Lazy-initialize the global reference pool to reduce its overhead when unused (#4178)

* Add benchmarks exercising the global reference count decrement pool.

* Lazy-initialize the global reference pool to reduce its overhead when unused

Cargo.toml

@@ -18,6 +18,7 @@ rust-version = "1.63"
 cfg-if = "1.0"
 libc = "0.2.62"
 memoffset = "0.9"
+once_cell = "1"
 
 # ffi bindings to the python interpreter, split into a separate crate so they can be used independently
 pyo3-ffi = { path = "pyo3-ffi", version = "=0.22.0-dev" }

newsfragments/4178.changed.md

@@ -0,0 +1 @@
+The global reference pool (to track pending reference count decrements) is now initialized lazily to avoid the overhead of taking a mutex upon function entry when the functionality is not actually used.

pyo3-benches/benches/bench_pyobject.rs

@@ -1,4 +1,12 @@
-use codspeed_criterion_compat::{criterion_group, criterion_main, Bencher, Criterion};
+use codspeed_criterion_compat::{criterion_group, criterion_main, BatchSize, Bencher, Criterion};
+
+use std::sync::{
+    atomic::{AtomicUsize, Ordering},
+    mpsc::channel,
+    Arc, Barrier,
+};
+use std::thread::spawn;
+use std::time::{Duration, Instant};
 
 use pyo3::prelude::*;
 
@@ -6,14 +14,108 @@ fn drop_many_objects(b: &mut Bencher<'_>) {
     Python::with_gil(|py| {
         b.iter(|| {
             for _ in 0..1000 {
-                std::mem::drop(py.None());
+                drop(py.None());
             }
         });
     });
 }
 
+fn drop_many_objects_without_gil(b: &mut Bencher<'_>) {
+    b.iter_batched(
+        || {
+            Python::with_gil(|py| {
+                (0..1000)
+                    .map(|_| py.None().into_py(py))
+                    .collect::<Vec<PyObject>>()
+            })
+        },
+        |objs| {
+            drop(objs);
+
+            Python::with_gil(|_py| ());
+        },
+        BatchSize::SmallInput,
+    );
+}
+
+fn drop_many_objects_multiple_threads(b: &mut Bencher<'_>) {
+    const THREADS: usize = 5;
+
+    let barrier = Arc::new(Barrier::new(1 + THREADS));
+
+    let done = Arc::new(AtomicUsize::new(0));
+
+    let sender = (0..THREADS)
+        .map(|_| {
+            let (sender, receiver) = channel();
+
+            let barrier = barrier.clone();
+
+            let done = done.clone();
+
+            spawn(move || {
+                for objs in receiver {
+                    barrier.wait();
+
+                    drop(objs);
+
+                    done.fetch_add(1, Ordering::AcqRel);
+                }
+            });
+
+            sender
+        })
+        .collect::<Vec<_>>();
+
+    b.iter_custom(|iters| {
+        let mut duration = Duration::ZERO;
+
+        let mut last_done = done.load(Ordering::Acquire);
+
+        for _ in 0..iters {
+            for sender in &sender {
+                let objs = Python::with_gil(|py| {
+                    (0..1000 / THREADS)
+                        .map(|_| py.None().into_py(py))
+                        .collect::<Vec<PyObject>>()
+                });
+
+                sender.send(objs).unwrap();
+            }
+
+            barrier.wait();
+
+            let start = Instant::now();
+
+            loop {
+                Python::with_gil(|_py| ());
+
+                let done = done.load(Ordering::Acquire);
+                if done - last_done == THREADS {
+                    last_done = done;
+                    break;
+                }
+            }
+
+            Python::with_gil(|_py| ());
+
+            duration += start.elapsed();
+        }
+
+        duration
+    });
+}
+
 fn criterion_benchmark(c: &mut Criterion) {
     c.bench_function("drop_many_objects", drop_many_objects);
+    c.bench_function(
+        "drop_many_objects_without_gil",
+        drop_many_objects_without_gil,
+    );
+    c.bench_function(
+        "drop_many_objects_multiple_threads",
+        drop_many_objects_multiple_threads,
+    );
 }
 
 criterion_group!(benches, criterion_benchmark);

src/gil.rs

@@ -5,6 +5,8 @@ use crate::impl_::not_send::{NotSend, NOT_SEND};
 #[cfg(pyo3_disable_reference_pool)]
 use crate::impl_::panic::PanicTrap;
 use crate::{ffi, Python};
+#[cfg(not(pyo3_disable_reference_pool))]
+use once_cell::sync::Lazy;
 use std::cell::Cell;
 #[cfg(all(feature = "gil-refs", debug_assertions))]
 use std::cell::RefCell;
@@ -227,7 +229,9 @@ impl GILGuard {
         let pool = mem::ManuallyDrop::new(GILPool::new());
 
         #[cfg(not(pyo3_disable_reference_pool))]
-        POOL.update_counts(Python::assume_gil_acquired());
+        if let Some(pool) = Lazy::get(&POOL) {
+            pool.update_counts(Python::assume_gil_acquired());
+        }
         GILGuard::Ensured {
             gstate,
             #[cfg(feature = "gil-refs")]
@@ -240,7 +244,9 @@ impl GILGuard {
         increment_gil_count();
         let guard = GILGuard::Assumed;
         #[cfg(not(pyo3_disable_reference_pool))]
-        POOL.update_counts(guard.python());
+        if let Some(pool) = Lazy::get(&POOL) {
+            pool.update_counts(guard.python());
+        }
         guard
     }
 
@@ -307,11 +313,14 @@ impl ReferencePool {
     }
 }
 
+#[cfg(not(pyo3_disable_reference_pool))]
+unsafe impl Send for ReferencePool {}
+
 #[cfg(not(pyo3_disable_reference_pool))]
 unsafe impl Sync for ReferencePool {}
 
 #[cfg(not(pyo3_disable_reference_pool))]
-static POOL: ReferencePool = ReferencePool::new();
+static POOL: Lazy<ReferencePool> = Lazy::new(ReferencePool::new);
 
 /// A guard which can be used to temporarily release the GIL and restore on `Drop`.
 pub(crate) struct SuspendGIL {
@@ -336,7 +345,9 @@ impl Drop for SuspendGIL {
 
             // Update counts of PyObjects / Py that were cloned or dropped while the GIL was released.
             #[cfg(not(pyo3_disable_reference_pool))]
-            POOL.update_counts(Python::assume_gil_acquired());
+            if let Some(pool) = Lazy::get(&POOL) {
+                pool.update_counts(Python::assume_gil_acquired());
+            }
         }
     }
 }
@@ -409,7 +420,9 @@ impl GILPool {
     pub unsafe fn new() -> GILPool {
         // Update counts of PyObjects / Py that have been cloned or dropped since last acquisition
         #[cfg(not(pyo3_disable_reference_pool))]
-        POOL.update_counts(Python::assume_gil_acquired());
+        if let Some(pool) = Lazy::get(&POOL) {
+            pool.update_counts(Python::assume_gil_acquired());
+        }
         GILPool {
             start: OWNED_OBJECTS
                 .try_with(|owned_objects| {