[PATCH 1/3] Add gc-estimated-time variable

[Spencer Baugh]

---
 src/alloc.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/src/alloc.c b/src/alloc.c
index 2b3643e35b..8d31903089 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -6129,10 +6129,19 @@ garbage_collect (void)
   /* Accumulate statistics.  */
   if (FLOATP (Vgc_elapsed))
     {
+      const struct timespec this_gc_elapsed = timespec_sub (current_timespec 
(), start);
       static struct timespec gc_elapsed;
-      gc_elapsed = timespec_add (gc_elapsed,
-                                timespec_sub (current_timespec (), start));
+      gc_elapsed = timespec_add (gc_elapsed, this_gc_elapsed);
       Vgc_elapsed = make_float (timespectod (gc_elapsed));
+      static double gc_time_ema;
+      if (gc_time_ema == 0.0) {
+        /* initialize exponential moving average to first GC time */
+        gc_time_ema = timespectod (this_gc_elapsed);
+      } else {
+        /* decay very fast, since heap size can change rapidly */
+        gc_time_ema = 0.5 * gc_time_ema + 0.5 * timespectod (this_gc_elapsed);
+      }
+      Vgc_estimated_time = make_float (gc_time_ema);
     }
 
   gcs_done++;
@@ -7528,6 +7537,11 @@ do hash-consing of the objects allocated to pure space.  
*/);
 The time is in seconds as a floating point value.  */);
   DEFVAR_INT ("gcs-done", gcs_done,
               doc: /* Accumulated number of garbage collections done.  */);
+  DEFVAR_LISP ("gc-estimated-time", Vgc_estimated_time,
+              doc: /* An estimate of the time required for the next garbage 
collection.
+The time is in seconds as a floating point value.
+This is calculated as an exponential moving average of the time
+required for previous garbage collections.  */);
 
   DEFVAR_INT ("integer-width", integer_width,
              doc: /* Maximum number N of bits in safely-calculated integers.
-- 
2.28.0