Fix Enumerable.Chunk throughput regression

[stephentoub]

We previously changed the implementation of Enumerable.Chunk to avoid significantly overallocating in the case of the chunk size being a lot larger than the actual number of elements. We instead switched to a doubling scheme ala List<T>, and I pushed for us to just use List<T> to keep things simple. However, in doing some perf measurements I noticed that for common cases Chunk is now around 20% slower in throughput than it was previously, which is a bit too much too swallow, and the code that just uses an array directly isn't all that much more complicated; it also affords the ability to avoid further overallocation when doubling the size of the storage, which should ideally be capped at the chunk size. This does so and fixes the throughput regression enough (not completely).

[ghost]

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

Issue Details

---

We previously changed the implementation of Enumerable.Chunk to avoid significantly overallocating in the case of the chunk size being a lot larger than the actual number of elements. We instead switched to a doubling scheme ala List<T>, and I pushed for us to just use List<T> to keep things simple. However, in doing some perf measurements I noticed that for common cases Chunk is now around 20% slower in throughput than it was previously, which is a bit too much too swallow, and the code that just uses an array directly isn't all that much more complicated; it also affords the ability to avoid further overallocation when doubling the size of the storage, which should ideally be capped at the chunk size. This does so and fixes the throughput regression.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Linq, tenet-performance
Milestone:	7.0.0

[stephentoub]

private IEnumerable<int> _source = new int[1_000];

[Benchmark]
[Arguments(10)]
[Arguments(100)]
public void Chunk(int chunkSize)
{
    foreach (int[] _ in _source.Chunk(chunkSize)) { }
}

Method	Toolchain	chunkSize	Mean	Ratio	Allocated	Alloc Ratio
Chunk	net6.0	10	5.276 us	1.00	6.34 KB	1.00
Chunk	\main\corerun.exe	10	6.764 us	1.28	6.57 KB	1.04
Chunk	\pr\corerun.exe	10	5.911 us	1.12	6.52 KB	1.03
Chunk	net6.0	100	4.342 us	1.00	4.23 KB	1.00
Chunk	\main\corerun.exe	100	5.100 us	1.16	5.41 KB	1.28
Chunk	\pr\corerun.exe	100	4.906 us	1.12	5.27 KB	1.24