#28239 acpmux: a chat opened on screen reads ahead of agent work

Leo: "but i also wonder how much of this is simply putting the desktop at higher priority?" / "i also like the way it just loads in the subagent stuff instantly :(" / "as in i want ours to load fast too :("

What

cmux#27587 set the daemon's disk I/O, process wide, to IOPOL_UTILITY so agent work yields to the app. That also covers the daemon's reads for a chat being opened on screen: the chat's log load and the first page of a visible _acpmux/attach. Under load those reads wait behind agent compiles, even though they are what the person is waiting on.

This adds Tier::OnScreen (Role::VisibleOpen) to the one QoS policy in qos.rs, and qos::on_screen(work). The calling thread runs at QOS_CLASS_USER_INITIATED with disk I/O at IOPOL_IMPORTANT (thread scope) for the read. Afterwards it is put back to the class and thread I/O policy it had, because tokio reuses the thread. A visible attach runs both its session_log load and its events_page read through it. Hidden attaches, _acpmux/events paging and every other caller are unchanged. ACPMUX_QOS=off turns it off with the rest of the policy. On Linux it does nothing.

Tests

• qos::tests::an_on_screen_read_raises_the_thread_and_puts_it_back (macOS): inside the guard the thread reports user-initiated and IOPOL_IMPORTANT, and afterwards it is back where it started.
• the_daemon_keeps_its_cpu_and_agents_yield now covers the new role.

Performance

metric	before	after	delta	harness
visible attach read: thread QoS / disk tier	default / utility (process)	user-initiated / important	the open's reads no longer queue behind agent I/O	unit test above
click to first content, all-core load (fleet)	pending	pending	pending	chat-open bench (cmux#27625), loaded condition

The fleet numbers will be added once the chat-open bench lands on feat-cmux-next.

Risk

Low. The raise is scoped to one synchronous read per visible open and restored in Drop. The daemon's CPU priority and every other path are unchanged.

🤖 Generated with Claude Code

Agent session: agent session

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Summary by cubic

Makes a chat opened on screen read ahead of agent work, so the log load and first page no longer queue behind agent compiles under load.

Marks the daemon's reads for a visible _acpmux/attach (its session_log load and first page) as on-screen work. The calling thread runs at user-initiated QoS with important disk I/O (thread scope) for the read, then returns to its prior class and I/O policy since tokio reuses the thread. Hidden attaches, _acpmux/events paging, and all other paths are unchanged; ACPMUX_QOS=off disables it; Linux is a no-op. macOS only.

• Adds Tier::OnScreen (Role::VisibleOpen) to the QoS policy in qos.rs and the qos::on_screen guard, with restore-on-drop.
• Adds tests covering the thread raise/restore and wires the visible attach's two reads through the guard.