Rust: Dropping heavy things in another thread can make your code 10000x faster

Re:Rust: Dropping heavy things in another thread can make yo... by justinpombrio on Saturday May 30, 2020 @03:52PM (#23364061)

EDIT: See kevincox's reply. Rust will bitwise copy the containing type, which is typically very cheap. For example, it you move a String, it will copy the String struct, which contains a couple pointers and a length (or something along those lines). Importantly, it will not copy the underlying char array.

I was thinking of the following code, where I believe the assignment to y is actually free. Though apparently this isn't called a "move".

    let x = <<large owned type like [char; 1000]>>;
    let y = x;

More info: https://doc.rust-lang.org/rust-by-example/scope/move.html

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Re:Rust: Dropping heavy things in another thread can make yo... by kevincox on Saturday May 30, 2020 @04:19PM (#23364304)

Semantically it is a bitwise copy.

In the example from the article it is probably actually a copy because the value was originally on the parent thread's stack, which will be reused after the function returns, so the value will need to be copied to the new thread's stack.

However it is important to not that it isn't a deep/recursive bitwise copy. It just needs to copy the HashMap itself (which is probably a handful of words).

So yes, it is doing a bitwise copy, but this is also very cheap. It will be much, much cheaper than spawning the thread.

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Re:Rust: Dropping heavy things in another thread can make yo... by ahupp on Saturday May 30, 2020 @04:52PM (#23364609)

That's not exactly true. In C++ terms, the example code is moving a value:

  std::map<...> foo;
  someFunction(std::move(foo));

And not moving a pointer like:

  std::unique_ptr<std::map<...>> foo = ...;
  someFunction(std::move(foo));

So it copies sizeof(std::map<...>), not a pointer.

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Re:Rust: Dropping heavy things in another thread can make yo... by pjmlp on Monday June 01, 2020 @03:23AM (#23376073)

Not necessary, std::map::map(&&) is free to move just the internal pointers and bookkeeping data, only dumb implementations would move the memory blocks where the map data is actually stored, as per ISO C++ requirements.

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Re:Rust: Dropping heavy things in another thread can make yo... by ahupp on Monday June 01, 2020 @02:01PM (#23381603)

The comment originally said it was the cost of a pointer copy, and I was pointing out it's not a pointer. eg., sizeof(std::map<std::string,std::string>) is 48, which is the "pointers and bookeeping" you mention.

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Re:Rust: Dropping heavy things in another thread can make yo... by pjmlp on Tuesday June 02, 2020 @12:52AM (#23387593)

Ah sorry for the misinterpretation. Should have payed more attention.

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