From 5499bc305826fc0401a8911026c45d7d621049a1 Mon Sep 17 00:00:00 2001
From: Nikolai Vazquez <hello@nikolaivazquez.com>
Date: Wed, 27 Nov 2024 20:32:12 -0500
Subject: [PATCH] Reduce thread pool memory usage

The array-based channel that was created for a capacity of 1 would
allocate many more kilobytes than waiting for the auxiliary thread to
accept the task.
---
 CHANGELOG.md       |  5 +++++
 src/thread_pool.rs | 21 ++++++++++++++-------
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d731156..3f8e173 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,11 @@ Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Changed
+
+- Reduced thread pool memory usage by many kilobytes by using rendezvous
+  channels instead of array-based channels.
+
 ## [0.1.16] - 2024-11-25
 
 ### Added
diff --git a/src/thread_pool.rs b/src/thread_pool.rs
index a1ff277..c607a93 100644
--- a/src/thread_pool.rs
+++ b/src/thread_pool.rs
@@ -223,14 +223,13 @@ fn spawn(additional: NonZeroUsize, threads: &mut Vec<mpsc::SyncSender<Task>>) {
     let next_thread_id = threads.len() + 1;
 
     threads.extend((next_thread_id..(next_thread_id + additional.get())).map(|thread_id| {
-        // Create single-task channel.
+        // Create single-task channel. Unless another benchmark is running, the
+        // current thread will be immediately unblocked after the auxiliary
+        // thread accepts the task.
         //
-        // If we used 0 (rendezvous channel), then the main thread would block
-        // while sending a task until the auxiliary thread accepts the task.
-        // Using a capacity of 1 allows the main thread to immediately progress
-        // onto finishing its local work, including sending the task to all
-        // remaining threads.
-        let (sender, receiver) = mpsc::sync_channel::<Task>(1);
+        // This uses a rendezvous channel (capacity 0) instead of other standard
+        // library channels because it reduces memory usage by many kilobytes.
+        let (sender, receiver) = mpsc::sync_channel::<Task>(0);
 
         let work = move || {
             // Abort the process if the caught panic error itself panics when
@@ -379,4 +378,12 @@ mod benches {
 
         bencher.bench(benched);
     }
+
+    /// Benchmarks using `ThreadPool` once.
+    #[crate::bench(crate = crate, args = aux_thread_counts(), sample_size = 1)]
+    fn broadcast_once(bencher: crate::Bencher, aux_threads: usize) {
+        bencher
+            .with_inputs(ThreadPool::new)
+            .bench_refs(|pool| pool.broadcast(aux_threads, crate::black_box_drop));
+    }
 }