Add reservation benchmark

[jerbaroo]

This PR adds a benchmark which can be run with just bench-chunks-select. It produces the SVG contained in this PR. Output from the Rust bench is written to a CSV (not committed) which is then read by Python to produce a graph. The Rust bench also prints the information to stdout, which looks like:

SHAPE                          STRATEGY                            SIZE         MEDIAN     P10 / P90
---------------------------------------------------------------------------------------------------------
Seeding database with shape=FewSubmissionsManyChunks and total_chunks=100:
    Metadata values: 10
    Submissions per metadata value: 1
    Chunks per submission: 10
Beginning benchmark of strategy Random
FewSubmissionsManyChunks       Random                                 100          56.5       51.4 / 65.9
Beginning benchmark of strategy PreferDistinct { meta_key: "metadata_value", underlying: Oldest }
FewSubmissionsManyChunks       PreferDistinct(metadata_value, Oldest) 100          111.1      103.1 / 130.8
Seeding database with shape=FewSubmissionsManyChunks and total_chunks=500:
    Metadata values: 10
    Submissions per metadata value: 1
    Chunks per submission: 50
Beginning benchmark of strategy Random
FewSubmissionsManyChunks       Random                                 500          55.8       53.1 / 66.6
Beginning benchmark of strategy PreferDistinct { meta_key: "metadata_value", underlying: Oldest }
FewSubmissionsManyChunks       PreferDistinct(metadata_value, Oldest) 500          182.6      174.3 / 256.1
Seeding database with shape=FewSubmissionsManyChunks and total_chunks=1000:
    Metadata values: 10
    Submissions per metadata value: 1
    Chunks per submission: 100
Beginning benchmark of strategy Random
FewSubmissionsManyChunks       Random                                 1000         55.9       52.1 / 63.2
Beginning benchmark of strategy PreferDistinct { meta_key: "metadata_value", underlying: Oldest }
FewSubmissionsManyChunks       PreferDistinct(metadata_value, Oldest) 1000         344.3      260.8 / 384.3
Seeding database with shape=FewSubmissionsManyChunks and total_chunks=2000:
    Metadata values: 10
    Submissions per metadata value: 1
    Chunks per submission: 200
Beginning benchmark of strategy Random
FewSubmissionsManyChunks       Random                                 2000         55.3       50.9 / 62.7
Beginning benchmark of strategy PreferDistinct { meta_key: "metadata_value", underlying: Oldest }
FewSubmissionsManyChunks       PreferDistinct(metadata_value, Oldest) 2000         499.6      438.2 / 681.2
Seeding database with shape=FewSubmissionsManyChunks and total_chunks=4000:
    Metadata values: 10
    Submissions per metadata value: 1
    Chunks per submission: 400
Beginning benchmark of strategy Random

[ReinierMaas]

I didn't review opsqueue/benches/chunks_select.rs yet, only the top block comment. But I think I already have some good advice that can be helpful.

[ReinierMaas]

LGTM

I left a few smaller changes you could still make.

[ReinierMaas]

Using an underscore here _ drop the receiving side instantly. The sending side will receive errors when it tries to send anything over this channel. I haven't looked at the underlying code but does the reservation hold if we drop this receiving side of the channel? We could opt to return the receiving side together with the ChunkId and retain them until we finish our reservation to ensure that we can receive messages from the view of the reserver.

[jerbaroo]

The reservation holds if we drop the receiving side of the channel.

The sending side of the channel is not used during the reservation except for being stored in the Reserver so that it can be used when the reservation expires.

[jerbaroo]

On reservation timeout if a sending error occurs it is just dropped:

                    // Only error case is if receiver is no longer listening
                    // In that case, nobody cares about the value being evicted anymore.
                    // So `let _ =` is correct here.
                    let _ = val.1.send(val.0);

[ReinierMaas]

What are we dropping here? Should we retain that until we finish our chunk reservation?

[jerbaroo]

In fetch_and_reserve_chunks the stream of chunk that are successfully reserved are joined with submission info. reserved is a Vec<(Chunk, Submission)>, but here with as_slice the second value of the tuple is only a reference to the Submission, so nothing is dropped.