Use a dedicated fiber for heap size monitoring

guix-data-service/jobs/load-new-guix-revision.scm

@@ -614,12 +614,6 @@
      '(define unsupported-cross-compilation-target-error? (const #f))
      inf))
 
-  (format (current-error-port)
-          "heap size: ~a MiB~%"
-          (round
-           (/ (assoc-ref (gc-stats) 'heap-size)
-              (expt 2. 20))))
-
   (catch
     'match-error
     (lambda ()
@@ -2588,6 +2582,23 @@ SKIP LOCKED")
 
      (exec-query conn "BEGIN")
 
+     (spawn-fiber
+      (lambda ()
+        (while #t
+          (sleep 30)
+
+          (let ((stats (gc-stats)))
+            (simple-format
+             (current-error-port)
+             "process-job heap: ~a MiB used (~a MiB heap)~%"
+             (round
+              (/ (- (assoc-ref stats 'heap-size)
+                    (assoc-ref stats 'heap-free-size))
+                 (expt 2. 20)))
+             (round
+              (/ (assoc-ref stats 'heap-size)
+                 (expt 2. 20))))))))
+
      (match (select-job-for-update conn id)
        (((id commit source git-repository-id))