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romanett
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Follow-ups from the "Not changed" list of #4720 (client per-request allocation and CPU): three changes that the paired A/B shows a gain for, plus a write-up of the fourth (discarded `ResponseHeader`), which is not changed. ## Results Paired A/B, each change against the commit before it: 4 alternating rounds, medians, 20 sessions, 24-core machine shared with other sessions. Client = the harness process, server = the reference server child. Allocated bytes per operation are deterministic; throughput and latency carry the run-to-run noise noted below. | Change | Scenario | Throughput | p50 | p90 | Client CPU / op | Client alloc / op | Server CPU / op | |---|---|---|---|---|---|---|---| | Receive staging | Read, 1 value | +4% | **−9%** | **−13%** | ≈ | ±0 | ≈ | | | Read, 1 value, SignAndEncrypt | +6% | −6% | −11% | ≈ | ±0 | ≈ | | | Read, 100 values | +2% | −4% | ≈ | ≈ | ±0 | ≈ | | | Data changes | ≈ | ≈ | ≈ | ≈ | ±0 | ≈ | | ManagedSession lock | Read, 1 value, ManagedSession | ≈ | ≈ | ≈ | ≈ | **−125 B** (2632 → 2507) | ≈ | | | Same, inflight 8 | ≈ | ≈ | ≈ | ≈ | −133 B | ≈ | | Write coalescing | Read, 1 value, 20 sessions on one shared channel, inflight 4 | **+13%** | **−13%** | −3% | −6% | ±0 | −4% | | | Same, inflight 1 | +20% ¹ | −8% | ¹ | −9% | ±0 | −13% | | | Read, 1 value, inflight 8 (one channel per session) | +2% | −4% | −6% | ≈ | ±0 | ≈ | | | Read, 100 values, inflight 8 | +3% | −4% | −8% | ≈ | ±0 | ≈ | | | Data changes | ≈ | ≈ | ≈ | ≈ | ±0 | ≈ | ≈: within run-to-run noise (about ±5% for CPU per op here). ¹ Rounds ranged from −42% to +32%; the inflight-4 runs were better in all four rounds. ## Changes - **`TcpByteTransport` receive**: `ReceiveChunkAsync` read the 8-byte chunk header and then the body, two `recv` calls per chunk. It now waits for data with a small staging buffer (8 KB, allocated on the first receive, not from the `BufferManager` budget). One receive returns the header and, when the chunk is small and already there, the whole chunk; bytes of following chunks are kept for the next call. A larger chunk is completed with a receive into the rented buffer, as before. - The receive buffer is still rented only after the header has been validated (message type, size against the negotiated limit), so an idle peer holds none of the budget. - A receive cancelled while its bytes are in the staging buffer keeps them for the next call; on .NET Framework the staging receive takes no token, so cancellation is observed once data arrives. - A receive that fails while reading the body of a chunk larger than the staging buffer has consumed part of the chunk; later receives then fail with `BadTcpInternalError` rather than read body bytes as a header. Before this PR such a failure lost the header and body bytes silently. The channel cancels receives only when it closes. - Server listener channels use the same transport. - **`ManagedSession` service calls**: each call was an async wrapper (await the reader lock, await the inner call), one more state machine box per request. - When services are available and no writer holds or waits for the lock, the reader lock is taken synchronously (`AsyncReaderWriterLock.TryEnterReader`). It is released by a cached continuation on the inner call's task, registered before the caller's await, so the lock is free again when the caller resumes. - Otherwise the call waits for the lock as before, so reconnect and failover still block new calls and drain the ones in flight. - The remaining 88 B per call is the task's continuation list (two continuations). - **Write coalescing** (`UaSCUaBinaryChannel`): when more writes for the same transport are queued behind the one being sent, the writer takes up to 16 of them and sends their segments in one vectored send. - Each write is then reported on its own, in order, with its size. A failed send is reported for every write in it, and their buffers are returned. - Writes still never start on the caller's stack, and one writer sends one batch at a time, so chunk order is unchanged. - Only byte-stream transports opt in (`TcpByteTransport`, also through the server's admission lease, via the internal `IUaSCChunkCoalescing`). WebSocket and capture decorators keep one send per chunk. ### Behaviour notes - Each TCP connection that has received holds an 8 KB staging array (pinned on .NET 5+), outside the `BufferManager` budget. The previous transport held an 8-byte header array. - `ManagedSession` service calls: if the inner call completes while the release continuation is being registered, the reader lock is released on the thread pool shortly after, instead of before the caller resumes. - Concurrent `ReceiveChunkAsync` calls on one `TcpByteTransport` now throw `InvalidOperationException`. The channel has a single receive loop, and interleaved receives on a stream were never valid. ## Not changed: discarded `ResponseHeader` Every generated structure's `Initialize()` constructs its structure-typed fields: `new ResponseHeader()` (112 B), `new RequestHeader()` (104 B). The decoders always create the instance (`new T()` or the activator) and then assign every field in `Decode`, so the default is thrown away. That is 4.6% of the client's 2.4 KB per Read and about 3% on the server, which discards a `RequestHeader` per decoded request and a `ResponseHeader` per response it constructs. `PublishResponse` also discards a default `NotificationMessage`. Options considered: - **Lazy getter** (`get => m_x ??= new X()`): `IsEqual`/`GetHashCode` compare fields, so a never-read default (null) would no longer equal an explicit default. Two threads reading an unset header could get different instances. Pooled instances reset through `Initialize()` anyway. - **Decode-only constructor** called by the activator: only the plain `Class`/`DerivedClass` templates assign every field in `Decode`, not optional-field or union types. A derived type's constructor must chain to a base built by an older generator in another assembly, which lacks it (the same problem `ForeignDerivedPooledExtensionClass` works around). It also needs a new public marker type in `Opc.Ua.Types`. - **Decode into the existing instance**: needs a new `IDecoder` member in all decoders. It also changes behaviour when a caller decodes into an object whose header it still references. Each touches every generated type for about 100 B per message, so this is left as is. ## Harness - `--client shared`: classic sessions whose secure channels come from one `ClientChannelManager`. - Created sequentially (`--connect-parallel`, default 1). Concurrent `Session.CreateAsync` calls on one manager fail with `BadSecureChannelClosed`, a separate bug. - 20 sessions on one channel reach about 45–70k req/s, against about 140k on 20 channels. - `ab.sh`: `CANDIDATE=` selects the candidate build. ## Testing - New: `TcpByteTransportTests`: chunks sent together, a chunk split across sends (inside the next header), a chunk larger than the staging buffer, cancellation keeping staged bytes. `UaSCBinaryChannelWriteQueueTests`: coalesced in order (1 + 16 + 16 + 7 sends), framing transport not coalesced, a failed batch reported for every write and later writes still sent. `AsyncReaderWriterLockTests`: `TryEnterReader` against a writer, the reader held until the operation completes and released on failure and on a completed operation, the awaiter resuming after the release. - Before the rebase onto master, net10.0, run sequentially: Core Stack+Security 4689 passed, 1 failed (`ClientBaseTests.ActivityTraceFlags_Metrics_ShouldRecordMetricsOnly`, a meter-listener flake fixed on master by #4708; passes 3/3 in isolation); Client 2768; Sessions 851 (379 skipped); Server 8398; Subscriptions 643. - After the rebase: Core Stack+Security 4811 passed; Client 2781; Stress Contract+Integration 45 passed; all 0 failed. Sessions, Server and Subscriptions were not rerun on the new base. - net48 build of `UA.slnx`: clean (0 warnings, 0 errors); `dotnet format whitespace` clean. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>