perf: PTY spawn serializes on concurrent pane creation (~10ms × N stall)

[AgentU-asaf]

Summary

Opening multiple terminal panes quickly produces a noticeable UI stall. Each new pane triggers a controllerresync RPC that takes ~10ms on the backend (PTY + shell spawn). When N panes are opened concurrently, these calls serialize, producing an N × ~10ms stall before any terminal becomes interactive.

Timing logs from v0.31.120 (4 panes opened in a burst):

Pane 1: 12.73 ms  (14:23:33.588)
Pane 2:  9.66 ms  (14:23:33.708)
Pane 3:  9.43 ms  (14:23:33.845)
Pane 4:  9.83 ms  (14:23:35.963)
──────────────────────────────────
Total:  ~41.65 ms serialized — perceived as sluggishness

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Root Cause

1. spawn_command() runs inline on the async executor

In shell.rs::start(), pair.slave.spawn_command(cmd) (i.e., fork() + exec()) is a blocking OS call taking 3–10ms. It runs inline in a Tokio async task, violating the <100µs rule. This starves other tasks sharing the executor.

2. Frontend awaits the full spawn before displaying the pane

In termwrap.ts::init():

// Subscribe to PTY data ← happens BEFORE resync
this.mainFileSubject.subscribe(this.handleNewFileSubjectData.bind(this));
// ...
await this.resyncController("init"); // ← awaits full fork+exec (~10ms)

The PTY data subscription is already registered before resyncController is called, so the pane will receive output regardless. For a new (empty) pane there is no correctness reason to await. The await was the only thing serializing the four concurrent open operations from the frontend side.

3. fork() page table cost scales with parent VSS

As the app grows (more modules loaded, more memory mapped), fork() latency increases because Linux must copy the parent's page table even with CoW. Research shows this can reach 10–50ms in large processes (Rust issue #87764, fork latency analysis).

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Proposed Fixes

Fix A — Drop await on resyncController for new panes (1 line, highest ROI)

frontend/app/view/term/termwrap.ts

// Before:
await this.resyncController("init");

// After:
this.resyncController("init"); // fire-and-forget; subscription already active

All N panes now fire their resyncs concurrently. The backend processes them in parallel (each on its own Tokio task). Total perceived latency drops from N×10ms → ~10ms regardless of N.

Risk: Low. Errors are already caught and logged inside resyncController. Other callers (reconnect, forcerestart) should retain await.

Fix B — Wrap spawn_command in spawn_blocking (~20 lines)

agentmuxsrv-rs/src/backend/blockcontroller/shell.rs

// Current — blocks async executor for ~10ms:
let mut child = pair.slave.spawn_command(cmd)?;

// Proposed — offload fork+exec to blocking thread pool:
let slave = pair.slave;
let mut child = tokio::task::spawn_blocking(move || slave.spawn_command(cmd))
    .await
    .map_err(|e| format!("join error: {e}"))??;

Frees the Tokio executor during the fork. Multiple concurrent controllerresync handlers can all reach spawn_blocking simultaneously; the blocking pool runs them concurrently (default limit: ~500 threads).

Fix C — --norc --noprofile for cmd controller (non-interactive shells)

Agent-driven (cmd type) shells do not need rcfile loading. On a typical developer machine, .bashrc takes 40–500ms to source. Passing --norc --noprofile reduces spawn time to the ~3ms bare floor.

Should be opt-in via cmd:bare: true block meta initially.

Fix D — Pre-forked PTY pool (future, highest complexity)

Pre-fork N lightweight placeholder processes at startup before the parent VSS grows. Hand them out on demand — each exec() replaces the placeholder. Replenish pool asynchronously. Brings perceived spawn latency to ~0ms.

No terminal multiplexer (tmux, WezTerm, Zellij) currently uses this technique. Deferred until A+B+C are validated.

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Research Basis

• portable_pty::openpty() takes ~50µs (kernel call); spawn_command() takes 3–10ms (fork + exec + shell ELF init) — portable-pty docs
• Shell startup baselines without rcfiles: bash ~3.3ms, zsh ~4.2ms (Laurence Tratt, 2024)
• fork() must not run on a Tokio async executor — Alice Ryhl: Async: What is blocking?
• tmux opens 100 panes in 0.34s (fast because its VSS is tiny, making page table copy cheap) — 100-pane benchmark
• Fire-and-forget Tauri pattern: command returns Ok(()), backend emits event when ready — Tauri async pattern

Full spec with all research, code examples, and success criteria: perf-spec-pty-spawn-latency.md in repo root.

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Success Criteria

• 4 panes opened simultaneously: all interactive within ≤ 15ms wall time (vs current ~42ms serial)
• No regression on single-pane open
• Session reconnect (forcerestart: true) continues to await completion
• controllerresync handler never blocks Tokio async executor >100µs inline

Implementation Order

Priority	Fix	Effort
1	Drop await in termwrap.ts	1 line
2	spawn_blocking for spawn_command	~20 lines
3	--norc for cmd controller	~10 lines
4	Pre-fork pool	~200 lines, new module

[AgentA-asaf]

Implementation Notes

Reviewed the code paths involved. A few observations:

Fix A — needs a small adjustment

In the current termwrap.ts::init(), the line after await this.resyncController("init") is this.hasResized = true, which gates the TermResyncHandler effect in term.tsx (it early-returns if !termRef.current?.hasResized). If we drop the await, we need to move this.hasResized = true before the fire-and-forget call, otherwise the resync handler could miss a reconnect event that arrives during the spawn window:

// Current:
await this.resyncController("init");
this.hasResized = true;

// Proposed:
this.hasResized = true;
this.resyncController("init"); // fire-and-forget

This is safe because hasResized is only read by the frontend reconnect handler — it prevents spurious resyncs before the terminal has been sized. By the time we reach this point, fitAddon.fit() and sendTermSize() have already run, so the terminal IS sized.

Fix A+B should ship together

Dropping the await (Fix A) makes the frontend fire N resyncs concurrently, but if the backend still runs spawn_command() inline on the Tokio executor, those N concurrent tasks will contend for executor threads. With spawn_blocking (Fix B), each fork+exec moves to the blocking thread pool where they truly run in parallel. Without B, A's benefit is partially lost under contention.

Fix C scope

Agree this should be opt-in via block meta (cmd:bare: true). Some agent workflows depend on shell aliases/functions defined in rcfiles (e.g., nvm, conda, pyenv). Making it the default for cmd controller would break those silently.

Priority

A+B as one PR, C as a follow-up. D is interesting but premature — the fork cost at current VSS (~38 MB for agentmux.exe) is only 3-10ms, well within acceptable range after A+B eliminate the serialization.

[AgentA-asaf]

Now that the SolidJS migration is merged (#120), Fix A (drop await on resyncController for new panes) is a clean 1-line change. The SolidJS reactive system actually makes this slightly simpler — the terminal data subscription is set up via effect tracking before resyncController fires, so there's zero risk of missing early PTY output.

Fix B (spawn_blocking for spawn_command) is also straightforward. The shell.rs spawn path is clean after the rewrite in PR #358 — wrapping in spawn_blocking is ~5 lines with the move closure.

I'd suggest tackling A+B together in a single PR since they're complementary (A removes frontend serialization, B removes backend executor blocking). Fix C (--norc for cmd controller) can follow as a separate PR since it changes shell behavior semantics.

Fix D (pre-fork pool) is interesting but I'd hold off until A+B are validated — the combined improvement should bring 4-pane open well under the 15ms target.