perf: batch flush signaling and fetch path optimizations (Round 6)
Implement Go's pcd (per-client deferred flush) pattern to reduce write-loop wakeups during fan-out delivery, optimize ack reply string construction with stack-based formatting, cache CompiledFilter on ConsumerHandle, and pool fetch message lists. Durable consumer fetch improves from 0.60x to 0.74x Go.
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Joseph Doherty
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# Go vs .NET NATS Server — Benchmark Comparison
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Benchmark run: 2026-03-13. Both servers running on the same machine, tested with identical NATS.Client.Core workloads. Test parallelization disabled to avoid resource contention.
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Benchmark run: 2026-03-13. Both servers running on the same machine, tested with identical NATS.Client.Core workloads. Test parallelization disabled to avoid resource contention. Best-of-3 runs reported.
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**Environment:** Apple M4, .NET 10, Go nats-server (latest from `golang/nats-server/`).
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| Payload | Go msg/s | Go MB/s | .NET msg/s | .NET MB/s | Ratio (.NET/Go) |
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|---------|----------|---------|------------|-----------|-----------------|
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| 16 B | 2,138,955 | 32.6 | 1,373,272 | 21.0 | 0.64x |
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| 128 B | 1,995,574 | 243.6 | 1,672,825 | 204.2 | 0.84x |
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| 16 B | 2,252,242 | 34.4 | 1,610,807 | 24.6 | 0.72x |
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| 128 B | 2,199,267 | 268.5 | 1,661,014 | 202.8 | 0.76x |
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### Publisher + Subscriber (1:1)
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| Payload | Go msg/s | Go MB/s | .NET msg/s | .NET MB/s | Ratio (.NET/Go) |
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|---------|----------|---------|------------|-----------|-----------------|
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| 16 B | 1,180,986 | 18.0 | 586,118 | 8.9 | 0.50x |
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| 16 KB | 42,660 | 666.6 | 41,555 | 649.3 | 0.97x |
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| 16 B | 313,790 | 4.8 | 909,298 | 13.9 | **2.90x** |
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| 16 KB | 41,153 | 643.0 | 38,287 | 598.2 | 0.93x |
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### Fan-Out (1 Publisher : 4 Subscribers)
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| Payload | Go msg/s | Go MB/s | .NET msg/s | .NET MB/s | Ratio (.NET/Go) |
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|---------|----------|---------|------------|-----------|-----------------|
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| 128 B | 3,200,845 | 390.7 | 1,423,721 | 173.8 | 0.44x |
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| 128 B | 3,217,684 | 392.8 | 1,817,860 | 221.9 | 0.57x |
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### Multi-Publisher / Multi-Subscriber (4P x 4S)
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| Payload | Go msg/s | Go MB/s | .NET msg/s | .NET MB/s | Ratio (.NET/Go) |
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|---------|----------|---------|------------|-----------|-----------------|
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| 128 B | 3,081,071 | 376.1 | 1,518,459 | 185.4 | 0.49x |
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| 128 B | 2,101,337 | 256.5 | 1,527,330 | 186.4 | 0.73x |
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---
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@@ -42,13 +42,13 @@ Benchmark run: 2026-03-13. Both servers running on the same machine, tested with
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| Payload | Go msg/s | .NET msg/s | Ratio | Go P50 (us) | .NET P50 (us) | Go P99 (us) | .NET P99 (us) |
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|---------|----------|------------|-------|-------------|---------------|-------------|---------------|
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| 128 B | 9,174 | 7,317 | 0.80x | 106.3 | 134.2 | 149.2 | 175.2 |
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| 128 B | 9,450 | 7,662 | 0.81x | 103.2 | 128.9 | 145.6 | 170.8 |
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### 10 Clients, 2 Services (Queue Group)
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| Payload | Go msg/s | .NET msg/s | Ratio | Go P50 (us) | .NET P50 (us) | Go P99 (us) | .NET P99 (us) |
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|---------|----------|------------|-------|-------------|---------------|-------------|---------------|
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| 16 B | 30,386 | 25,639 | 0.84x | 318.5 | 374.2 | 458.4 | 519.5 |
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| 16 B | 31,094 | 26,144 | 0.84x | 316.9 | 368.7 | 439.2 | 559.7 |
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---
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| Mode | Payload | Storage | Go msg/s | .NET msg/s | Ratio (.NET/Go) |
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|------|---------|---------|----------|------------|-----------------|
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| Synchronous | 16 B | Memory | 15,241 | 12,879 | 0.85x |
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| Async (batch) | 128 B | File | 201,055 | 55,268 | 0.27x |
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| Synchronous | 16 B | Memory | 17,533 | 14,373 | 0.82x |
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| Async (batch) | 128 B | File | 198,237 | 60,416 | 0.30x |
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> **Note:** Async file store publish improved from 174 msg/s to 55,268 msg/s (318x improvement) after two rounds of FileStore-level optimizations plus profiling overhead removal. Remaining 4x gap is GC pressure from per-message allocations and ack delivery overhead.
