Add memory overhead test for the datastore

Part of #6066

crates/re_data_store/Cargo.toml

@@ -51,9 +51,11 @@ web-time.workspace = true
 
 [dev-dependencies]
 re_types = { workspace = true, features = ["datagen", "testing"] }
+re_format.workspace = true
 
 anyhow.workspace = true
 criterion.workspace = true
+insta.workspace = true
 mimalloc.workspace = true
 rand.workspace = true
 similar-asserts.workspace = true

crates/re_data_store/tests/memory_test.rs

@@ -0,0 +1,107 @@
+//! Measures the memory overhead of the data store.
+
+use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};
+
+thread_local! {
+    static LIVE_BYTES_IN_THREAD: AtomicUsize = AtomicUsize::new(0);
+}
+
+pub struct TrackingAllocator {
+    allocator: std::alloc::System,
+}
+
+#[global_allocator]
+pub static GLOBAL_ALLOCATOR: TrackingAllocator = TrackingAllocator {
+    allocator: std::alloc::System,
+};
+
+#[allow(unsafe_code)]
+// SAFETY:
+// We just do book-keeping and then let another allocator do all the actual work.
+unsafe impl std::alloc::GlobalAlloc for TrackingAllocator {
+    #[allow(clippy::let_and_return)]
+    unsafe fn alloc(&self, layout: std::alloc::Layout) -> *mut u8 {
+        LIVE_BYTES_IN_THREAD.with(|bytes| bytes.fetch_add(layout.size(), Relaxed));
+
+        // SAFETY:
+        // Just deferring
+        unsafe { self.allocator.alloc(layout) }
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: std::alloc::Layout) {
+        LIVE_BYTES_IN_THREAD.with(|bytes| bytes.fetch_sub(layout.size(), Relaxed));
+
+        // SAFETY:
+        // Just deferring
+        unsafe { self.allocator.dealloc(ptr, layout) };
+    }
+}
+
+fn live_bytes() -> usize {
+    LIVE_BYTES_IN_THREAD.with(|bytes| bytes.load(Relaxed))
+}
+
+/// Assumes all allocations are on the calling thread.
+///
+/// The reason we use thread-local counting is so that
+/// the counting won't be confused by any other running threads (e.g. other tests).
+fn memory_use<R>(run: impl Fn() -> R) -> usize {
+    let used_bytes_start = live_bytes();
+    let ret = run();
+    let bytes_used = live_bytes() - used_bytes_start;
+    drop(ret);
+    bytes_used
+}
+
+// ----------------------------------------------------------------------------
+
+use re_data_store::{DataStore, DataStoreConfig};
+use re_log_types::{DataRow, RowId, TimePoint, TimeType, Timeline};
+use re_types::components::{InstanceKey, Scalar};
+use re_types_core::Loggable as _;
+
+/// The memory overhead of storing many scalars in the store.
+#[test]
+fn scalar_memory_overhead() {
+    re_log::setup_logging();
+
+    const NUM_SCALARS: usize = 1024 * 1024;
+
+    let total_mem_use = memory_use(|| {
+        let mut store = DataStore::new(
+            re_log_types::StoreId::random(re_log_types::StoreKind::Recording),
+            InstanceKey::name(),
+            DataStoreConfig::default(),
+        );
+
+        for i in 0..NUM_SCALARS {
+            let entity_path = re_log_types::entity_path!("scalar");
+            let timepoint =
+                TimePoint::default().with(Timeline::new("log_time", TimeType::Time), i as i64);
+            let num_instances = 1;
+            let row = DataRow::from_cells1_sized(
+                RowId::new(),
+                entity_path,
+                timepoint,
+                num_instances,
+                vec![Scalar(i as f64)],
+            )
+            .unwrap();
+            store.insert_row(&row).unwrap();
+        }
+
+        store
+    });
+
+    insta::assert_debug_snapshot!(
+        "scalars_on_one_timeline",
+        [
+            format!("{NUM_SCALARS} scalars"),
+            format!("{} in total", re_format::format_bytes(total_mem_use as _)),
+            format!(
+                "{} per row",
+                re_format::format_bytes(total_mem_use as f64 / NUM_SCALARS as f64)
+            ),
+        ]
+    );
+}

crates/re_data_store/tests/snapshots/memory_test__scalars_on_one_timeline.snap

@@ -0,0 +1,10 @@
+---
+source: crates/re_data_store/tests/memory_test.rs
+assertion_line: 93
+expression: "[format!(\"{NUM_SCALARS} scalars\"),\n        format!(\"{} in total\", re_format::format_bytes(total_mem_use as _)),\n        format!(\"{} per row\",\n            re_format::format_bytes(total_mem_use as f64 / NUM_SCALARS as\n            f64))]"
+---
+[
+    "1048576 scalars",
+    "936 MiB in total",
+    "936 B per row",
+]