subscriber: make Registry::enter/exit much faster

tracing-subscriber/Cargo.toml

@@ -83,3 +83,7 @@ harness = false
 [[bench]]
 name = "fmt"
 harness = false
+
+[[bench]]
+name = "enter"
+harness = false

tracing-subscriber/benches/enter.rs

@@ -0,0 +1,64 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+use tracing_subscriber::prelude::*;
+
+fn enter(c: &mut Criterion) {
+    let mut group = c.benchmark_group("enter");
+    let _subscriber = tracing_subscriber::fmt()
+        .with_max_level(tracing::Level::INFO)
+        .finish()
+        .set_default();
+    group.bench_function("enabled", |b| {
+        let span = tracing::info_span!("foo");
+        b.iter_with_large_drop(|| span.enter())
+    });
+    group.bench_function("disabled", |b| {
+        let span = tracing::debug_span!("foo");
+        b.iter_with_large_drop(|| span.enter())
+    });
+}
+
+fn enter_exit(c: &mut Criterion) {
+    let mut group = c.benchmark_group("enter_exit");
+    let _subscriber = tracing_subscriber::fmt()
+        .with_max_level(tracing::Level::INFO)
+        .finish()
+        .set_default();
+    group.bench_function("enabled", |b| {
+        let span = tracing::info_span!("foo");
+        b.iter(|| span.enter())
+    });
+    group.bench_function("disabled", |b| {
+        let span = tracing::debug_span!("foo");
+        b.iter(|| span.enter())
+    });
+}
+
+fn enter_many(c: &mut Criterion) {
+    let mut group = c.benchmark_group("enter_many");
+    let _subscriber = tracing_subscriber::fmt()
+        .with_max_level(tracing::Level::INFO)
+        .finish()
+        .set_default();
+    group.bench_function("enabled", |b| {
+        let span1 = tracing::info_span!("span1");
+        let _e1 = span1.enter();
+        let span2 = tracing::info_span!("span2");
+        let _e2 = span2.enter();
+        let span3 = tracing::info_span!("span3");
+        let _e3 = span3.enter();
+        let span = tracing::info_span!("foo");
+        b.iter_with_large_drop(|| span.enter())
+    });
+    group.bench_function("disabled", |b| {
+        let span1 = tracing::info_span!("span1");
+        let _e1 = span1.enter();
+        let span2 = tracing::info_span!("span2");
+        let _e2 = span2.enter();
+        let span3 = tracing::info_span!("span3");
+        let _e3 = span3.enter();
+        let span = tracing::debug_span!("foo");
+        b.iter_with_large_drop(|| span.enter())
+    });
+}
+criterion_group!(benches, enter, enter_exit, enter_many);
+criterion_main!(benches);

tracing-subscriber/src/registry/sharded.rs

@@ -202,19 +202,21 @@ impl Subscriber for Registry {
     fn event(&self, _: &Event<'_>) {}
 
     fn enter(&self, id: &span::Id) {
-        self.current_spans
+        if self
+            .current_spans
             .get_or_default()
             .borrow_mut()
-            .push(self.clone_span(id));
+            .push(id.clone())
+        {
+            self.clone_span(id);
+        }
     }
 
     fn exit(&self, id: &span::Id) {
-        if let Some(id) = self
-            .current_spans
-            .get()
-            .and_then(|spans| spans.borrow_mut().pop(id))
-        {
-            dispatcher::get_default(|dispatch| dispatch.try_close(id.clone()));
+        if let Some(spans) = self.current_spans.get() {
+            if spans.borrow_mut().pop(id) {
+                dispatcher::get_default(|dispatch| dispatch.try_close(id.clone()));
+            }
         }
     }