Backport of ocaml#12318 to the 4.14 LTS branch.

Changes

@@ -8,6 +8,13 @@ OCaml 4.14 maintenance version
 
 ### Bug fixes:
 
+### Runtime system:
+
+- #12322: GC: simplify the meaning of custom_minor_max_size: blocks with
+  out-of-heap memory above this limit are now allocated directly in
+  the major heap.
+  (Damien Doligez, report by Stephen Dolan, review by ?)
+
 OCaml 4.14.1 (20 December 2022)
 ------------------------------
 

runtime/caml/config.h

@@ -264,7 +264,7 @@ typedef uint64_t uintnat;
 /* Default setting for maximum size of custom objects counted as garbage
    in the minor heap.
    Documented in gc.mli */
-#define Custom_minor_max_bsz_def 8192
+#define Custom_minor_max_bsz_def 70000
 
 /* Default allocation policy. */
 #define Allocation_policy_def caml_policy_best_fit

runtime/caml/custom.h

@@ -55,17 +55,26 @@ extern "C" {
 #endif
 
 
+CAMLextern uintnat caml_custom_major_ratio;
+
 CAMLextern value caml_alloc_custom(struct custom_operations * ops,
                                    uintnat size, /*size in bytes*/
                                    mlsize_t mem, /*resources consumed*/
                                    mlsize_t max  /*max resources*/);
 
+/* [caml_alloc_custom_mem] allocates a custom block with dependent memory
+   (memory outside the heap that will be reclaimed when the block is
+   finalized). If [mem] is greater than [custom_minor_max_size] (see gc.mli)
+   the block is allocated directly in the major heap. */
 CAMLextern value caml_alloc_custom_mem(struct custom_operations * ops,
                                        uintnat size, /*size in bytes*/
                                        mlsize_t mem  /*memory consumed*/);
 
 CAMLextern void caml_register_custom_operations(struct custom_operations * ops);
 
+/* Return the current [max] factor for [caml_alloc_custom_mem] allocations. */
+CAMLextern mlsize_t caml_custom_get_max_major (void);
+
 /* Global variable moved to Caml_state in 4.10 */
 #define caml_compare_unordered (Caml_state_field(compare_unordered))
 

runtime/caml/memory.h

@@ -50,6 +50,7 @@ CAMLextern value caml_alloc_shr_no_track_noexc (mlsize_t, tag_t);
 CAMLextern value caml_alloc_shr_for_minor_gc (mlsize_t, tag_t, header_t);
 
 CAMLextern void caml_adjust_gc_speed (mlsize_t, mlsize_t);
+CAMLextern void caml_adjust_minor_gc_speed (mlsize_t, mlsize_t);
 CAMLextern void caml_alloc_dependent_memory (mlsize_t bsz);
 CAMLextern void caml_free_dependent_memory (mlsize_t bsz);
 CAMLextern void caml_modify (value *, value);

runtime/custom.c

@@ -30,41 +30,55 @@ uintnat caml_custom_major_ratio = Custom_major_ratio_def;
 uintnat caml_custom_minor_ratio = Custom_minor_ratio_def;
 uintnat caml_custom_minor_max_bsz = Custom_minor_max_bsz_def;
 
