Fix spurious major GC slices.

Changes

@@ -25,6 +25,10 @@ _______________
 - #13003: new, more consistent names for array-creation C functions
   (Gabriel Scherer, review by Olivier Nicole)
 
+- #13086: Avoid spurious major GC slices.
+  (Damien Doligez, report by Stephen Dolan, review by Gabriel Scherer
+   and Stephen Dolan)
+
 ### Code generation and optimizations:
 
 - #13014: Enable compile-time option -function-sections on all previously

runtime/caml/domain_state.tbl

@@ -74,6 +74,12 @@ DOMAIN_STATE(uintnat, sweeping_done)
 /* Is sweeping done for the current major cycle. */
 
 DOMAIN_STATE(uintnat, allocated_words)
+/* Number of words promoted or allocated directly to the major heap since
+   latest slice. */
+
+DOMAIN_STATE(uintnat, allocated_words_direct)
+/* Number of words allocated directly to the major heap since the latest
+   slice. (subset of allocated_words) */
 
 DOMAIN_STATE(uintnat, swept_words)
 

runtime/domain.c

@@ -702,6 +702,7 @@ static void domain_create(uintnat initial_minor_heap_wsize,
   domain_state->gc_regs = NULL;
 
   domain_state->allocated_words = 0;
+  domain_state->allocated_words_direct = 0;
   domain_state->swept_words = 0;
 
   domain_state->local_roots = NULL;

runtime/intern.c

@@ -434,11 +434,12 @@ static value intern_alloc_obj(struct caml_intern_state* s, caml_domain_state* d,
   } else {
     p = caml_shared_try_alloc(d->shared_heap, wosize, tag,
                               0 /* no reserved bits */);
-    d->allocated_words += Whsize_wosize(wosize);
     if (p == NULL) {
       intern_cleanup (s);
       caml_raise_out_of_memory();
     }
+    d->allocated_words += Whsize_wosize(wosize);
+    d->allocated_words_direct += Whsize_wosize(wosize);
     Hd_hp(p) = Make_header (wosize, tag, caml_global_heap_state.MARKED);
     caml_memprof_sample_block(Val_hp(p), wosize,
                               Whsize_wosize(wosize),

runtime/major_gc.c

@@ -627,16 +627,18 @@ static void update_major_slice_work(intnat howmuch,
 {
   double heap_words;
   intnat alloc_work, dependent_work, extra_work, new_work;
-  intnat my_alloc_count, my_dependent_count;
+  intnat my_alloc_count, my_alloc_direct_count, my_dependent_count;
   double my_extra_count;
   caml_domain_state *dom_st = Caml_state;
   uintnat heap_size, heap_sweep_words, total_cycle_work;
 
   my_alloc_count = dom_st->allocated_words;
+  my_alloc_direct_count = dom_st->allocated_words_direct;
   my_dependent_count = dom_st->dependent_allocated;
   my_extra_count = dom_st->extra_heap_resources;
   dom_st->stat_major_words += dom_st->allocated_words;
   dom_st->allocated_words = 0;
+  dom_st->allocated_words_direct = 0;
   dom_st->dependent_allocated = 0;
   dom_st->extra_heap_resources = 0.0;
   /*
@@ -708,6 +710,9 @@ static void update_major_slice_work(intnat howmuch,
   caml_gc_message (0x40, "allocated_words = %"
                          ARCH_INTNAT_PRINTF_FORMAT "u\n",
                    my_alloc_count);
+  caml_gc_message (0x40, "allocated_words_direct = %"
+                         ARCH_INTNAT_PRINTF_FORMAT "u\n",
+                   my_alloc_direct_count);
   caml_gc_message (0x40, "alloc work-to-do = %"
                          ARCH_INTNAT_PRINTF_FORMAT "d\n",
                    alloc_work);
@@ -2015,6 +2020,7 @@ void caml_finish_marking (void)
     caml_shrink_mark_stack();
     Caml_state->stat_major_words += Caml_state->allocated_words;
     Caml_state->allocated_words = 0;
+    Caml_state->allocated_words_direct = 0;
     CAML_EV_END(EV_MAJOR_FINISH_MARKING);
   }
 }

runtime/memory.c

@@ -434,7 +434,8 @@ Caml_inline value alloc_shr(mlsize_t wosize, tag_t tag, reserved_t reserved,
   }
 
   dom_st->allocated_words += Whsize_wosize(wosize);
-  if (dom_st->allocated_words > dom_st->minor_heap_wsz / 5) {
+  dom_st->allocated_words_direct += Whsize_wosize(wosize);
+  if (dom_st->allocated_words_direct > dom_st->minor_heap_wsz / 5) {
     CAML_EV_COUNTER (EV_C_REQUEST_MAJOR_ALLOC_SHR, 1);
     caml_request_major_slice(1);
   }