GC improvements 5: Store{Diff,Event} optimizations (#4399)

Optimize the creation of `StoreDiff`s and `StoreEvent`s, which turns out
to be a major cost in time series use cases, when it is common to
generate several millions of those on any single GC run.

Once again some pretty significant wins.

### Benchmarks

Compared to `main`:
```
group                                                     gc_improvements_0                       gc_improvements_5
-----                                                     -----------------                       -----------------
.../plotting_dashboard/drop_at_least=0.3/bucketsz=1024    13.00  1084.0±4.47ms 54.1 KElem/sec     1.00     83.4±1.16ms 702.9 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=2048    25.37      2.1±0.02s 27.6 KElem/sec     1.00     83.7±0.61ms 700.0 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=256     5.55    465.8±2.50ms 125.8 KElem/sec    1.00     84.0±0.50ms 697.8 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=512     7.94    655.3±2.61ms 89.4 KElem/sec     1.00     82.5±1.33ms 710.0 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/default          8.02    652.8±4.12ms 89.8 KElem/sec     1.00     81.4±0.94ms 720.0 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=1024         35.87      2.4±0.05s 24.2 KElem/sec     1.00     67.5±2.21ms 867.5 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=2048         35.91      2.4±0.03s 24.1 KElem/sec     1.00     67.8±1.86ms 863.9 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=256          37.02      2.5±0.08s 23.5 KElem/sec     1.00     67.5±1.43ms 868.2 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=512          35.47      2.4±0.02s 24.5 KElem/sec     1.00     67.4±1.40ms 869.4 KElem/sec
.../timeless_logs/drop_at_least=0.3/default               36.00      2.4±0.03s 24.4 KElem/sec     1.00     66.8±0.85ms 877.3 KElem/sec
```

Compared to previous PR:
```
group                                                     gc_improvements_4                       gc_improvements_5
-----                                                     -----------------                       -----------------
.../plotting_dashboard/drop_at_least=0.3/bucketsz=1024    1.26    105.0±0.91ms 558.1 KElem/sec    1.00     83.4±1.16ms 702.9 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=2048    1.28    107.3±0.83ms 546.2 KElem/sec    1.00     83.7±0.61ms 700.0 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=256     1.27    106.3±0.74ms 551.3 KElem/sec    1.00     84.0±0.50ms 697.8 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/bucketsz=512     1.29    106.4±0.94ms 550.6 KElem/sec    1.00     82.5±1.33ms 710.0 KElem/sec
.../plotting_dashboard/drop_at_least=0.3/default          1.26    102.9±0.75ms 569.4 KElem/sec    1.00     81.4±0.94ms 720.0 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=1024         1.00     65.3±0.81ms 897.6 KElem/sec    1.03     67.5±2.21ms 867.5 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=2048         1.00     64.9±1.07ms 903.2 KElem/sec    1.05     67.8±1.86ms 863.9 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=256          1.00     64.4±0.99ms 910.2 KElem/sec    1.05     67.5±1.43ms 868.2 KElem/sec
.../timeless_logs/drop_at_least=0.3/bucketsz=512          1.00     64.6±1.08ms 906.9 KElem/sec    1.04     67.4±1.40ms 869.4 KElem/sec
.../timeless_logs/drop_at_least=0.3/default               1.00     65.3±1.29ms 897.3 KElem/sec    1.02     66.8±0.85ms 877.3 KElem/sec
```

---

Part of the GC improvements series:
- #4394
- #4395
- #4396
- #4397
- #4398
- #4399
- #4400
- #4401

crates/re_arrow_store/src/store_event.rs

@@ -112,11 +112,17 @@ pub struct StoreDiff {
     /// one addition and (optionally) one deletion (in that order!).
     pub row_id: RowId,
 
-    /// The [`TimePoint`] associated with that row.
+    /// The time data associated with that row.
     ///
     /// Since insertions and deletions both work on a row-level basis, this is guaranteed to be the
     /// same value for both the insertion and deletion events (if any).
-    pub timepoint: TimePoint,
+    ///
+    /// This is not a [`TimePoint`] for performance reasons.
+    //
+    // NOTE: Empirical testing shows that a SmallVec isn't any better in the best case, and can be a
+    // significant performant drop at worst.
+    // pub times: SmallVec<[(Timeline, TimeInt); 5]>, // "5 timelines ought to be enough for anyone"
+    pub times: Vec<(Timeline, TimeInt)>,
 
