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perf(tui): instrument stdout drain — rule out terminal parse bottleneck

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Adds four fields to FrameEvent.phases and the matching profile
summary:

  optimizedPatches  post-optimize patch count (what's actually
                    written to stdout; the .patches field is
                    pre-optimize)
  writeBytes        UTF-8 byte count of the write this frame
  backpressure      true when Node's stdout.write returned false
                    (Writable buffer full — outer terminal can't
                    keep up)
  prevFrameDrainMs  end-to-end drain time of the PREVIOUS frame's
                    write, captured from stdout.write's 2-arg
                    callback.  Reported on the next frame so the
                    measurement reflects "time until OS flushed
                    the bytes to the terminal fd", not "time until
                    queued in Node".

writeDiffToTerminal() now returns { bytes, backpressure } and
accepts an optional onDrain callback.  Only attached on TTY with
diff; piped/non-TTY stdout bypasses flow control so the callback
would fire synchronously anyway.

Initial measurements under hold-wheel_up against 1106-msg session
(30Hz for 6s):

  patches total    28,888
  optimized total  16,700   (ratio 0.58 — optimizer cuts ~42%)
  writeBytes       42 KB / 10s = 4.2 KB/s throughput
  drainMs p50      0.14 ms   terminal accepts bytes instantly
  drainMs p99      0.85 ms
  backpressure     0% of frames

This rules out the terminal-parse hypothesis — Cursor's xterm.js
drains our output in sub-millisecond time at only 4 KB/s.  The
remaining lag has to be in the render pipeline, not the wire.
Profile output now includes the bytes+drain+backpressure lines to
keep this visible on every subsequent iteration.
This commit is contained in:
Brooklyn Nicholson 2026-04-26 17:06:22 -05:00
parent d3dedf10aa
commit f823535db2
6 changed files with 126 additions and 4 deletions
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11
ui-tui/packages/hermes-ink/src/ink/frame.ts
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@ -46,6 +46,17 @@ export type FrameEvent = {
write: number
/** Pre-optimize patch count (proxy for how much changed this frame) */
patches: number
/** Post-optimize patch count — what was actually written to stdout. */
optimizedPatches: number
/** Bytes written to stdout this frame (escape sequences + payload). */
writeBytes: number
/** Whether stdout.write returned false (backpressure = outer terminal slow). */
backpressure: boolean
/** ms from this frame's stdout.write until the write-callback fired.
* Populated on the NEXT frame (async), so this field reflects the
* PREVIOUS frame's terminal-drain time. 0 = callback already fired
* before next frame started (drained in sub-ms). */
prevFrameDrainMs: number
/** yoga calculateLayout() time (runs in resetAfterCommit, before onRender) */
yoga: number
/** React reconcile time: scrollMutated → resetAfterCommit. 0 if no commit. */