perf: replace O(n) TCP RX buffer scan with SIMD memchr + carry buffer (Sprint 3)

This commit fixes the most significant hot-path bottleneck in the client: the tcp_client_rx_loop was scanning up to 256KB byte-by-byte on every read() call looking for interleaved 12-byte status messages. Changes: - client.rs (tcp_client_rx_loop): Replace the O(n) for-loop scan with a three-stage approach: 1. Split-message check: An 11-byte carry buffer stores trailing bytes from the previous read. We check every possible alignment where a status message (0x07 + cpu_byte) could span the carry and the start of the current buffer. This fixes a latent bug where the old code would miss status messages split across TCP read boundaries. 2. Fast scan: memchr::memchr (AVX2/NEON SIMD) finds 0x07 bytes in the 256KB buffer. On all-zero data packets this exits in ~4096 SIMD-width operations instead of 262,144 byte compares. ~64x faster scan path. 3. Carry save: Save up to 11 trailing bytes for the next read. - client.rs (unit tests): Add scan_status_message() helper and five unit tests covering: - Status message fully within buffer - Status message split across reads (5+7 bytes) - Status message split at boundary (1+11 bytes) - All-zero buffer (no false positive) - Short buffer (no panic) - Cargo.toml / Cargo.lock: Add memchr as an explicit dependency. Verified against live MikroTik RouterOS (TCP both + receive modes with EC-SRP5 auth). Status messages detected correctly. No wire protocol changes — 100% MikroTik compatible.
2026-04-30 20:46:34 +04:00
parent 205030ce33
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+# btest-rs Performance Audit
+
+**Date:** 2026-04-30  
+**Scope:** Full codebase (`src/`, `tests/`, `Cargo.toml`)  
+**Methodology:** Static code analysis, hot-path tracing, lock/contention review, algorithmic complexity analysis  
+
+---
+
+## Executive Summary
+
+The codebase is generally well-structured for a network I/O tool, with good use of atomics in the per-packet hot path and zero-allocation protocol serialization. However, **three critical bottlenecks** significantly limit throughput and scalability:
+
+1. **O(n) buffer scan on every TCP read** in the client RX loop (up to 256KB scanned per `read()` call)
+2. **Expensive EC curve reconstruction on every authentication** (heavy `BigUint` modular arithmetic)
+3. **Single SQLite connection mutex** serializing all DB operations in `server_pro`
+
+Additionally, there is **no benchmark or profiling infrastructure** in the project, making it impossible to measure improvements or catch regressions.
+
+---
+
+## Severity Legend
+
+| Icon | Severity | Impact |
+|------|----------|--------|
+| 🔴 | **Critical** | Direct throughput/latency hit in hot path; fix immediately |
+| 🟠 | **High** | Significant overhead under load; fix in next sprint |
+| 🟡 | **Medium** | Noticeable at scale or under specific conditions |
+| 🟢 | **Low** | Cosmetic / easy wins; batch with other work |
+
+---
+
+## 🔴 Critical Bottlenecks
+
+### 1. O(n) Linear Buffer Scan in `tcp_client_rx_loop` (`src/client.rs:210-216`)
+
+**Problem:** On every TCP `read()` call (up to 256KB), the client performs a byte-by-byte scan looking for interleaved 12-byte status messages:
+
+```rust
+for i in 0..=(n - STATUS_MSG_SIZE) {
+    if buf[i] == STATUS_MSG_TYPE && buf[i + 1] >= 0x80 {
+        // ...
+    }
+}
+```
+
+Since data packets are all zeros and status bytes are extremely rare, this **almost always scans the entire 256KB buffer** uselessly. At high bandwidth (many reads/sec), this wastes massive CPU cycles and pollutes cache lines.
+
+**Impact:** CPU-bound slowdown on the client RX side during bidirectional TCP tests. The compiler *may* auto-vectorize the simple loop, but it still processes ~256K bytes per read.
+
+**Fix Options (pick one):**
+- **Best:** Use `memchr` (crate or `std::slice::memchr` on nightly) to find `0x07` bytes. On all-zero buffers this exits after a few SIMD-width checks.
+- **Alternative:** Since status messages are injected at `write_all` boundaries and data is all zeros, maintain a small 12-byte sliding ring buffer across reads. Process the stream with a tiny state machine instead of scanning the whole buffer.
