[coreutils] draft [PATCH] sort: explicit --parallel=N now overrides environment

[Paul Eggert]

It seems to me that this code in sort.c:

      unsigned long int np2 = num_processors (NPROC_CURRENT_OVERRIDABLE);
      if (!nthreads || nthreads > np2)
        nthreads = np2;

is now obsolete.  It was written assuming spin locks, but now that
we use mutexes, shouldn't we respect an explicit --parallel=N
flag?  Something like the following, say?   This would let the user
override the environment in the command line, which is normally what
people would expect.

>From bbc60da9222e38bb7983464cec35c42ad41f2eb8 Mon Sep 17 00:00:00 2001
From: Paul Eggert <egg...@cs.ucla.edu>
Date: Sat, 11 Dec 2010 01:02:57 -0800
Subject: [PATCH] sort: explicit --parallel=N now overrides environment

* NEWS: Document this.
* doc/coreutils.texi (sort invocation): Likewise.
* src/sort.c (main): Implement it.
---
 NEWS               |    4 ++++
 doc/coreutils.texi |    3 +--
 src/sort.c         |   13 +++++++------
 3 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/NEWS b/NEWS
index 9f55cbb..d70de31 100644
--- a/NEWS
+++ b/NEWS
@@ -20,6 +20,10 @@ GNU coreutils NEWS                                    -*- 
outline -*-
 
 ** New features
 
+  sort --parallel=N now respects values of N greater than the number
+  of processors available, thus overriding any settings in the
+  OMP_NUM_THREADS environment variable.
+
   split accepts the --number option to generate a specific number of files.
 
 
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 34d9ff0..3ecc4d2 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -4128,8 +4128,7 @@ disks and controllers.
 @opindex --parallel
 @cindex multithreaded sort
 Limit the number of sorts run in parallel to @var{n}. By default,
-...@var{n} is set to the number of available processors, and values
-greater than that are reduced to that limit. Also see
+...@var{n} is set to the number of available processors.  Also see
 @ref{nproc invocation}.
 
 @item -u
diff --git a/src/sort.c b/src/sort.c
index 2c0f852..babc437 100644
--- a/src/sort.c
+++ b/src/sort.c
@@ -4624,14 +4624,15 @@ main (int argc, char **argv)
     }
   else
     {
-      unsigned long int np2 = num_processors (NPROC_CURRENT_OVERRIDABLE);
-      if (!nthreads || nthreads > np2)
-        nthreads = np2;
-
       /* Avoid integer overflow later.  */
       size_t nthreads_max = SIZE_MAX / (2 * sizeof (struct merge_node));
-      if (nthreads_max < nthreads)
-        nthreads = nthreads_max;
+      if (nthreads)
+        nthreads = MIN (nthreads, nthreads_max);
+      else
+        {
+          unsigned long int np2 = num_processors (NPROC_CURRENT_OVERRIDABLE);
+          nthreads = MIN (np2, nthreads_max);
+        }
 
       sort (files, nfiles, outfile, nthreads);
     }
-- 
1.7.2