     /// The [`EntityPath`] associated with that row.
     ///
@@ -137,7 +143,7 @@ impl StoreDiff {
         Self {
             kind: StoreDiffKind::Addition,
             row_id: row_id.into(),
-            timepoint: TimePoint::timeless(),
+            times: Default::default(),
             entity_path: entity_path.into(),
             cells: Default::default(),
         }
@@ -148,75 +154,43 @@ impl StoreDiff {
         Self {
             kind: StoreDiffKind::Deletion,
             row_id: row_id.into(),
-            timepoint: TimePoint::timeless(),
+            times: Default::default(),
             entity_path: entity_path.into(),
             cells: Default::default(),
         }
     }
 
     #[inline]
-    pub fn at_timepoint(mut self, timepoint: impl Into<TimePoint>) -> StoreDiff {
-        self.timepoint = self.timepoint.union_max(&timepoint.into());
+    pub fn at_timepoint(&mut self, timepoint: impl Into<TimePoint>) -> &mut Self {
+        self.times.extend(timepoint.into());
         self
     }
 
     #[inline]
     pub fn at_timestamp(
-        mut self,
+        &mut self,
         timeline: impl Into<Timeline>,
         time: impl Into<TimeInt>,
-    ) -> StoreDiff {
-        self.timepoint.insert(timeline.into(), time.into());
+    ) -> &mut Self {
+        self.times.push((timeline.into(), time.into()));
         self
     }
 
     #[inline]
-    pub fn with_cells(mut self, cells: impl IntoIterator<Item = DataCell>) -> Self {
+    pub fn with_cells(&mut self, cells: impl IntoIterator<Item = DataCell>) -> &mut Self {
         self.cells
             .extend(cells.into_iter().map(|cell| (cell.component_name(), cell)));
         self
     }
 
-    /// Returns the union of two [`StoreDiff`]s.
-    ///
-    /// They must share the same [`RowId`], [`EntityPath`] and [`StoreDiffKind`].
     #[inline]
-    pub fn union(&self, rhs: &Self) -> Option<Self> {
-        let Self {
-            kind: lhs_kind,
-            row_id: lhs_row_id,
-            timepoint: lhs_timepoint,
-            entity_path: lhs_entity_path,
-            cells: lhs_cells,
-        } = self;
-        let Self {
-            kind: rhs_kind,
-            row_id: rhs_row_id,
-            timepoint: rhs_timepoint,
-            entity_path: rhs_entity_path,
-            cells: rhs_cells,
-        } = rhs;
-
-        let same_kind = lhs_kind == rhs_kind;
-        let same_row_id = lhs_row_id == rhs_row_id;
-        let same_entity_path = lhs_entity_path == rhs_entity_path;
-
-        (same_kind && same_row_id && same_entity_path).then(|| Self {
-            kind: *lhs_kind,
-            row_id: *lhs_row_id,
-            timepoint: lhs_timepoint.clone().union_max(rhs_timepoint),
-            entity_path: lhs_entity_path.clone(),
-            cells: [lhs_cells.values(), rhs_cells.values()]
-                .into_iter()
-                .flatten()
-                .map(|cell| (cell.component_name(), cell.clone()))
-                .collect(),
-        })
+    pub fn timepoint(&self) -> TimePoint {
+        self.times.clone().into_iter().collect()
     }
 
     #[inline]
     pub fn is_timeless(&self) -> bool {
-        self.timepoint.is_timeless()
+        self.times.is_empty()
     }
 
