Reduce allocation in HTTP/2 requests by ~30%

[stephentoub]

Reduces total number and size of allocations on my particular test by ~30%. The test just repeatedly hits the microsoft.com en-us home page:

using System; using System.IO; using System.Net; using System.Net.Http; using System.Threading; using System.Threading.Tasks; class Program { static async Task Main() { var client = new HttpClient(new SocketsHttpHandler() { UseCookies = false }) { Timeout = Timeout.InfiniteTimeSpan, DefaultRequestVersion = HttpVersion.Version20 }; var uri = new Uri("https://www.microsoft.com/en-us/"); for (int i = 0; i < 1000; i++) { using (HttpResponseMessage r = await client.GetAsync(uri, HttpCompletionOption.ResponseHeadersRead)) using (Stream s = await r.Content.ReadAsStreamAsync()) { await s.CopyToAsync(Stream.Null); } } } }

Primary changes:

• Adds a CopyToAsync override on the HTTP/2 response content stream.
• Stops forcing the Expect and TransferEncoding request headers into existence. They should now only be allocated if the dev explicitly sets them on the request.
• Removes string allocations from Uri.IdnHost, while also reducing the amount of unsafe code used.
• Removes the Http2Connection.ReadAtLeastAsync method, putting the relevant logic into its sole caller, EnsureIncomingBytesAsync, without increasing its size (and reducing overall number of lines of code).
• Reduces the number of calls to ReadFrameAsync that actually need to yield, such that more complete synchronously and never need to allocate a state machine.
• Reduce the size of the Http2Connection.SendAsync state machine.
• Add additional common header values to KnownHeaders

Additionally:

• When first measuring, I neglected to disable cookies, and the microsoft.com home page triggers a bunch of related functionality. Before disabling that in the test, I removed a bunch of temporarily allocations related to cookies, like unnecessary enumerator allocations, also removing some associated interface dispatch.

cc: @scalablecory, @davidsh, @MihaZupan, @JamesNK
Contributes to #31235

[MihaZupan]

As far as Uri is concerned, IdnEquivalent is doing a lot of work for nothing - that entire loop has no side-effects, it can be cut out entirely. Example

This method is only called from the Uri.IdnHost property getter.
IdnHost is currently not marked as nullable, but will still return null for empty hosts.
This change changes that to return string.Empty instead.

[stephentoub]

that entire loop has no side-effects

Which loop? Can you link to it?

[MihaZupan]

Loop

Looking at other methods in that file, UnicodeEquivalent seems to be doing busy work as well to set ref parameters that are ignored by all call sites.

[JamesNK]

Nice!

gRPC is always a POST with a request body. It would be great if you could look at that scenario as well.

Here is an example of a gRPC request using HttpClient directly - https://github.com/grpc/grpc-dotnet/blob/d1b2b6496b6aa73ddff80f294ee1068cc472d369/perf/benchmarkapps/BenchmarkClient/Worker/GrpcRawUnaryWorker.cs#L71-L88

[stephentoub]

Loop

Yeah, that does appear to be dead, after dotnet/corefx#40546. I've removed it.

[stephentoub]

Looking at other methods in that file, UnicodeEquivalent seems to be doing busy work as well to set ref parameters that are ignored by all call sites.

I'll leave that to you to clean up separately. Thanks.