QUIC: Reuse QuicStream instances on a connection

[JamesNK]

In Kestrel we want to reduce per-request HTTP/3 allocations to the minimum possible. Ideally that would be zero.

With HTTP/1.1 and HTTP/2 Kestrel is able to reuse per-request instances. For example, after a HTTP/2 request is gracefully completed (i.e. wasn't aborted and finished writing a frame with the close stream bit set) the stream is returned to a pool on the connection. Future requests on the connection will reuse the pooled stream by reseting it back to its initial state instead of creating a new one. This is discussed in a blog post.

At the moment a new QuicStream is created for each HTTP/3 request on a connection:

public class QuicConnection
{
    // Existing
    QuicStream OpenUnidirectionalStream();
    QuicStream OpenBidirectionalStream();
    ValueTask<QuicStream> AcceptStreamAsync();

    // New?
    void ReturnCompletedStream(QuicStream stream);
}

Support should be added to System.Net.Quic for optionally returning a QuicStream back to the connection when an app is no longer using it. The instance can then be reused by one of the factory methods above.

Extra things to consider:

• Validate QuicStream is in a valid state before pooling it.
• Maximum number of pooled streams. In Kestrel there is a max of 100 HTTP/2 pooled streams allowed on a connection. I don't think this needs to be configurable.
• Remove unused streams after a certain amount of time. In Kestrel the HTTP/2 pooled streams on a connection are removed if not used within a certain amount of time. In place so idle connections don't unnessarily hold onto memory. I don't think this needs to be configurable.

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Issue Details

---

In Kestrel we want to reduce per-request HTTP/3 connections to the minimum possible. Ideally that would be zero.

With HTTP/1.1 and HTTP/2 Kestrel is able to reuse per-request instances. For example, after a HTTP/2 request is gracefully completed (i.e. wasn't aborted and finished writing a frame with the close stream bit set) the stream is returned to a pool on the connection. Future requests on the connection will reuse the pooled stream by reseting it back to its initial state instead of creating a new one. This is discussed in a blog post.

At the moment a new QuicStream is created for each HTTP/3 request on a connection:

public class QuicConnection
{
    // Existing
    QuicStream OpenUnidirectionalStream();
    QuicStream OpenBidirectionalStream();
    ValueTask<QuicStream> AcceptStreamAsync();

    // New?
    void ReturnCompletedStream(QuicStream stream);
}

Support should be added to System.Net.Quic for optionally returning a QuicStream back to the connection when an app is no longer using it. The instance can then be reused by one of the factory methods above.

Author:	JamesNK
Assignees:	-
Labels:	area-System.Net.Quic
Milestone:	-

[karelz]

Triage: Leave it in Future, if it pops in perf analysis, we can move it to 7.0.

[wegylexy]

Unlike a TCP stream, which is the connection, QUIC multiplexes streams over a single connection.
Correct me if I am wrong. In HTTP/2, requests are multiplexed with frames on the same stream. In HTTP/3, each request has its own lightweight stream exclusive for that particular request. Resetting a QUIC stream aborts it. This is how HTTP/3 works over QUIC.
Nevertheless, it may be possible to reuse the .NET wrapper on a future instance of native MsQuic stream, which is super lightweight. See also dotnet/aspnetcore#33680.

[ManickaP]

it may be possible to reuse the .NET wrapper on a future instance of native MsQuic stream

That's how I understood the issue. I think it's highly unlikely msquic would support re-using their stream object, but maybe I'm wrong, @nibanks?

[nibanks]

MsQuic stream objects are highly optimized for creation and deletion (most of the time it's just pushing and popping from a pool list). So don't really need/want to support reuse. Now, if that's not easy to do in C#, perhaps you could support reuse by just deleting the old one and opening a new one.

[ManickaP]

Yeah, I thought so, and since native memory doesn't influence GC, recreating the msquic object wouldn't have any negative impact on pooling our managed wrapper.

[wfurt]

We could add API that allows to assign new stream to existing QuicStream instead of always allocations new one.
like stream.Close(); stream = QuicConnection.AcceptAsync(stream, token);
We can certainly use pool allocation behind the scenes but that has also some cost and does not give choices to the caller.