     /// `-1` for deletions, `+1` for additions.
@@ -297,7 +271,7 @@ mod tests {
                 if event.is_timeless() {
                     self.timeless += delta;
                 } else {
-                    for (&timeline, &time) in &event.timepoint {
+                    for &(timeline, time) in &event.times {
                         *self.timelines.entry(timeline).or_default() += delta;
                         *self.times.entry(time).or_default() += delta;
                     }

crates/re_arrow_store/src/store_gc.rs

@@ -240,7 +240,7 @@ impl DataStore {
                         table.try_drop_row(&self.cluster_cell_cache, row_id, time.as_i64());
                     if let Some(inner) = diff.as_mut() {
                         if let Some(removed) = removed {
-                            diff = inner.union(&removed);
+                            inner.times.extend(removed.times);
                         }
                     } else {
                         diff = removed;
@@ -258,7 +258,7 @@ impl DataStore {
                         table.try_drop_row(&self.cluster_cell_cache, row_id);
                     if let Some(inner) = diff.as_mut() {
                         if let Some(removed) = removed {
-                            diff = inner.union(&removed);
+                            inner.times.extend(removed.times);
                         }
                     } else {
                         diff = removed;
@@ -476,7 +476,7 @@ impl DataStore {
                         .entry(row_id)
                         .or_insert_with(|| StoreDiff::deletion(row_id, entity_path.clone()));
 
-                    diff.timepoint.insert(bucket.timeline, time.into());
+                    diff.times.push((bucket.timeline, time.into()));
 
                     for column in &mut inner.columns.values_mut() {
                         let cell = column[i].take();
@@ -657,10 +657,9 @@ impl IndexedBucketInner {
                     if let Some(inner) = diff.as_mut() {
                         inner.cells.insert(cell.component_name(), cell);
                     } else {
-                        diff = StoreDiff::deletion(removed_row_id, ent_path.clone())
-                            .at_timestamp(timeline, time)
-                            .with_cells([cell])
-                            .into();
+                        let mut d = StoreDiff::deletion(removed_row_id, ent_path.clone());
+                        d.at_timestamp(timeline, time).with_cells([cell]);
+                        diff = Some(d);
                     }
                 }
             }
@@ -752,9 +751,9 @@ impl PersistentIndexedTable {
                     if let Some(inner) = diff.as_mut() {
                         inner.cells.insert(cell.component_name(), cell);
                     } else {
-                        diff = StoreDiff::deletion(removed_row_id, ent_path.clone())
-                            .with_cells([cell])
-                            .into();
+                        let mut d = StoreDiff::deletion(removed_row_id, ent_path.clone());
+                        d.cells.insert(cell.component_name(), cell);
+                        diff = Some(d);
                     }
                 }
             }

crates/re_arrow_store/src/store_write.rs

@@ -189,8 +189,8 @@ impl DataStore {
             }
         }
 
-        let diff = StoreDiff::addition(*row_id, entity_path.clone())
-            .at_timepoint(timepoint.clone())
+        let mut diff = StoreDiff::addition(*row_id, entity_path.clone());
+        diff.at_timepoint(timepoint.clone())
             .with_cells(cells.iter().cloned());
 
         // TODO(#4220): should we fire for auto-generated data?

crates/re_data_store/src/entity_tree.rs

@@ -105,7 +105,7 @@ impl CompactedStoreEvents {
                         event.delta().unsigned_abs();
                 }
             } else {
-                for (&timeline, &time) in &event.timepoint {
+                for &(timeline, time) in &event.times {
                     let per_timeline = this.timeful.entry(event.entity_path.hash()).or_default();
                     let per_component = per_timeline.entry(timeline).or_default();
                     for component_name in event.cells.keys() {
@@ -205,7 +205,7 @@ impl EntityTree {
             let leaf = self.create_subtrees_recursively(
                 event.diff.entity_path.as_slice(),
                 0,
-                &event.diff.timepoint,
+                &event.diff.times,
                 event.num_components() as _,
             );
 
@@ -235,7 +235,7 @@ impl EntityTree {
                     pending_clears = self.flat_clears.clone().into_iter().collect_vec();
                     Default::default()
                 });
-            per_component.add(&store_diff.timepoint, 1);
+            per_component.add(&store_diff.times, 1);
 
             // Is the newly added component under the influence of previously logged `Clear`
             // component?
@@ -343,7 +343,7 @@ impl EntityTree {
                     next,
                     is_recursive,
                     store_diff.row_id,
-                    store_diff.timepoint.clone(),
+                    store_diff.timepoint(),
                 ));
                 stack.extend(next.children.values_mut().collect::<Vec<&mut Self>>());
             }
@@ -352,7 +352,7 @@ impl EntityTree {
                 self,
                 is_recursive,
                 store_diff.row_id,
-                store_diff.timepoint.clone(),
+                store_diff.timepoint(),
             ));
         }
 
