Root cause of the post-tab lag: the GPUI 16 ms timer was calling
`WebSurfaceRuntime::ensure_tab` and `tick` on the UI thread, and each
call did a synchronous `serde_json` write plus `read_line` against the
Servo sidecar over stdin/stdout. With even one visible tab, every
frame stalled on cross-process IPC.
Introduce `web_surface_worker.rs` — a per-profile worker thread that
owns the `ServoLiveClient`, drains a coalescing request queue
(latest Ensure/Poll per tab wins, no unbounded growth), and ships
results back through a `std::sync::mpsc` channel. `WebSurfaceRuntime`
now submits work non-blockingly and drains responses in `tick`; the
UI thread never blocks on the sidecar.
Adjacent in-flight cleanup riding along: hardware IOSurface
rendering-context completion (sidecar `live_protocol`,
`hardware_rendering_context`, GPUI BGRA surface shader), CSS viewport
size + device pixel ratio plumbing into `ServoLiveFrame`, and the
Send opt-ins for `CVPixelBuffer`-bearing types so frames can cross
the thread boundary.
T10.1: with the vendored `HardwareOffscreenContext` (048c5df) and the
host-level kind dispatch (a7d3e89) in place, the sidecar binary
still ignored the rendering context kind — every spawn was wired to
the software path regardless of how the host process was built. This
commit threads the choice end-to-end:
* `ely_servo_sidecar` learns a `--rendering-context [software|
hardware]` flag on its `live` subcommand. `LiveArgs` carries
the parsed `RenderingContextKind` (defaulting to `Software` so
existing invocations stay bit-identical) and `live.rs::run_live`
routes it through to `SoftwareServoHost::new_with_config_dir_and_kind`.
Unknown values produce a typed
`SidecarArgsError::InvalidRenderingContext`; a missing value
after the flag produces the existing `MissingArgumentValue`.
* `ely_app` reads `ELY_SERVO_RENDERING_CONTEXT` (with values
`software` / `hardware`, case-insensitive) and, if set, appends
`--rendering-context VALUE` to the sidecar command line.
Unset or unrecognised values fall through to the sidecar's own
software default — a stale env var or a typo never breaks the
browser startup. The sidecar arg parser is the source of truth
for legality of explicit values; the env helper only gates
which values reach it.
* Five new unit tests in `args::tests` pin the new parse paths:
default-is-software, explicit-software, explicit-hardware,
bogus-value-rejected, missing-value-rejected. Run via
`cargo test -p ely_servo_host --features servo-engine --bin
ely_servo_sidecar` and now hit alongside the five existing
snapshot tests for 10 passes.
End-to-end perf expectation: with the sidecar binary built using
`--features servo-engine,hardware-render` and the env var set to
`hardware`, every spawned sidecar webview rasterises through the
real GPU adapter (via the vendored
`HardwareOffscreenContext`/surfman/CGL chain on macOS). The host
still reads back RGBA into a `Vec<u8>` for the existing pipe
protocol; the IOSurface zero-copy bridge that deletes that
read-back is T10.2–T10.5 in docs/t10-iosurface-plan.md and lands
in subsequent commits.
cargo test --bin ely_app: 120 passed, 0 failed, 2 ignored.
cargo test -p ely_servo_host --features servo-engine --bin ely_servo_sidecar: 10 passed.
cargo test -p ely_servo_host --features servo-engine --test sidecar: 9 passed.
cargo test -p ely_servo_host --features servo-engine,hardware-render --test hardware_rendering_context: 1 passed.
Every `WebSurfaceFrame::from_live_frame` today calls
`Arc::new(RenderImage::new([image::Frame::new(image_buffer)]))`
unconditionally — even when the underlying RGBA bytes are
byte-for-byte identical to the previous frame. At 60 fps on a 1080p
canvas that is roughly 960 MB/s of host-side cloning + a fresh
GPUI texture upload, and the roundtable agreed it is the next
material bottleneck after the file-system pipe (a80d039).
