BufferedStream LOH allocations

[ladeak]

Description

Using some measurements, BrotliStream seems to more efficiently when the written data is in larger chunks. This is also described in issue #36245

Hence, it seems reasonable to combined writing with BufferedStream, that can buffer data in chunks to the brotli compression.

Based on measurements with a certain type of data, Brotli stream was the most efficient with writes of ~300KB chunks.

However, when such value is set as the buffer size in BufferedStream ctor, the buffer is allocated on the LOH. This results that after a dozen or so compressed streams written (each with a new BufferedStream), a Gen2 GC compaction is triggered. On a hot path, eventually causing performance issues as the GC spends a lot of time collecting garbage.

Configuration

.NET 9, x64, JIT

Regression?

No

Analysis

It seems each new BufferedStream is allocating a byte[] as a backing buffer. Ideally this could be backed by ArrayPool<byte>.Shared instead of new allocations - at least for the case when the buffer is allocated on the LOH. It seems BufferedStream already has knowledge about SOH and LOH as noted in some source code comments.

Workaround

1. Pooling BufferedStream objects.
2. Creating a custom stream object that buffers backed by ArrayPool. However, having a general implementation seems to be relatively complicated.

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[adamsitnik]

Hi @ladeak

We had a similar discussion about the buffer allocated internally by the FileStream #15088 (comment).

The conclusion then was to wait:

@tannergooding mentioned that there may be a more generalized allocator/management feature in the works, so rather than accepting a buffer now as well as an allocator "soon", we feel that the right answer for now is just to take the buffer size, not a user provided buffer.

@tannergooding Have we made any progress on the "generalized allocator/management feature"?

[ladeak]

@tannergooding do you recall the status of this the above mentioned allocator?