LOH in StreamCopier

[Kahbazi]

Why is the buffer size is 81920 in StreamCopier?

https://github.com/microsoft/reverse-proxy/blob/ca46305dc23519598d831c0fca64e0136476c89b/src/ReverseProxy.Core/Service/Proxy/StreamCopier.cs#L19

Is it to avoid having Large Heap Object? If so I think it still creates a LOH since ArrayPool<byte>.Shared is creating arrays with the length of power of 2, and Renting with length of 81920 would get a array with size of 131072.
Or is it ok to have pooled LOH with ArrayPool?

[ManickaP]

The same size is in runtime: https://github.com/dotnet/runtime/blob/f7d251fa684dc0c432525eb455be6e06f9629911/src/libraries/System.Private.CoreLib/src/System/IO/Stream.cs#L30-L33 with the argument of fitting under the LOH threshold.

So if this really ends up in LOH, problems are in other places as well.

Confirmed, it does end up in LOH, which is exactly the thing this constant was trying to prevent.

[ManickaP]

From the dotnet/runtime#43179 (comment):

After that I experimented with changing it, but decreasing the buffer size makes a measurable impact on throughout in some cases, and the buffer ends up getting pooled a lot of the time, so it wasn't clear that decreasing it was actually the right call.

So I'm not sure if we'd want to change it. And if we do, we should start with benchmarks and measure the impact before we change anything.

@Kahbazi: Has this caused you any issues, or is it more of that you just noticed it in the code?

[Kahbazi]

@ManickaP I have no issue, I was just curios!

[ManickaP]

I'll close this for now. If we discover any performance issues due to this we can revisit and adjust the value.