Find unnecessary Vec reallocations

Hi all,

Is there a nice way to ensure that I allocate my vectors with sufficient capacity? I would basically like something which can help remind me to use


where possible.

I know that I can inspect Vec::capacity before and after my operations to see if the vector was enlarged — however, this is a manual process which requires me to sprinkle temporary tracking code here and there.

Is there a more systematic way to do this? Perhaps some way of profiling a binary and detect calls to Vec::reserve (or the equivalent function which does the low-level reallocation)?

There's the massif heap profiler tool which is part of the Valgrind software suite:
I haven't actually learned how to use massif yet, but it seems to be featureful. After profiling, there's both a CLI and a GUI tool to analyze the profile data.

There's also which profiles memory by writing a tracing allocator that wraps the system allocator. It involves more changes to your code itself, and in part, was done for fun.


Thank for the links and thanks for reminding me to look more into Valgrind! I read a bit more about it and found a nice guide about optimization of Rust programs.

I also found the Rust Performance Book, which is full of tips and tricks for optimization. Lots of stuff to study :slight_smile:

I don't know of any automatic method. You could create your own newtype wrapper for Vec that logs its backtrace (or #[track_caller] info!) when constructing a Vec, and logs capacity just before the Vec is dropped. Then you could correlate these two.

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Or if you're really serious about this, you could create a clone of Vec that never reallocates, and panics when it runs out of space.

Thanks for the idea, that sounds quite doable!

I guess that would be equivalent to swapping my Vecs for ArrayVec via the arrayvec crate... I'll try and experiment a bit with that idea.

Ah, no, I misunderstood how the crate works: an ArrayVec has a statically known size, whereas I was thinking I could specify the size dynamically.

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