http3: a request allocates about 1.7 KB of objects that could be reused per connection, 1 KiB of it a new arena #488

Each buffered request in ioxide.http3, the managed HTTP/3, allocates about 1.85 KB:

• about 1.66 KB in its request stream and request, which are new for every stream; 1 KiB of that is the request's arena;
• about 185 B in the QUIC send queue's per-stream objects (ngtcp2: every QUIC stream that sends allocates an OutStream with its own Queue, about 185 B per HTTP/3 response #487).

Measured on main eb5f515 (paths under src/protocols/ioxide.http3/ unless named):

• App: Playground/Http3/ManagedBuffered, path /.
• Setup: 1 reactor pinned to one core, and h2load --alpn-list=h3 -c 32 -m 16 -t 1 pinned to another physical core: 616K req/s.
• Method: a 10 s dotnet-trace --profile gc-verbose window, allocations grouped by type and first ioxide frame.

The streamed sample (Playground/Http3/ManagedStreamedBoth, 8 chunks of 1 KiB) allocates 2.78 KB per request:

• the same request objects;
• an Http3BodyReader (139 B) and its chunk Queue (38 B) per request;
• larger send-queue arrays (ngtcp2: every QUIC stream that sends allocates an OutStream with its own Queue, about 185 B per HTTP/3 response #487).

Its flushes allocate nothing measurable. They wait only when the QUIC send queue is full, and this load never fills it.

Possible fix: the one #486 applies to ioxide.http2.

• Keep a ReqStream on its connection once its handler is done, and reuse it for the next stream with its Http3Request, arena, lists and carry.
  • Today a stream leaves _requests at dispatch (Http3Connection.cs:642, :667) or on reset (:258, :503).
• The request object is then reused once its handler is done, as in http2: a streamed flush that waits for the write pump allocates nothing, and a connection reuses its request objects #486. It was already only valid during the handler.

In ioxide.http2, the same change took a request from 793 B to 89 B, and server CPU per request down by 17%. An HTTP/3 request costs more CPU (QUIC crypto and packets), so the share here needs measuring.