cli: Tweak default thread count logic

CHANGELOG.md

@@ -5,15 +5,18 @@
 - 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
 
 - Fix `NO_COLOR` support, see #1421 (@acuteenvy)
 
 ## 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)
+- Performance has been significanly improved, both due to optimizations in the underlying `ignore`
+  crate (#1429), and in `fd` itself (#1422)
+
+- The default number of threads is now more constrained. We use all available cores and hyperthreads up
+  to a maximum of 8. Past that, we use only one thread per physical core, up to a limit of 64. This
+  should improve performance and startup time on systems with many CPU cores. (#1203, #1412, #1430)
 
 ## Other
 

Cargo.toml

@@ -49,6 +49,7 @@ normpath = "1.1.1"
 crossbeam-channel = "0.5.8"
 clap_complete = {version = "4.4.4", optional = true}
 faccess = "0.2.4"
+num_cpus = "1.16.0"
 
 [dependencies.clap]
 version = "4.4.7"

src/cli.rs

@@ -715,24 +715,25 @@ impl Opts {
 fn default_num_threads() -> NonZeroUsize {
     // If we can't get the amount of parallelism for some reason, then
     // default to a single thread, because that is safe.
-    // Note that the minimum value for a NonZeroUsize is 1.
-    // Unfortunately, we can't do `NonZeroUsize::new(1).unwrap()`
-    // in a const context.
-    const FALLBACK_PARALLELISM: NonZeroUsize = NonZeroUsize::MIN;
-    // 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,
-    )
+    let fallback = NonZeroUsize::MIN;
+    // fd generally scales well up to 8 threads. Past that, it's better to limit
+    // ourselves to the number of physical cores, ignoring hyperthreads.
+    let threshold = NonZeroUsize::new(8).unwrap();
+    // To limit startup overhead on massively parallel machines, don't use more
+    // than 64 threads.
+    let limit = NonZeroUsize::new(64).unwrap();
+
+    // Get the total number of CPUs available, including hyperthreads
+    let threads = std::thread::available_parallelism().unwrap_or(fallback);
+    if threads <= threshold {
+        return threads;
+    }
+
+    // Compute min(threads, max(cores, threshold), limit). Avoid calling
+    // num_cpus::get_physical() if we don't have to, since it costs a bit of
+    // startup time to parse /proc/cpuinfo.
+    let cores = NonZeroUsize::new(num_cpus::get_physical()).unwrap();
+    threads.min(cores.clamp(threshold, limit))
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, ValueEnum)]