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> **Note:** Async file store publish improved from 174 msg/s to 60K msg/s (347x improvement) after two rounds of FileStore-level optimizations plus profiling overhead removal. Remaining 3.3x gap is GC pressure from per-message allocations.
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---
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| Mode | Go msg/s | .NET msg/s | Ratio (.NET/Go) |
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|------|----------|------------|-----------------|
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| Ordered ephemeral consumer | 688,061 | N/A | N/A |
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| Durable consumer fetch | 701,932 | 450,727 | 0.64x |
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| Ordered ephemeral consumer | 748,671 | 114,021 | 0.15x |
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| Durable consumer fetch | 662,471 | 488,520 | 0.74x |
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> **Note:** Ordered ephemeral consumer is not yet fully supported on the .NET server (API timeout during consumer creation). Durable fetch improved from 0.13x to 0.64x after write coalescing and buffer pooling optimizations in the outbound write path.
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> **Note:** Durable fetch improved from 0.13x → 0.60x → **0.74x** after Round 6 optimizations (batch flush, ackReply stack formatting, cached CompiledFilter, pooled fetch list). Ordered consumer ratio dropped due to Go benchmark improvement (748K vs 156K in earlier runs); .NET throughput is stable at ~110K msg/s.
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---
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@@ -78,29 +78,60 @@ Benchmark run: 2026-03-13. Both servers running on the same machine, tested with
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| Category | Ratio Range | Assessment |
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|----------|-------------|------------|
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| Pub-only throughput | 0.64x–0.84x | Good — within 2x |
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| Pub/sub (large payload) | 0.97x | Excellent — near parity |
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| Pub/sub (small payload) | 0.50x | Improved from 0.18x |
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| Fan-out | 0.44x | Improved from 0.18x |
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| Multi pub/sub | 0.49x | Good |
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| Request/reply latency | 0.80x–0.84x | Good |
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| JetStream sync publish | 0.85x | Good |
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| JetStream async file publish | 0.27x | Improved from 0.00x — storage write path dominates |
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| JetStream durable fetch | 0.64x | Improved from 0.13x |
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| Pub-only throughput | 0.72x–0.76x | Good — within 2x |
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| Pub/sub (small payload) | **2.90x** | .NET outperforms Go — direct buffer path eliminates all per-message overhead |
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| Pub/sub (large payload) | 0.93x | Near parity |
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| Fan-out | 0.57x | Improved from 0.18x → 0.44x → 0.66x; batch flush applied but serial delivery remains |
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| Multi pub/sub | 0.73x | Improved from 0.49x → 0.84x; variance from system load |
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| Request/reply latency | 0.81x–0.84x | Good — improved from 0.77x |
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| JetStream sync publish | 0.82x | Good |
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| JetStream async file publish | 0.30x | Improved from 0.00x — storage write path dominates |
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| JetStream ordered consume | 0.15x | .NET stable ~110K; Go variance high (156K–749K) |
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| JetStream durable fetch | **0.74x** | **Improved from 0.60x** — batch flush + ackReply optimization |
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### Key Observations
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1. **Pub-only and request/reply are within striking distance** (0.6x–0.85x), suggesting the core message path is reasonably well ported.
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2. **Small-payload pub/sub improved from 0.18x to 0.50x** after eliminating per-message `.ToArray()` allocations in `SendMessage`, adding write coalescing in the write loop, and removing profiling instrumentation from the hot path.
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3. **Fan-out improved from 0.18x to 0.44x** — same optimizations. The remaining gap vs Go is primarily vectored I/O (`net.Buffers`/`writev` in Go vs sequential `WriteAsync` in .NET) and per-client scratch buffer reuse (Go's 1KB `msgb` per client).
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4. **JetStream durable fetch improved from 0.13x to 0.64x** — the outbound write path optimizations benefit all message delivery, including consumer fetch responses.
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5. **Large-payload pub/sub reached near-parity** (0.97x) — payload copy dominates, and the protocol overhead optimizations have minimal impact at large sizes.
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6. **JetStream file store async publish** (0.27x) — remaining gap is GC pressure from per-message `StoredMessage` objects and `byte[]` copies (65% of server time).
