# HG changeset patch
# User Raphaël Gomès <rgomes@octobus.net>
# Date 2022-03-18 15:15:44
# Node ID e2f8ed37201ce2be0b11721898a45964f75f0963
# Parent  4a8eff64860a4ed68021b454fa66a246f0261665

rust-status: cap the number of concurrent threads to 16

During benchmarking it was determined that the use of more threads is very
advantageous... until we use more than 16. This is most likely due to some
resource contention (thrashing, etc.). Until we have time to figure out and
fix the underlying cause, let's just cap at 16 threads.

Differential Revision: https://phab.mercurial-scm.org/D12384

diff --git a/rust/hg-core/src/dirstate_tree/status.rs b/rust/hg-core/src/dirstate_tree/status.rs
--- a/rust/hg-core/src/dirstate_tree/status.rs
+++ b/rust/hg-core/src/dirstate_tree/status.rs
@@ -47,6 +47,17 @@ pub fn status<'tree, 'on_disk: 'tree>(
     ignore_files: Vec<PathBuf>,
     options: StatusOptions,
 ) -> Result<(DirstateStatus<'on_disk>, Vec<PatternFileWarning>), StatusError> {
+    // Force the global rayon threadpool to not exceed 16 concurrent threads.
+    // This is a stop-gap measure until we figure out why using more than 16
+    // threads makes `status` slower for each additional thread.
+    // We use `ok()` in case the global threadpool has already been
+    // instantiated in `rhg` or some other caller.
+    // TODO find the underlying cause and fix it, then remove this.
+    rayon::ThreadPoolBuilder::new()
+        .num_threads(16)
+        .build_global()
+        .ok();
+
     let (ignore_fn, warnings, patterns_changed): (IgnoreFnType, _, _) =
         if options.list_ignored || options.list_unknown {
             let mut hasher = Sha1::new();