+- **Alternative:** Track read bytes modulo expected packet size. Status messages are injected between full packets, so they will appear at predictable offsets *if* the client knows the server's `effective_size`. This requires protocol coordination.
+
+---
+
+### 2. `WCurve::new()` Recomputes Generator on Every Auth (`src/ecsrp5.rs:363,499`)
+
+**Problem:** Every EC-SRP5 authentication (client and server) calls `WCurve::new()`, which performs `lift_x(9)` → `prime_mod_sqrt()` — heavy `BigUint` modular arithmetic to derive the curve generator point.
+
+```rust
+pub async fn client_authenticate<S: ...>(stream: &mut S, username: &str, password: &str) -> Result<()> {
+    let w = WCurve::new();  // <-- expensive, same result every time
+    // ...
+}
+```
+
+The curve constants (`P`, `CURVE_ORDER`, `WEIERSTRASS_A`) are already cached as `LazyLock` statics, but the generator point is not.
+
+**Impact:** Auth latency spikes, especially on the server under many concurrent connections. Each auth does redundant `BigUint` allocations and modular square roots.
+
+**Fix:** Cache `WCurve` (or at least the generator point) in a global `LazyLock`:
+
+```rust
+static WCURVE: std::sync::LazyLock<WCurve> = std::sync::LazyLock::new(WCurve::new);
+```
+
+Then use `&*WCURVE` in both `client_authenticate` and `server_authenticate`.
+
+---
+
+### 3. Single SQLite Mutex Serializes All DB Operations (`src/server_pro/user_db.rs:15-18`)
+
+**Problem:** The entire `server_pro` database layer uses a single shared `Connection` behind a `std::sync::Mutex`:
+
+```rust
+pub struct UserDb {
+    conn: Arc<Mutex<Connection>>,
+}
+```
+
+While SQLite WAL mode is enabled (allowing readers to proceed during writes), **the Rust mutex still serializes all access to the connection object**. Under concurrent load with many tests starting/finishing, this becomes the primary bottleneck.
+
+**Critical sub-issue:** `QuotaManager::remaining_budget()` (`src/server_pro/quota.rs:387`) performs **up to 15 separate SQLite queries** in sequence, locking the mutex 15+ times per pre-test check.
+
+**Impact:** Connection setup/teardown latency increases linearly with concurrency. Quota checks and usage recording block each other.
+
+**Fix Options:**
+- **Connection pooling:** Use `r2d2_sqlite` or `deadpool-sqlite` to maintain a small pool of connections (SQLite handles this well in WAL mode).
+- **Separate read/write paths:** Open a read-only connection for quota checks (`remaining_budget`) and a dedicated write connection for usage recording. SQLite WAL allows this.
+- **Batch quota checks:** Cache quota results for a few seconds per user/IP to avoid redundant queries.
+- **Channel-based writer:** Use a single dedicated DB writer task with an `mpsc` channel so only one task ever touches the connection, eliminating lock contention entirely.
+
+---
+
+## 🟠 High Severity Issues
+
+### 4. 100ms Busy-Poll Wait in Multi-Connection TCP (`src/server.rs:313-332`)
+
+**Problem:** When waiting for secondary TCP connections to join a multi-connection session, the primary connection busy-polls the session map every 100ms:
+
+```rust
+loop {
+    let count = { let map = sessions.lock().await; ... };
+    if count + 1 >= conn_count as usize { break; }
+    tokio::time::sleep(Duration::from_millis(100)).await;
+}
+```
+
+This adds up to **100ms of unnecessary latency** to every multi-connection test startup. It also hammers the async mutex needlessly.
+
+**Fix:** Replace with `tokio::sync::Notify`. When a secondary connection registers itself, it calls `notify_one()`. The primary waits on `notified().await` with a timeout, waking instantly when ready.
+
+---
+
+### 5. FreeBSD CPU Sampling Spawns Process Every Second (`src/cpu.rs:142`)
+
+**Problem:** On FreeBSD, `get_cpu_times()` spawns `sysctl -n kern.cp_time` via `std::process::Command` every second:
+
+```rust
+fn get_cpu_times() -> (u64, u64) {
+    if let Ok(output) = std::process::Command::new("sysctl")
+        .arg("-n").arg("kern.cp_time").output()
+    { ... }
+}
+```
+
+`fork()` + `exec()` is extremely expensive relative to the work being done (reading 5 integers).
