ARROW-11048: [Rust] Added bench to MutableBuffer

[jorgecarleitao]

This bench compares the behavior of MutableBuffer vs allocating Vec<u8> and using extend_from_slice.

On my computer:

mutable time: [579.24 us 580.21 us 581.34 us] mutable prepared time: [614.98 us 616.15 us 617.42 us] from_slice time: [1.2945 ms 1.3262 ms 1.3607 ms] from_slice prepared time: [1.0161 ms 1.0472 ms 1.0881 ms] 

I.e. growing a MutableBuffer seems to be 2x faster than creating a Vec and using Buffer::from to convert it to a Buffer.

It is odd that creating a buffer with the correct capacity takes longer, though. Any ideas @jhorstmann , @Dandandan ?

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11048

[Dandandan]

I am not sure what is causing the difference, I would expect it to be faster just as from_slice prepared is faster.
The main difference is that initializing with capacity 0 uses memory::allocate_aligned while the other uses a few calls to memory::reallocate. I guess the first is (quite) a bit slower somehow?

[jhorstmann]

I think Buffer::from currently does another copy of the vec contents because it does not take ownership and because of the current alignment and padding requirements. We probably could add a zero-copy Buffer::from_vec method if we loosen those restrictions. Another interesting benchmark would be another version of from_slice that returns a Vec instead of Buffer, to see whether the extend/reallocate logic itself could be optimized.

[jorgecarleitao]

just FYI, I found one reason. alloc_zeroed is expensive. I am preparing a PR where this is addressed. There is some code atm that relies on MutableBuffer::reserve and MutableBuffer::new to allocate zeros (and not undefined), which I need to handle.

[Dandandan]

LGTM, nice to be able to measure this / remove inefficiencies 👍

[alamb]

The full set of Rust CI tests did not run on this PR :(

Can you please rebase this PR against apache/master to pick up the changes in #9056 so that they do?

I apologize for the inconvenience.

[codecov-io]

Codecov Report

Merging #9032 (7a0fa18) into master (51672b2) will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #9032 +/- ##
=======================================
Coverage 82.61% 82.61% =======================================
Files 202 202 Lines 50048 50048 =======================================
Hits 41347 41347 Misses 8701 8701 

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[alamb]

I believe these benches were already added as part of #8997 (which I found out while trying to merge this PR -- LOL)

[jorgecarleitao]

Closing as this code was merged as part of another PR