sort: accept new option --batch-size=NMERGE

* src/sort.c: (static unsigned int nmerge) Replace constant NMERGE.
(specify_nmerge) Validate and apply new option.
(mergefps) Replace some arrays with pointers to xnmalloc'd storage.
* tests/misc/sort-merge: Test new option.
* doc/coreutils.texi: Describe new option.
* NEWS: Advertise new option.

1 files changed, 23 insertions, 0 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index e4a979e61..f7b9e7844 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -3804,6 +3804,29 @@ multiple fields.
 Example:  To sort on the second field, use @option{--key=2,2}
 (@option{-k 2,2}).  See below for more examples.
 
+@item --batch-size=@var{nmerge}
+@opindex --batch-size
+@cindex number of inputs to merge, nmerge
+Merge at most @var{nmerge} inputs at once.
+
+When @command{sort} has to merge more than @var{nmerge} inputs,
+it merges them in groups of @var{nmerge}, saving the result in
+a temporary file, which is then used as an input in a subsequent merge.
+
+A large value of @var{nmerge} may improve merge performance and decrease
+temporary storage utilization at the expense of increased memory usage
+and I/0.  Conversely a small value of @var{nmerge} may reduce memory
+requirements and I/0 at the expense of temporary storage consumption and
+merge performance.
+
+The value of @var{nmerge} must be at least 2.
+
+The value of @var{nmerge} may be bounded by a resource limit for open
+file descriptors.  Try @samp{ulimit -n} or @samp{getconf OPEN_MAX} to
+to display the limit for a particular system.
+If the value of @var{nmerge} exceeds this limit, then @command{sort} will
+issue a warning to standard error and exit with a nonzero status.
+
 @item -o @var{output-file}
 @itemx --output=@var{output-file}
 @opindex -o