@@ -387,7 +387,7 @@ impl EntityTree {
                 .entry(component_path.entity_path().clone())
                 .or_default();
 
-            *timepoint = store_diff.timepoint.clone().union_max(timepoint);
+            *timepoint = store_diff.timepoint().union_max(timepoint);
             component_paths.insert(component_path.clone());
         }
     }
@@ -433,7 +433,7 @@ impl EntityTree {
         for event in filtered_events.iter().filter(|e| &e.entity_path == path) {
             for component_name in event.cells.keys() {
                 if let Some(histo) = self.time_histograms_per_component.get_mut(component_name) {
-                    histo.remove(&event.timepoint, 1);
+                    histo.remove(&event.timepoint(), 1);
                     if histo.is_empty() {
                         self.time_histograms_per_component.remove(component_name);
                     }
@@ -442,7 +442,7 @@ impl EntityTree {
         }
 
         for event in &filtered_events {
-            recursive_time_histogram.remove(&event.timepoint, event.num_components() as _);
+            recursive_time_histogram.remove(&event.timepoint(), event.num_components() as _);
         }
 
         children.retain(|_, child| {
@@ -458,10 +458,10 @@ impl EntityTree {
         &mut self,
         full_path: &[EntityPathPart],
         depth: usize,
-        timepoint: &TimePoint,
+        times: &[(Timeline, TimeInt)],
         num_components: u32,
     ) -> &mut Self {
-        self.recursive_time_histogram.add(timepoint, num_components);
+        self.recursive_time_histogram.add(times, num_components);
 
         match full_path.get(depth) {
             None => {
@@ -473,7 +473,7 @@ impl EntityTree {
                 .or_insert_with(|| {
                     EntityTree::new(full_path[..depth + 1].into(), self.recursive_clears.clone())
                 })
-                .create_subtrees_recursively(full_path, depth + 1, timepoint, num_components),
+                .create_subtrees_recursively(full_path, depth + 1, times, num_components),
         }
     }
 

crates/re_data_store/src/time_histogram_per_timeline.rs

@@ -1,7 +1,7 @@
 use std::collections::BTreeMap;
 
 use re_arrow_store::{StoreEvent, StoreSubscriber};
-use re_log_types::{TimePoint, Timeline};
+use re_log_types::{TimeInt, TimePoint, Timeline};
 
 // ---
 
@@ -51,8 +51,8 @@ impl TimeHistogramPerTimeline {
         self.num_timeless_messages
     }
 
-    pub fn add(&mut self, timepoint: &TimePoint, n: u32) {
-        if timepoint.is_timeless() {
+    pub fn add(&mut self, times: &[(Timeline, TimeInt)], n: u32) {
+        if times.is_empty() {
             self.num_timeless_messages = self
                 .num_timeless_messages
                 .checked_add(n as u64)
@@ -61,11 +61,11 @@ impl TimeHistogramPerTimeline {
                     u64::MAX
                 });
         } else {
-            for (timeline, time_value) in timepoint.iter() {
+            for &(timeline, time) in times {
                 self.times
-                    .entry(*timeline)
+                    .entry(timeline)
                     .or_default()
-                    .increment(time_value.as_i64(), n);
+                    .increment(time.as_i64(), n);
             }
         }
     }

crates/re_data_store/src/times_per_timeline.rs

@@ -54,7 +54,7 @@ impl StoreSubscriber for TimesPerTimeline {
         re_tracing::profile_function!(format!("num_events={}", events.len()));
 
         for event in events {
-            for (&timeline, &time) in &event.timepoint {
+            for &(timeline, time) in &event.times {
                 let per_time = self.0.entry(timeline).or_default();
                 let count = per_time.entry(time).or_default();
 

examples/rust/custom_store_subscriber/src/main.rs

@@ -161,7 +161,7 @@ impl StoreSubscriber for TimeRangesPerEntity {
 
     fn on_events(&mut self, events: &[StoreEvent]) {
         for event in events {
-            for (&timeline, &time) in &event.timepoint {
+            for &(timeline, time) in &event.times {
                 // update counters
                 let per_timeline = self.times.entry(event.entity_path.clone()).or_default();
                 let per_time = per_timeline.entry(timeline).or_default();