BytesMut::freeze is not zero-cost

[Jasper-Bekkers]

If self.kind() == KIND_VEC is true, freeze ends up calling vec.into() which appears to not be zero-cost at all, and ends up doing a huge memcpy somewhere in the bowels of Vec

[Jasper-Bekkers]

It looks like the original from_vec implementation didn't used to call into_boxed_slice before the refactoring in #298 https://github.com/tokio-rs/bytes/pull/298/files#diff-d272e47b868896a26bf5ddcfa9be0ea2L1836

[fanatid]

@Jasper-Bekkers out of interest, how you get this trace? Is there a way do nearly same on Linux?

[Jasper-Bekkers]

It's profiled with Superluminal Profiler; https://superluminal.eu I don't think it supports Linux yet.

[Matthias247]

By looking at it this comes from calling Vec::into_boxed_slice(), which calls Vec::shrink_to_fit().

That again means freeze is not zero cost if the BytesMut wasn't written up to it's capacity. If you reserve the exact amount upfront, it might not be an issue. If you split off anything upfront (which promotes to an Arc), then it also wouldn't be an issue.

This is all fixable. However a tricky question is whether people are all ok with unused memory in the BytesMut being "lost" while Bytes elements refer to a subset of it. For myself - I would be - given that I don't see Bytes as an ideal long term storage anyway and everyone should strive for releasing the chunks as soon as no longer required to avoid references keeping bigger buffers alive for longer than necessary.

[Matthias247]

After thinking a bit more about it: I think we can solve this easily by adding an additional shrink_and_freeze() method for people which care about potential waste of memory, and make sure freeze() is always allocation free.

[Jasper-Bekkers]

After thinking a bit more about it: I think we can solve this easily by adding an additional shrink_and_freeze() method for people which care about potential waste of memory, and make sure freeze() is always allocation free.

Did this ever end up getting implemented, and if so, should we close this issue? ☺️