perf(sdk): add optional foldhash for ValueMap HashMaps in metrics hot path

opentelemetry-sdk/CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## vNext
 
+- Add optional `metrics-use-foldhash` feature flag to use [foldhash](https://github.com/orlp/foldhash) for `ValueMap` HashMaps in the metrics hot path, providing ~30% improvement over the default std `SipHash` hasher. [#3388](https://github.com/open-telemetry/opentelemetry-rust/pull/3388)
 - Add 32-bit platform support by using `portable-atomic` for `AtomicI64` and `AtomicU64` in the metrics module. This enables compilation on 32-bit ARM targets (e.g., `armv5te-unknown-linux-gnueabi`, `armv7-unknown-linux-gnueabihf`).
 - `Aggregation` enum and `StreamBuilder::with_aggregation()` are now stable and no longer require the `spec_unstable_metrics_views` feature flag.
 - Fix `service.name` Resource attribute fallback to follow OpenTelemetry

opentelemetry-sdk/Cargo.toml

@@ -12,6 +12,7 @@ autobenches = false
 
 [dependencies]
 opentelemetry = { workspace = true }
+foldhash = { version = "0.1", optional = true }
 opentelemetry-http = { workspace = true, optional = true }
 futures-channel = { workspace = true }
 futures-executor = { workspace = true }
@@ -59,6 +60,7 @@ experimental_logs_batch_log_processor_with_async_runtime = ["logs", "experimenta
 experimental_logs_concurrent_log_processor = ["logs"]
 experimental_trace_batch_span_processor_with_async_runtime = ["tokio/sync", "trace", "experimental_async_runtime"]
 experimental_metrics_disable_name_validation = ["metrics"]
+metrics-use-foldhash = ["metrics", "foldhash"]
 bench_profiling = []
 
 [[bench]]

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

@@ -9,7 +9,7 @@ use core::fmt;
 #[cfg(not(target_has_atomic = "64"))]
 use portable_atomic::{AtomicI64, AtomicU64};
 use std::cmp::min;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 use std::mem::swap;
 use std::ops::{Add, AddAssign, DerefMut, Sub};
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
@@ -24,6 +24,23 @@ use opentelemetry::{otel_warn, KeyValue};
 use super::data::{AggregatedMetrics, MetricData};
 use super::pipeline::DEFAULT_CARDINALITY_LIMIT;
 
+#[cfg(feature = "metrics-use-foldhash")]
+type HashMap<K, V> = std::collections::HashMap<K, V, foldhash::fast::RandomState>;
+#[cfg(not(feature = "metrics-use-foldhash"))]
+type HashMap<K, V> = std::collections::HashMap<K, V>;
+
+#[cfg(feature = "metrics-use-foldhash")]
+fn new_hashmap<K, V>(capacity: usize) -> HashMap<K, V> {
+    std::collections::HashMap::with_capacity_and_hasher(
+        capacity,
+        foldhash::fast::RandomState::default(),
+    )
+}
+#[cfg(not(feature = "metrics-use-foldhash"))]
+fn new_hashmap<K, V>(capacity: usize) -> HashMap<K, V> {
+    std::collections::HashMap::with_capacity(capacity)
+}
+
 // TODO Replace it with LazyLock once it is stable
 pub(crate) static STREAM_OVERFLOW_ATTRIBUTES: OnceLock<Vec<KeyValue>> = OnceLock::new();
 
@@ -85,7 +102,7 @@ where
 {
     fn new(config: A::InitConfig, cardinality_limit: usize) -> Self {
         ValueMap {
-            trackers: RwLock::new(HashMap::with_capacity(
+            trackers: RwLock::new(new_hashmap(
                 1 + min(DEFAULT_CARDINALITY_LIMIT, cardinality_limit),
             )),
             trackers_for_collect: OnceLock::new(),
@@ -100,7 +117,7 @@ where
     #[inline]
     fn trackers_for_collect(&self) -> &RwLock<HashMap<Vec<KeyValue>, Arc<A>>> {
         self.trackers_for_collect.get_or_init(|| {
-            RwLock::new(HashMap::with_capacity(
+            RwLock::new(new_hashmap(
                 1 + min(DEFAULT_CARDINALITY_LIMIT, self.cardinality_limit),
             ))
         })