Preallocate and keep memory for HashMap in Metric aggregation

opentelemetry-sdk/src/metrics/internal/aggregate.rs

@@ -12,11 +12,11 @@ use super::{
     precomputed_sum::PrecomputedSum, sum::Sum, Number,
 };
 
-const STREAM_CARDINALITY_LIMIT: u32 = 2000;
+pub(crate) const STREAM_CARDINALITY_LIMIT: usize = 2000;
 
 /// Checks whether aggregator has hit cardinality limit for metric streams
 pub(crate) fn is_under_cardinality_limit(size: usize) -> bool {
-    size < STREAM_CARDINALITY_LIMIT as usize
+    size < STREAM_CARDINALITY_LIMIT
 }
 
 /// Receives measurements to be aggregated.

opentelemetry-sdk/src/metrics/internal/mod.rs

@@ -7,12 +7,12 @@ mod sum;
 
 use core::fmt;
 use std::collections::{HashMap, HashSet};
-use std::mem::take;
+use std::mem::swap;
 use std::ops::{Add, AddAssign, DerefMut, Sub};
 use std::sync::atomic::{AtomicBool, AtomicI64, AtomicU64, AtomicUsize, Ordering};
 use std::sync::{Arc, RwLock};
 
-use aggregate::is_under_cardinality_limit;
+use aggregate::{is_under_cardinality_limit, STREAM_CARDINALITY_LIMIT};
 pub(crate) use aggregate::{AggregateBuilder, ComputeAggregation, Measure};
 pub(crate) use exponential_histogram::{EXPO_MAX_SCALE, EXPO_MIN_SCALE};
 use once_cell::sync::Lazy;
@@ -51,6 +51,11 @@ where
 {
     /// Trackers store the values associated with different attribute sets.
     trackers: RwLock<HashMap<Vec<KeyValue>, Arc<A>>>,
+
+    /// Used by collect exclusively. The data type must match the one used in
+    /// `trackers` to allow mem::swap.
+    trackers_for_collect: RwLock<HashMap<Vec<KeyValue>, Arc<A>>>,
+
     /// Number of different attribute set stored in the `trackers` map.
     count: AtomicUsize,
     /// Indicates whether a value with no attributes has been stored.
@@ -67,7 +72,10 @@ where
 {
     fn new(config: A::InitConfig) -> Self {
         ValueMap {
-            trackers: RwLock::new(HashMap::new()),
+            trackers: RwLock::new(HashMap::with_capacity(1 + STREAM_CARDINALITY_LIMIT)),
+            // TODO: For cumulative, this is not required, so avoid this
+            // pre-allocation.
+            trackers_for_collect: RwLock::new(HashMap::with_capacity(1 + STREAM_CARDINALITY_LIMIT)),
             has_no_attribute_value: AtomicBool::new(false),
             no_attribute_tracker: A::create(&config),
             count: AtomicUsize::new(0),
@@ -170,19 +178,23 @@ where
             ));
         }
 
-        let trackers = match self.trackers.write() {
-            Ok(mut trackers) => {
+        if let Ok(mut trackers_collect) = self.trackers_for_collect.write() {
+            if let Ok(mut trackers_current) = self.trackers.write() {
+                swap(trackers_collect.deref_mut(), trackers_current.deref_mut());
                 self.count.store(0, Ordering::SeqCst);
-                take(trackers.deref_mut())
+            } else {
+                otel_warn!(name: "MeterProvider.InternalError", message = "Metric collection failed. Report this issue in OpenTelemetry repo.", details ="ValueMap trackers lock poisoned");
+                return;
             }
-            Err(_) => todo!(),
-        };
 
-        let mut seen = HashSet::new();
-        for (attrs, tracker) in trackers.into_iter() {
-            if seen.insert(Arc::as_ptr(&tracker)) {
-                dest.push(map_fn(attrs, tracker.clone_and_reset(&self.config)));
+            let mut seen = HashSet::new();
+            for (attrs, tracker) in trackers_collect.drain() {
+                if seen.insert(Arc::as_ptr(&tracker)) {
+                    dest.push(map_fn(attrs, tracker.clone_and_reset(&self.config)));
+                }
             }
+        } else {
+            otel_warn!(name: "MeterProvider.InternalError", message = "Metric collection failed. Report this issue in OpenTelemetry repo.", details ="ValueMap trackers for collect lock poisoned");
         }
     }
 }