Add the gc watcher from the build coordinator

guix-data-service/web/server.scm

@@ -21,6 +21,7 @@
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-71)
   #:use-module (ice-9 match)
+  #:use-module (ice-9 format)
   #:use-module (ice-9 threads)
   #:use-module (web http)
   #:use-module (web request)
@@ -127,6 +128,30 @@
       (setrlimit 'nofile 4096 4096))
     #:unwind? #t)
 
+  (call-with-new-thread
+   (lambda ()
+     (set-thread-name "gc watcher")
+
+     (add-hook!
+      after-gc-hook
+      (let ((last-gc-time-taken
+             (assq-ref (gc-stats) 'gc-time-taken)))
+        (lambda ()
+          (let* ((gc-time-taken
+                  (assq-ref (gc-stats) 'gc-time-taken))
+                 (time-since-last
+                  (/ (- gc-time-taken
+                        last-gc-time-taken)
+                     internal-time-units-per-second)))
+            (when (> time-since-last 0.1)
+              (format (current-error-port)
+                      "after gc (additional time taken: ~f)\n"
+                      time-since-last))
+            (set! last-gc-time-taken
+                  (assq-ref (gc-stats) 'gc-time-taken))))))
+     (while #t
+       (sleep 0.1))))
+
   (let ((finished?         (make-condition))
         (request-scheduler #f))
     (call-with-sigint