Merge pull request #1412 from tmccombs/limit-default-threads

Limit default threads

CHANGELOG.md

@@ -5,10 +5,14 @@
 - Breaking: `.git/` is now ignored by default when using `--hidden` / `-H`, use `--no-ignore` / `-I` or
   `--no-ignore-vcs` to override, see #1387 and #1396 (@skoriop)
 
+
 ## Bugfixes
 
 ## Changes
 
+- The default number of threads is now constrained to be at most 16. This should improve startup time on
+  systems with many CPU cores. (#1203)
+
 ## Other
 
 # v8.7.1

src/cli.rs

@@ -719,7 +719,20 @@ fn default_num_threads() -> NonZeroUsize {
     // Unfortunately, we can't do `NonZeroUsize::new(1).unwrap()`
     // in a const context.
     const FALLBACK_PARALLELISM: NonZeroUsize = NonZeroUsize::MIN;
-    std::thread::available_parallelism().unwrap_or(FALLBACK_PARALLELISM)
+    // As the number of threads increases, the startup time suffers from
+    // initializing the threads, and we get diminishing returns from additional
+    // parallelism. So set a maximum number of threads to use by default.
+    //
+    // This value is based on some empirical observations, but the ideal value
+    // probably depends on the exact hardware in use.
+    //
+    // Safety: The literal "20" is known not to be zero.
+    const MAX_DEFAULT_THREADS: NonZeroUsize = unsafe { NonZeroUsize::new_unchecked(20) };
+
+    std::cmp::min(
+        std::thread::available_parallelism().unwrap_or(FALLBACK_PARALLELISM),
+        MAX_DEFAULT_THREADS,
+    )
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, ValueEnum)]