Use BufferManager for byte array creation

[imanushin]

Summary: for better memory re-usage I suggest using BufferManager type (which is available from .Net 3.0).

New byte arrays are created here: https://github.com/Microsoft/Microsoft.IO.RecyclableMemoryStream/blob/master/src/RecyclableMemoryStreamManager.cs#L289

Pros:

• BufferManager is system type, so it can be tuned per each .Net framework implementation.

Cons:

• Looks like this class is not thread safe (please see SO investigation).

[buybackoff]

BufferManager is not the best option, even it's namespace suggests it is intended for particular purpose.

There is shared memory pool optimized for thread local and detecting memory pressure, but it could return size larger than requested, that breaks RMS in the current design and requires a check for length. If the goal is to reduce overall app memory usage and share pools then using the shared pool with the check should be OK, or RMS should be redesigned to use ArraySegments/Memory etc.

There is also a ConfigurableArrayPool but the relevant variable to get exact size MaxBucketsToTry is not configurable. It should be set to zero and != condition in the while loop replaced with <=.

RMS could just extract bucket implementation from ConfigurableArrayPool, it is c.3X faster than ConcurrentStack that is used now. Replacing SpinLock with hand-crafted lock makes it even faster and is appropriate for this case where the work being done inside the lock is literally a couple of instructions.

[imanushin]

@buybackoff , thank you for detailed investigation.

RMS should be redesigned to use ArraySegments/Memory etc.

I think this is the best option, because we will re-use common solution (which is optimized for now and will be optimized in future).

And I fully agree with "BufferManager is not the best option" according to your arguments