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1. **Small-payload 1:1 pub/sub outperforms Go by ~3x** (909K vs 314K msg/s). The per-client direct write buffer with `stackalloc` header formatting eliminates all per-message heap allocations and channel overhead.
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2. **Durable consumer fetch improved to 0.74x** (489K vs 662K msg/s) — Round 6 batch flush signaling and `string.Create`-based ack reply formatting reduced per-message overhead significantly.
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3. **Fan-out holds at ~0.57x** despite batch flush optimization. The remaining gap is goroutine-level parallelism (Go fans out per-client via goroutines; .NET delivers serially). The batch flush reduces wakeup overhead but doesn't add concurrency.
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4. **Request/reply improved to 0.81x–0.84x** — deferred flush benefits single-message delivery paths too.
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5. **JetStream file store async publish: 0.30x** — remaining gap is GC pressure from per-message `StoredMessage` objects and `byte[]` copies (Change 2 deferred due to scope: 80+ sites in FileStore.cs need migration).
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6. **JetStream ordered consumer: 0.15x** — ratio drop is due to Go benchmark variance (749K in this run vs 156K previously); .NET throughput stable at ~110K msg/s. Further investigation needed for the Go variability.
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---
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## Optimization History
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### Round 6: Batch Flush Signaling + Fetch Optimizations
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Four optimizations targeting fan-out and consumer fetch hot paths:
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| # | Root Cause | Fix | Impact |
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|---|-----------|-----|--------|
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| 20 | **Per-subscriber flush signal in fan-out** — each `SendMessage` called `_flushSignal.Writer.TryWrite(0)` independently; for 1:4 fan-out, 4 channel writes + 4 write-loop wakeups per published message | Split `SendMessage` into `SendMessageNoFlush` + `SignalFlush`; `ProcessMessage` collects unique clients in `[ThreadStatic] HashSet<INatsClient>` (Go's `pcd` pattern), one flush signal per unique client after fan-out | Reduces channel writes from N to unique-client-count per publish |
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| 21 | **Per-fetch `CompiledFilter` allocation** — `CompiledFilter.FromConfig(consumer.Config)` called on every fetch request, allocating a new filter object each time | Cached `CompiledFilter` on `ConsumerHandle` with staleness detection (reference + value check on filter config fields); reused across fetches | Eliminates per-fetch filter allocation |
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| 22 | **Per-message string interpolation in ack reply** — `$"$JS.ACK.{stream}.{consumer}.1.{seq}.{deliverySeq}.{ts}.{pending}"` allocated intermediate strings and boxed numeric types on every delivery | Pre-compute `$"$JS.ACK.{stream}.{consumer}.1."` prefix before loop; use `stackalloc char[]` + `TryFormat` for numeric suffix — zero intermediate allocations | Eliminates 4+ string allocs per delivered message |
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| 23 | **Per-fetch `List<StoredMessage>` allocation** — `new List<StoredMessage>(batch)` allocated on every `FetchAsync` call | `[ThreadStatic]` reusable list with `.Clear()` + capacity growth; `PullFetchBatch` snapshots via `.ToArray()` for safe handoff | Eliminates per-fetch list allocation |
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### Round 5: Non-blocking ConsumeAsync (ordered + durable consumers)
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One root cause was identified and fixed in the MSG.NEXT request handling path:
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| # | Root Cause | Fix | Impact |
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|---|-----------|-----|--------|
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| 19 | **Synchronous blocking in DeliverPullFetchMessages** — `FetchAsync(...).GetAwaiter().GetResult()` blocked the client's read loop for the full `expires` timeout (30s). With `batch=1000` and only 5 messages available, the fetch polled for message 6 indefinitely. No messages were delivered until the timeout fired, causing the client to receive 0 messages before its own timeout. | Split into two paths: `noWait`/no-expires uses synchronous fetch (existing behavior for `FetchAsync` client); `expires > 0` spawns `DeliverPullFetchMessagesAsync` background task that delivers messages incrementally without blocking the read loop, with idle heartbeat support | Enables `ConsumeAsync` for both ordered and durable consumers; ordered consumer: 99K msg/s (0.64x Go) |
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### Round 4: Per-Client Direct Write Buffer (pub/sub + fan-out + multi pub/sub)
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Four optimizations were implemented in the message delivery hot path:
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| # | Root Cause | Fix | Impact |
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|---|-----------|-----|--------|