+
+**Fix:** Use `libc::sysctl()` via FFI, matching the macOS implementation style. Cache the `mib` array and call the syscall directly.
+
+---
+
+### 6. Per-Call Timer Registration in UDP RX Loops (`src/client.rs:393`, `src/server.rs:925`)
+
+**Problem:** Both UDP RX loops create a new `tokio::time::timeout` timer on **every single `recv`/`recv_from` call**:
+
+```rust
+match tokio::time::timeout(Duration::from_secs(5), socket.recv(&mut buf)).await
+```
+
+At high packet rates (e.g., 100K pps), registering and canceling timers on the Tokio timer wheel adds measurable overhead.
+
+**Fix:** Use `tokio::select!` with a long-lived `tokio::time::sleep` future that is reset, or use the socket's built-in SO_RCVTIMEO if available via `socket2`. Alternatively, since UDP is connectionless, consider whether a 5-second timeout is needed on every call or if the outer test duration timer is sufficient.
+
+---
+
+## 🟡 Medium Severity Issues
+
+### 7. String Error Matching with Allocation (`src/server_pro/enforcer.rs:157-161`)
+
+```rust
+match format!("{}", e).as_str() {
+    s if s.contains("daily") => ...
+}
+```
+
+`format!("{}", e)` allocates a `String` from the error just to do substring matching. Use `e.to_string().contains(...)` or match on error types directly if possible.
+
+---
+
+### 8. `ip.to_string()` Called Repeatedly in Quota Checks (`src/server_pro/quota.rs:389`)
+
+```rust
+let ip_str = ip.to_string();
+// ... used in 6+ DB calls
+```
+
+This allocates a `String` on every quota check. Accept `&str` or `IpAddr` directly in DB methods, or cache the string.
+
+---
+
+### 9. `chrono_date_today()` Recomputes Calendar from Epoch (`src/server_pro/user_db.rs:617-638`)
+
+A hand-rolled date calculation loops through years from 1970 and months every time it's called (which is before almost every DB write). The `chrono` crate is already used indirectly by `rusqlite`; add it as a direct dependency and replace with `chrono::Local::now().format("%Y-%m-%d")`.
+
+---
+
+## 🟢 Low Severity / Easy Wins
+
+### 10. CSV File Reopened on Every Write (`src/csv_output.rs:77`)
+
+```rust
+if let Ok(mut f) = OpenOptions::new().append(true).open(path) {
+    let _ = writeln!(f, "{}", row);
+}
+```
+
+Called once per test, not per-packet, but still suboptimal. Consider keeping a lazily-initialized `Mutex<Option<File>>` or using `std::fs::OpenOptions` once at init and storing the handle.
+
+---
+
+### 11. Global Syslog Mutex Held During I/O (`src/syslog_logger.rs`)
+
+```rust
+static SYSLOG: Mutex<Option<SyslogSender>> = Mutex::new(None);
+```
+
+The global `std::sync::Mutex` is held while formatting the timestamp (expensive manual calendar math) and sending UDP. Switch to `parking_lot::Mutex` (faster) or `tokio::sync::Mutex` if async, and format the message outside the lock. Better yet, use `tracing`'s built-in syslog integration or a structured appender.
+
+---
+
+### 12. `hash_password()` Uses `format!` + `format!` in Hex Loop (`src/server_pro/user_db.rs:612-614`)
+
+```rust
+hasher.update(format!("{}:{}", username, password).as_bytes());
+result.iter().map(|b| format!("{:02x}", b)).collect()  // N allocs for N bytes
+```
+
+The hex encoding allocates one `String` per byte. Use a small fixed buffer or `hex` crate (already used elsewhere in `ecsrp5.rs`).
+
+---
+
+### 13. Redundant `Instant::now()` Calls in TX Loop (`src/server.rs:593,606`)
+
+```rust
+if send_status && Instant::now() >= next_status {
+    // ...
+    next_status = Instant::now() + Duration::from_secs(1);
+}
+```
+
+Two monotonic clock reads per loop iteration. Cache `let now = Instant::now();` at the top of the loop.
+
+---
+
+## Architecture Observations
+
+### What the Code Does Well
+- **Zero-allocation protocol layer:** `serialize()` returns fixed-size stack arrays (`[u8; 12]`, `[u8; 16]`). Excellent.
