From: Pádraig Brady <P@draigBrady.com>
Date: Mon, 10 Nov 2025 23:17:24 +0000 (+0000)
Subject: sort: avoid pthread calls with --parallel=1
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e8dcdb993202fb76a97424abd45b1ccc83e9153d;p=thirdparty%2Fcoreutils.git

sort: avoid pthread calls with --parallel=1

Before this change there were the following unneeded pthread calls:

  $ seq 1e6 | ltrace -c -e 'pthread*' sort --parallel=1 | wc -l
  % time     seconds  usecs/call     calls      function
  ------ ----------- ----------- --------- --------------------
   39.13    0.031757          67       468 pthread_mutex_lock
   37.96    0.030811          65       468 pthread_mutex_unlock
   13.17    0.010691          65       162 pthread_cond_signal
    2.15    0.001747          64        27 pthread_mutex_destroy
    2.00    0.001620          60        27 pthread_mutex_init
    0.70    0.000565          62         9 pthread_cond_destroy
    0.64    0.000518          57         9 pthread_cond_init
  ------ ----------- ----------- --------- --------------------
  100.00    0.081159                  1170 total

* src/sort.c (sort): Avoid merge tree overhead when single threaded.
---

diff --git a/src/sort.c b/src/sort.c
index 05d00cc119..ed16c22172 100644
--- a/src/sort.c
+++ b/src/sort.c
@@ -4189,16 +4189,26 @@ sort (char *const *files, size_t nfiles, char const *output_file,
             }
           if (1 < buf.nlines)
             {
-              struct merge_node_queue queue;
-              queue_init (&queue, nthreads);
-              struct merge_node *merge_tree =
-                merge_tree_init (nthreads, buf.nlines, line);
+              if (nthreads > 1)
+                {
+                  struct merge_node_queue queue;
+                  queue_init (&queue, nthreads);
+                  struct merge_node *merge_tree =
+                    merge_tree_init (nthreads, buf.nlines, line);
 
-              sortlines (line, nthreads, buf.nlines, merge_tree + 1,
-                         &queue, tfp, temp_output);
+                  sortlines (line, nthreads, buf.nlines, merge_tree + 1,
+                             &queue, tfp, temp_output);
 
-              merge_tree_destroy (nthreads, merge_tree);
-              queue_destroy (&queue);
+                  merge_tree_destroy (nthreads, merge_tree);
+                  queue_destroy (&queue);
+                }
+              else
+                {
+                  sequential_sort (line, buf.nlines,
+                                   line - buf.nlines, false);
+                  for (size_t i = 0; i < buf.nlines; i++)
+                    write_unique (line - i - 1, tfp, temp_output);
+                }
             }
           else
             write_unique (line - 1, tfp, temp_output);