Rollup merge of #154419 - zetanumbers:take-first-group, r=nnethercote

Take first task group for further execution

Continuing from https://github.com/rust-lang/rust/pull/153768#discussion_r2938828363.

I thought that storing a first group of tasks for immediate execution instead of pushing and immediately poping it from rayon's local task queue in par_slice would avoid overwhelming work stealing potentially blocking the original thread. So I've implemented this change.

8 threads benchmarks:

<table><tr><td rowspan="2">Benchmark</td><td colspan="1"><b>baseline~~9</b></th><td colspan="2"><b>new~take-first-group~1</b></th></tr><tr><td align="right">Time</td><td align="right">Time</td><td align="right">%</th></tr><tr><td>🟣 <b>hyper</b>:check</td><td align="right">0.1110s</td><td align="right">0.1086s</td><td align="right">💚  -2.13%</td></tr><tr><td>🟣 <b>hyper</b>:check:initial</td><td align="right">0.1314s</td><td align="right">0.1298s</td><td align="right">💚  -1.23%</td></tr><tr><td>🟣 <b>hyper</b>:check:unchanged</td><td align="right">0.0771s</td><td align="right">0.0755s</td><td align="right">💚  -2.14%</td></tr><tr><td>🟣 <b>clap</b>:check</td><td align="right">0.3787s</td><td align="right">0.3757s</td><td align="right"> -0.80%</td></tr><tr><td>🟣 <b>clap</b>:check:initial</td><td align="right">0.4680s</td><td align="right">0.4564s</td><td align="right">💚  -2.48%</td></tr><tr><td>🟣 <b>clap</b>:check:unchanged</td><td align="right">0.2337s</td><td align="right">0.2301s</td><td align="right">💚  -1.52%</td></tr><tr><td>🟣 <b>syn</b>:check</td><td align="right">0.4321s</td><td align="right">0.4265s</td><td align="right">💚  -1.31%</td></tr><tr><td>🟣 <b>syn</b>:check:initial</td><td align="right">0.5586s</td><td align="right">0.5401s</td><td align="right">💚  -3.31%</td></tr><tr><td>🟣 <b>syn</b>:check:unchanged</td><td align="right">0.3434s</td><td align="right">0.3429s</td><td align="right"> -0.14%</td></tr><tr><td>🟣 <b>regex</b>:check</td><td align="right">0.2755s</td><td align="right">0.2661s</td><td align="right">💚  -3.40%</td></tr><tr><td>🟣 <b>regex</b>:check:initial</td><td align="right">0.3350s</td><td align="right">0.3347s</td><td align="right"> -0.11%</td></tr><tr><td>🟣 <b>regex</b>:check:unchanged</td><td align="right">0.1851s</td><td align="right">0.1832s</td><td align="right">💚  -1.01%</td></tr><tr><td>Total</td><td align="right">3.5296s</td><td align="right">3.4695s</td><td align="right">💚  -1.70%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">0.9837s</td><td align="right">💚  -1.63%</td></tr></table>

compiler/rustc_data_structures/src/sync/parallel.rs

@@ -145,7 +145,9 @@ fn par_slice<I: DynSend>(
 
         const MAX_GROUP_COUNT: usize = 128;
         let group_size = items.len().div_ceil(MAX_GROUP_COUNT);
-        let groups = items.chunks_mut(group_size);
+        let mut groups = items.chunks_mut(group_size);
+
+        let Some(first_group) = groups.next() else { return };
 
         // Reverse the order of the later functions since Rayon executes them in reverse
         // order when using a single thread. This ensures the execution order matches
@@ -159,6 +161,11 @@ fn par_slice<I: DynSend>(
                 }
             });
         }
+
+        // Run the first function without spawning to avoid overwhelming stealing.
+        for i in first_group.iter_mut() {
+            guard.run(|| for_each(i));
+        }
     });
 }