[bug#36630] [PATCH] guix: parallelize building the manual-database

[Arne Babenhauserheide <arne_bab@web.de>]

* guix/profiles.scm (manual-database): par-map over the entries.  This
distributes the load roughly equally over all cores and avoids blocking on
I/O.  The order of the entries stays the same since write-mandb-database sorts
them.
---
 guix/profiles.scm | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/guix/profiles.scm b/guix/profiles.scm
index f5c863945c..374f0f8a90 100644
--- a/guix/profiles.scm
+++ b/guix/profiles.scm
@@ -1312,15 +1312,11 @@ the entries in MANIFEST."
         #~(begin
             (use-modules (guix man-db)
                          (guix build utils)
+                         (ice-9 threads)
                          (srfi srfi-1)
                          (srfi srfi-19))

-            (define (compute-entries)
-              ;; This is the most expensive part (I/O and CPU, due to
-              ;; decompression), so report progress as we traverse INPUTS.
-              (let* ((inputs '#$(manifest-inputs manifest))
-                     (total  (length inputs)))
-                (append-map (lambda (directory count)
+            (define (compute-entry directory count total)
                               (format #t "\r[~3d/~3d] building list of \
 man-db entries..."
                                       count total)
@@ -1330,8 +1326,16 @@ man-db entries..."
                                 (if (directory-exists? man)
                                     (mandb-entries man)
                                     '())))
-                            inputs
-                            (iota total 1))))
+
+            (define (compute-entries)
+              ;; This is the most expensive part (I/O and CPU, due to
+              ;; decompression), so report progress as we traverse INPUTS.
+              (let* ((inputs '#$(manifest-inputs manifest))
+                     (total  (length inputs)))
+                (apply append (par-map compute-entry
+                                       inputs
+                                       (iota total 1)
+                                       (make-list total total)))))

             (define man-directory
               (string-append #$output "/share/man"))
--
2.22.0