Fast-path async in HttpResponseStreamWriter

[benaadams]

The ratio of non-async paths in WriteAsync of HttpResponseStreamWriter to async paths (including FlushAsync) for the RazorRendering benchmarkapp aspnet/Mvc#8366

For 1 request in the currently its 8774 non-async to 12 async.

This adds a fast-path to skip the async statemahchine creation for when no FlushAsync takes place.

It also means it can be detected upstream in MVC and allow more fast-paths to flow

/cc @davidfowl @rynowak

[JamesNK]

Looks good to me. How much time is spent in that method for a request? 8000k is a fair number of calls but does the state machine overhead add up to much?

[benaadams]

8000k statemachines with all their try/catchery

[benaadams]

8000k is a fair number of calls but does the state machine overhead add up to much?

Also this is for one request; so that's 8bn for 1M requests; or 8M for 1k requests (though they are larger responses)

[campersau]

This would now throw if value is null.

[benaadams]

fixed

[benaadams]

How much time is spent in that method for a request?

As CopyToCharBuffer is the workhorse part of the function; it does looks significant in terms of where the rest of the time is spent in this class:

[benaadams]

Any takers for a review/approve? /cc @rynowak

[rynowak]

OK it.

[rynowak]

Is it worth folding this together with L234?

[benaadams]

Done

[JamesNK]

I think the changes look good. Are there unit tests that hit both sync and async paths?

[davidfowl]

LGTM

[JunTaoLuo]

Merged in 800c79c. Thanks @benaadams !