+- **Atomic bandwidth tracking:** `BandwidthState` uses `AtomicU64` with `Relaxed` ordering in the per-packet path. No locks in the data plane.
+- **Buffer reuse:** TX/RX loops allocate `vec![0u8; ...]` once before the loop. Good.
+- **Aggressive release profile:** `lto = true`, `codegen-units = 1`, `opt-level = 3`.
+
+### Async Runtime Usage
+- `tokio` with `full` features is used. For a primarily I/O-bound tool, this is appropriate.
+- `tokio::task::yield_now().await` is used in unlimited-rate mode to prevent starving the runtime. This is correct but consider whether `tokio::task::spawn_blocking` or dedicated CPU pinning is needed for the EC-SRP5 math (which is CPU-bound and currently runs on the async runtime during auth).
+
+### Memory Safety
+- Several `unwrap()`/`expect()` calls in setup paths (socket binding, address parsing). These are acceptable for config errors but should use `?` propagation where possible to allow graceful degradation.
+
+---
+
+## Missing Performance Infrastructure
+
+| Infrastructure | Status | Recommendation |
+|----------------|--------|----------------|
+| **Benchmarks** | ❌ None | Add `criterion` + `benches/` for `BandwidthState`, protocol ser/de, and EC-SRP5 auth |
+| **Profiling hooks** | ❌ None | Add optional `pprof` or `dhat` dev-deps for heap profiling |
+| **Throughput regression tests** | ⚠️ Partial | Integration tests assert `tx > 0` and `rx > 0` but don't measure sustained throughput |
+| **Load tests** | ❌ None | Add a `benches/load_test.rs` that spawns 100+ concurrent tests against a local server |
+| **CI performance gates** | ❌ None | Consider a benchmark action that fails on >5% regression |
+
+---
+
+## Priority Action Plan
+
+### Phase 1: Hot-Path Fixes (1-2 days)
+1. Replace buffer scan with `memchr` or ring-buffer approach in `tcp_client_rx_loop`
+2. Cache `WCurve` in a global `LazyLock`
+3. Replace 100ms poll with `tokio::sync::Notify` in multi-conn wait
+
+### Phase 2: Scalability (2-3 days)
+4. Add SQLite connection pooling or channel-based writer in `server_pro`
+5. Cache `remaining_budget()` results for 5-10 seconds
+6. Fix FreeBSD CPU sampling to use `libc::sysctl` FFI
+
+### Phase 3: Polish & Tooling (1-2 days)
+7. Replace manual date arithmetic with `chrono`
+8. Add `criterion` benchmarks for auth and bandwidth state
+9. Fix low-severity allocation issues (CSV, syslog, hex encoding)
+
+---
+
+## Appendix: File-by-File Quick Reference
+
+| File | Lines | Hot Path? | Key Concern |
+|------|-------|-----------|-------------|
+| `src/client.rs` | 531 | ✅ Yes | O(n) 256KB scan per TCP read |
+| `src/server.rs` | 1094 | ✅ Yes | 100ms poll wait, status injection timing |
+| `src/ecsrp5.rs` | 660 | ✅ Yes (auth) | `WCurve::new()` recomputed per auth |
+| `src/bandwidth.rs` | 263 | ✅ Yes (atomics) | Well-designed; no issues |
+| `src/protocol.rs` | 214 | ✅ Yes (ser/de) | Zero-allocation; excellent |
+| `src/cpu.rs` | 215 | ⚠️ Periodic | FreeBSD `fork+exec` every second |
+| `src/server_pro/quota.rs` | 470 | ⚠️ Periodic | 15 DB queries per budget check |
+| `src/server_pro/user_db.rs` | 641 | ⚠️ All DB ops | Single mutex serializes everything |
+| `src/server_pro/server_loop.rs` | 449 | ✅ Yes | DB auth locks during connection setup |
+| `src/server_pro/enforcer.rs` | 411 | ⚠️ Periodic | String error matching allocates |
+| `src/csv_output.rs` | 86 | ❌ No | File reopen per write |
+| `src/syslog_logger.rs` | 154 | ❌ No | Global mutex + manual calendar math |
+| `src/auth.rs` | 164 | ⚠️ Auth only | Minor; double MD5 per auth |
+| `src/main.rs` | 243 | ❌ No | Entry point only |