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-- Copyright 2024 The Chromium Authors
-- Use of this source code is governed by a BSD-style license that can be
-- found in the LICENSE file.
-- This file implements the scrolling predictor jank metric, as is
-- implemented in cc/metrics/predictor_jank_tracker.cc. See comments in that
-- file to get additional context on how the metric is implemented.
--
-- "Delta" here refers to how much (in pixels) the page offset changed for a
-- given frame due to a scroll.
--
-- For more details please check the following document:
-- http://doc/1Y0u0Tq5eUZff75nYUzQVw6JxmbZAW9m64pJidmnGWsY.
INCLUDE PERFETTO MODULE chrome.scroll_jank.scroll_offsets;
-- The maximum delta value between three consecutive frames, used to determine
-- whether the sequence in the scroll is fast or slow; the sequence speed is
-- used to determine whether the sequence includes any predictor jank.
CREATE PERFETTO FUNCTION _get_slow_scroll_delta_threshold()
RETURNS DOUBLE AS
SELECT
7.0;
-- The threshold for the ratio of the delta of middle frame to tbe deltas of its
-- neighbors in a sequence of three frames, if the sequence is considered
-- "slow".
CREATE PERFETTO FUNCTION _get_slow_scroll_janky_threshold()
RETURNS DOUBLE AS
SELECT
1.4;
-- The threshold for the ratio of the delta of middle frame to tbe deltas of its
-- neighbors in a sequence of three frames, if the sequence is considered
-- "fast".
CREATE PERFETTO FUNCTION _get_fast_scroll_janky_threshold()
RETURNS DOUBLE AS
SELECT
1.2;
-- Determine the acceptable threshold (see _get_slow_scroll_janky_threshold()
-- and _get_fast_scroll_janky_threshold()) based on the maximum delta value
-- between three consecutive frames.
CREATE PERFETTO FUNCTION _get_scroll_jank_threshold(
d1 DOUBLE,
d2 DOUBLE,
d3 DOUBLE
)
RETURNS DOUBLE AS
SELECT
CASE
WHEN max(max($d1, $d2), $d3) <= _get_slow_scroll_delta_threshold()
THEN _get_slow_scroll_janky_threshold()
ELSE _get_fast_scroll_janky_threshold()
END;
-- Calculate the predictor jank of three consecutive frames, if it is above the
-- threshold. Anything below the threshold is not considered jank.
CREATE PERFETTO FUNCTION _get_predictor_jank(
d1 DOUBLE,
d2 DOUBLE,
d3 DOUBLE,
threshold DOUBLE
)
RETURNS DOUBLE AS
SELECT
CASE
WHEN $d2 / max($d1, $d3) >= $threshold
THEN $d2 / max($d1, $d3) - $threshold
WHEN min($d1, $d3) / $d2 >= $threshold
THEN min($d1, $d3) / $d2 - $threshold
ELSE 0.0
END;
CREATE PERFETTO TABLE _deltas_and_neighbors AS
SELECT
scroll_id,
scroll_update_id,
ts,
delta_y,
relative_offset_y,
lag(coalesce(delta_y, 0.0)) OVER (PARTITION BY scroll_id ORDER BY ts ASC) AS prev_delta,
lead(coalesce(delta_y, 0.0)) OVER (PARTITION BY scroll_id ORDER BY ts ASC) AS next_delta
FROM chrome_presented_scroll_offsets;
CREATE PERFETTO TABLE _deltas_and_neighbors_with_threshold AS
SELECT
scroll_id,
scroll_update_id,
ts,
delta_y,
relative_offset_y,
prev_delta,
next_delta,
_get_scroll_jank_threshold(abs(prev_delta), abs(delta_y), abs(next_delta)) AS delta_threshold
FROM _deltas_and_neighbors
WHERE
NOT delta_y IS NULL AND NOT prev_delta IS NULL AND next_delta IS NOT NULL;
-- The scrolling offsets and predictor jank values for the actual (applied)
-- scroll events.
CREATE PERFETTO TABLE chrome_predictor_error (
-- An ID that ties all EventLatencies in a particular scroll. (implementation
-- note: This is the EventLatency TraceId of the GestureScrollbegin).
scroll_id LONG,
-- An ID that ties this |event_latency_id| with the Trace Id (another
-- event_latency_id) that it was presented with.
scroll_update_id LONG,
-- Presentation timestamp.
present_ts TIMESTAMP,
-- The delta in raw coordinates between this presented EventLatency and the
-- previous presented frame.
delta_y DOUBLE,
-- The pixel offset of this presented EventLatency compared to the initial
-- one.
relative_offset_y DOUBLE,
-- The delta in raw coordinates of the previous scroll update event.
prev_delta DOUBLE,
-- The delta in raw coordinates of the subsequent scroll update event.
next_delta DOUBLE,
-- The jank value based on the discrepancy between scroll predictor
-- coordinates and the actual deltas between scroll update events.
predictor_jank DOUBLE,
-- The threshold used to determine if jank occurred.
delta_threshold DOUBLE
) AS
SELECT
scroll_id,
scroll_update_id,
ts AS present_ts,
delta_y,
relative_offset_y,
prev_delta,
next_delta,
_get_predictor_jank(abs(prev_delta), abs(delta_y), abs(next_delta), delta_threshold) AS predictor_jank,
delta_threshold
FROM _deltas_and_neighbors_with_threshold;
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