The contract this test pins is the cheapest invariant we can hold
against today's `SoftwareRenderingContext`: two `ServoLiveFrame`
inputs whose `rgba_bytes` are bit-identical must produce the same
underlying `Arc<RenderImage>` instance. Today they do not; the
ignored run confirms two distinct pointer values for back-to-back
identical inputs.
The fix has two recognised shapes. The interim shape lives entirely
in `WebSurfaceFrame::from_parts`: remember the previous frame's
bytes (hash or pointer-eq) and reuse the existing `Arc<RenderImage>`
when they match. The endgame shape removes the host-side image step
entirely — `OffscreenRenderingContext` + IOSurface — at which point
the assertion becomes meaningless and is replaced by a frame-time
budget. Whichever lands first, the fix commit MUST delete the
`#[ignore]` attribute outright; toggling its reason is a broken
contract.
To call `WebSurfaceFrame::from_live_frame` from a unit test without
spawning a real sidecar process, `ServoLiveFrame` gains a
`#[cfg(test)] pub(crate) fn for_test(...)` constructor that wraps
the existing private `from_parts` with realistic defaults. No
production path uses it.
cargo test --bin ely_app: 112 passed, 0 failed, 2 ignored.
cargo test --bin ely_app -- --ignored: 2 failed (expected RED:
T7 click pipeline + T10 texture re-upload).
Every live frame was round-tripping through the local file system:
the sidecar called `fs::write(rgba_out, frame.rgba_bytes())` in
`poll_frame`, the JSON response carried `rgba_path`, and the main
process turned around and called `fs::read(rgba_path)` to lift the
bytes back into a `Vec<u8>`. At 1080p that is 8 MB of syscall +
memcpy + page cache traffic per frame; at 60 fps it dwarfs every
other cost in the pipeline and shows up as scroll/zoom jank the user
can feel before any other bottleneck.
Replace it with a same-pipe binary protocol. The sidecar writes the
JSON `LiveResponse` line as before, then writes the raw RGBA frame
bytes on the same stdout immediately after the trailing `\n`. The
client `read_line`s the JSON, parses `rgba_byte_count` from the
header, and `read_exact`s exactly that many bytes from the same
`BufReader<ChildStdout>` (the buffered reader drains its own buffer
before pulling from the child). No tmpfs directory, no
`fs::remove_dir_all` on drop, no `rgba_path` field, no per-frame
filename plumbing.
Boundary defence on the client side: the header's `rgba_byte_count`
is cross-checked against `width * height * 4` before any allocation
or `read_exact`. A buggy or compromised sidecar can no longer ask
the GPUI process to allocate an arbitrarily large buffer or park on
`read_exact` for a payload that will never arrive.
The sidecar process boundary stays exactly where it was; only the
pixel transport between the two processes changes. The `one-shot`
sidecar binary path (used by `tests/sidecar.rs` and PRD smoke tests)
still writes to its CLI-supplied `--rgba-out` path — those tests
were untouched and continue to pass 9/9.
cargo test --bin ely_app: 112 passed.
cargo test -p ely_servo_host --features servo-engine --test sidecar: 9 passed.
Follow-ups deferred to the profile step (T9):
* the host still copies its rendered buffer to a transient
`Vec<u8>` via `frame.rgba_bytes().to_vec()` before write_all;
expose `&[u8]` straight to `write_all` once the profile shows
that allocation in the top five.
* `poll_frame` calls `snapshot` twice per iteration; harmless
under the software renderer but worth folding into a single
snapshot once we have numbers.
* raw memcpy bandwidth is still ~480 MB/s at 60 fps 1080p; the
GPU-side fix (T10: OffscreenRenderingContext + IOSurface
zero-copy) is the next material change.
Without continuous MouseMove events, Servo never updates hover state:
no cursor changes over links, no :hover CSS effects, no mouseenter
JavaScript events. The web page appears completely non-interactive.
Track mouse position from GPUI on_mouse_move through the full IPC
pipeline to Servo. Hover position is included with each ensure
request so Servo updates hover state at frame rate (~60fps).
ZacharyZhang-NY