Iris asked to look into incremental text rendering, hoping parley
supported it. It does not, by design: a `Layout` re-linebreaks and
re-aligns freely but "if the text content or the styles applied to that
content change then a new `Layout` must be created", its LRU cache holds
harfrust's per-font shaper data rather than shaped runs, and its own
`PlainEditor` rebuilds the whole layout from the whole buffer on every
keystroke.
The app already does what incremental layout would buy: `RowBlocks::
apply_delta` keeps one `TextEdit` per markdown block and re-shapes only
the one a delta landed in. Re-splitting the markdown to find it is 18µs
at 18,000 characters; comparing the blocks is 470ns.
What is left is one `TextBuffer::shape` of that block, linear in its
length at ~0.23ms per 1,000 characters here -- and the bench fixture's
streamed message is 14,888 characters in a *single* block, a run-on
paragraph with no blank line in it, so every delta reshapes all of it.
That is 3.5ms of the measured 3.86ms frame.
Real replies are not that: across 7,706 top-level blocks from 3,675 real
assistant messages on this machine (lengths only, no content copied
anywhere), p50 147 characters, p90 449, p99 836, largest 1,580, nothing
above 4,000; code fences p50 126, largest 589. At those sizes a reshape
is 48µs to 372µs here, roughly 0.12-0.93ms on the phone -- inside a
120Hz budget with no incremental anything.
So the recommendation is not to build it, and to give the fixture's
streamed message the paragraph structure a real reply has instead. Three
runs added to `frame_profile.rs` so none of this is re-derived: what
reshaping a growing message costs (including at the sizes real replies
reach), where a delta's cost is, and what the fixture actually streams.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Iris's phone came back "now THAT is smooth", and reading that run against
the bench's own timings found three things the report was getting wrong
-- two of them shipped yesterday in the fix for the last three.
`missed vsyncs` counted idleness. Every gap between frames was treated as
cadence, so the bench's own pauses read as stutter: 276 for sixteen 300ms
rests between flings, 2410 for twelve hundred 50ms keystroke gaps, 821
for four hundred 50ms stream gaps -- each within a few percent of the
arithmetic. A gap now measures anything only if the frame before it had
asked for another one.
`late` counted the swapchain wait as cost. A well-paced loop spends each
frame blocked in the acquire, so its total sits at exactly one refresh
period and every frame lands on the budget boundary -- 0.4ms of work and
5.7ms of waiting is not a late frame. It is judged on `FrameParts::work`.
And the refresh rate is the larger of what the platform claims and what
the run sustained, because each can only be wrong one way.
`Display.getRefreshRate()` answered 60 for a run that drew 3405 frames in
33.1s, since a phone that varies its rate answers with whatever mode it
is in when asked. The first attempt at measuring it instead took the
fastest tenth of the gaps and reported 88Hz for this repo's 60Hz
emulator, whose app manages 54 -- a budget no frame there could meet,
invented out of the app's best moments, and caught only by running the
corrected report on the emulator before shipping it. A sustained rate is
a floor and cannot do that. Both are printed when they disagree.
Also corrected in the docs: "103fps on a 120Hz screen" divided the fling
phase by its whole duration, rests included. Both runs sustained ~120.3fps
through the motion, so the callback ordering was never costing frames --
what changed is the clock, which moves no frame count at all, which is
exactly why nothing in a report could show it.
`fling_profile.rs` is `frame_profile.rs` and gained a stream run, which
says where the frame time now is: folding an arriving event is 0.35ms and
applying the diff 0.41ms, while the frame is 3.86ms here and 9.5ms on the
phone. 401 events move the item count 652 -> 654, so nearly every one is
a delta into the same row -- the cost is re-shaping one growing message,
not `fold_event`'s per-event clone, which was the hypothesis and is what
measuring it ruled out.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
irisandClaude Opus 5