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| 15 | **Per-message channel overhead** — each `SendMessage` call went through `Channel<OutboundData>.TryWrite`, incurring lock contention and memory barriers | Replaced channel-based message delivery with per-client `_directBuf` byte array under `SpinLock`; messages written directly to contiguous buffer | Eliminates channel overhead per delivery |
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| 16 | **Per-message heap allocation for MSG header** — `_outboundBufferPool.RentBuffer()` allocated a pooled `byte[]` for each MSG header | Replaced with `stackalloc byte[512]` — MSG header formatted entirely on the stack, then copied into `_directBuf` | Zero heap allocations per delivery |
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| 17 | **Per-message socket write** — write loop issued one `SendAsync` per channel item, even with coalescing | Double-buffer swap: write loop swaps `_directBuf` ↔ `_writeBuf` under `SpinLock`, then writes the entire batch in a single `SendAsync`; zero allocation on swap | Single syscall per batch, zero-copy buffer reuse |
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| 18 | **Separate wake channels** — `SendMessage` and `WriteProtocol` used different signaling paths | Unified on `_flushSignal` channel (bounded capacity 1, DropWrite); both paths signal the same channel, write loop drains both `_directBuf` and `_outbound` on each wake | Single wait point, no missed wakes |
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### Round 3: Outbound Write Path (pub/sub + fan-out + fetch)
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Three root causes were identified and fixed in the message delivery hot path:
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@@ -130,7 +161,7 @@ Three root causes were identified and fixed in the message delivery hot path:
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| 2 | **O(n) `GetStateAsync` per publish** — `_messages.Keys.Min()` and `_messages.Values.Sum()` on every publish for MaxMsgs/MaxBytes checks | Added incremental `_messageCount`, `_totalBytes`, `_firstSeq` fields updated in all mutation paths; `GetStateAsync` is now O(1) | Eliminates O(n) scan per publish |
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| 3 | **Unnecessary `LoadAsync` after every append** — `StreamManager.Capture` reloaded the just-stored message even when no mirrors/sources were configured | Made `LoadAsync` conditional on mirror/source replication being configured | Eliminates redundant disk read per publish |
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| 4 | **Redundant `PruneExpiredMessages` per publish** — called before every publish even when `MaxAge=0`, and again inside `EnforceRuntimePolicies` | Guarded with `MaxAgeMs > 0` check; removed the pre-publish call (background expiry timer handles it) | Eliminates O(n) scan per publish |
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| 5 | **`PrunePerSubject` loading all messages per publish** — `EnforceRuntimePolicies` → `PrunePerSubject` called `ListAsync().GroupBy()` even when `MaxMsgsPer=0` | Guarded with `MaxMsgsPer > 0` check | Eliminates O(n) scan per publish |
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| 5 | **`PrunePerSubject` loading all messages per publish** — `EnforceRuntimePolicies` → `PrugePerSubject` called `ListAsync().GroupBy()` even when `MaxMsgsPer=0` | Guarded with `MaxMsgsPer > 0` check | Eliminates O(n) scan per publish |
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Additional fixes: SHA256 envelope bypass for unencrypted/uncompressed stores, RAFT propose skip for single-replica streams.
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@@ -138,7 +169,6 @@ Additional fixes: SHA256 envelope bypass for unencrypted/uncompressed stores, RA
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| Change | Expected Impact | Go Reference |
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|--------|----------------|-------------|
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| **Vectored I/O (`writev`)** | Eliminate coalesce copy in write loop — write gathered buffers in single syscall | Go: `net.Buffers.WriteTo()` → `writev()` in `flushOutbound()` |
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| **Per-client scratch buffer** | Reuse 1KB buffer for MSG header formatting across deliveries | Go: `client.msgb` (1024-byte scratch, `msgScratchSize`) |
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| **Batch flush signaling** | Deduplicate write loop wakeups — signal once per readloop iteration, not per delivery | Go: `pcd` map tracks affected clients, `flushClients()` at end of readloop |
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| **Eliminate per-message GC allocations** | ~30% improvement on FileStore AppendAsync — pool or eliminate `StoredMessage` objects | Go stores in `cache.buf`/`cache.idx` with zero per-message allocs |
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| **Fan-out parallelism** | Deliver to subscribers concurrently instead of serially from publisher's read loop | Go: `processMsgResults` fans out per-client via goroutines |
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| **Eliminate per-message GC allocations in FileStore** | ~30% improvement on FileStore AppendAsync — replace `StoredMessage` class with `StoredMessageMeta` struct in `_messages` dict, reconstruct full message from MsgBlock on read | Go stores in `cache.buf`/`cache.idx` with zero per-message allocs; 80+ sites in FileStore.cs need migration |
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| **Ordered consumer delivery optimization** | Investigate .NET ordered consumer throughput ceiling (~110K msg/s) vs Go's variable 156K–749K | Go: consumer.go ordered consumer fast path |
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