less memory than Java and other GC languages. So the memory-savings are substantial. But CPU-power savings? I don't think that's a major concern. Maybe its just CPUs are so much faster today than before that its memory that we practically care about.
Only for things that you specifically want shared between threads – namely this (synchronized refcount) is an
std::sync::Arc. What you want to share really depends on the app; in database-backed web services it's quite common to have pretty much zero state shared across threads. Multithreaded environment doesn't imply sharing!The refcount absolutely is shared state across threads.
If Thread#1 thinks the refcount is 5, but Thread#2 thinks the refcount is 0, you've got problems.
Again, only for things that you specifically want shared between threads.
There's no "the" refcount in Rust, anyhow. If you just instantiate some container or your custom data struct, like
let mut x = Vec::new();– it's very local to where you are, it's on the stack, it's not reference counted at runtime at all, you cannot pass it between threads (if it's notSendit cannot EVER cross a thread boundary in safe Rust). The standard library provides two ref-counter containers.Rcis just a basic refcount that is not thread-safe and thus also is notSendand won't ever be allowed to cross the thread boundary in safe Rust.Arcimplements atomic-based thread-safe ref-counting and thus isSend, implementing what you're talking about, but as an opt-in per-object container, not as some behind-the-scenes global feature.I remember some old papers talking about Android’s runtime (which is garbage collected) x iOS (reference counted) in which Android was more efficient with high memory, but less efficient with lower available memory.