+mlsize_t caml_custom_get_max_major (void)
+{
+  /* The major ratio is a percentage relative to the major heap size.
+     A complete GC cycle will be done every time 2/3 of that much
+     memory is allocated for blocks in the major heap.  Assuming
+     constant allocation and deallocation rates, this means there are
+     at most [M/100 * major-heap-size] bytes of floating garbage at
+     any time.  The reason for a factor of 2/3 (or 1.5) is, roughly
+     speaking, because the major GC takes 1.5 cycles (previous cycle +
+     marking phase) before it starts to deallocate dead blocks
+     allocated during the previous cycle.  [heap_size / 150] is really
+     [heap_size * (2/3) / 100] (but faster). */
+  return Bsize_wsize (Caml_state->stat_heap_wsz) / 150
+         * caml_custom_major_ratio;
+}
+
+/* [mem] is an amount of out-of-heap resources, in the same units as
+   [max_major] and [max_minor]. When the cumulated amount of such
+   resources reaches [max_minor] (for resources held by the minor
+   heap) we do a minor collection; when it reaches [max_major] (for
+   resources held by the major heap), we guarantee that a major cycle
+   is done.
+
+   If [max_major] is 0, then [mem] is a number of bytes and the actual
+   limit is [caml_custom_get_max_major ()] computed at the
+   time when the custom block is promoted to the major heap.
+*/
 static value alloc_custom_gen (struct custom_operations * ops,
                                uintnat bsz,
                                mlsize_t mem,
                                mlsize_t max_major,
-                               mlsize_t mem_minor,
                                mlsize_t max_minor)
 {
   mlsize_t wosize;
   CAMLparam0();
   CAMLlocal1(result);
 
-  /* [mem] is the total amount of out-of-heap memory, [mem_minor] is how much
-     of it should be counted against [max_minor]. */
-  CAMLassert (mem_minor <= mem);
-
   wosize = 1 + (bsz + sizeof(value) - 1) / sizeof(value);
-  if (wosize <= Max_young_wosize) {
+  if (wosize <= Max_young_wosize && mem <= caml_custom_minor_max_bsz) {
     result = caml_alloc_small(wosize, Custom_tag);
     Custom_ops_val(result) = ops;
     if (ops->finalize != NULL || mem != 0) {
-      if (mem > mem_minor) {
-        caml_adjust_gc_speed (mem - mem_minor, max_major);
-      }
-      /* The remaining [mem_minor] will be counted if the block survives a
-         minor GC */
+      /* Record the extra resources in case the block gets promoted. */
       add_to_custom_table (Caml_state->custom_table, result,
-                           mem_minor, max_major);
+                           mem, max_major);
       /* Keep track of extra resources held by custom block in
          minor heap. */
-      if (mem_minor != 0) {
-        if (max_minor == 0) max_minor = 1;
-        Caml_state->extra_heap_resources_minor +=
-          (double) mem_minor / (double) max_minor;
-        if (Caml_state->extra_heap_resources_minor > 1.0)
-          caml_minor_collection ();
+      if (mem != 0) {
+        caml_adjust_minor_gc_speed (mem, max_minor);
       }
     }
   } else {
@@ -76,34 +90,27 @@ static value alloc_custom_gen (struct custom_operations * ops,
   CAMLreturn(result);
 }
 
+Caml_inline mlsize_t get_max_minor (void)
+{
+  return
+    Bsize_wsize (Caml_state->minor_heap_wsz) / 100 * caml_custom_minor_ratio;
+}
+
 CAMLexport value caml_alloc_custom(struct custom_operations * ops,
                                    uintnat bsz,
                                    mlsize_t mem,
                                    mlsize_t max)
 {
-  return alloc_custom_gen (ops, bsz, mem, max, mem, max);
+  mlsize_t max_major = max;
+  mlsize_t max_minor = max == 0 ? get_max_minor() : max;
+  return alloc_custom_gen (ops, bsz, mem, max_major, max_minor);
 }
 
 CAMLexport value caml_alloc_custom_mem(struct custom_operations * ops,
                                        uintnat bsz,
                                        mlsize_t mem)
 {
-  mlsize_t mem_minor =
-    mem < caml_custom_minor_max_bsz ? mem : caml_custom_minor_max_bsz;
-  mlsize_t max_major =
-    /* The major ratio is a percentage relative to the major heap size.
-       A complete GC cycle will be done every time 2/3 of that much memory
-       is allocated for blocks in the major heap.  Assuming constant
-       allocation and deallocation rates, this means there are at most
-       [M/100 * major-heap-size] bytes of floating garbage at any time.
-       The reason for a factor of 2/3 (or 1.5) is, roughly speaking, because
-       the major GC takes 1.5 cycles (previous cycle + marking phase) before
-       it starts to deallocate dead blocks allocated during the previous cycle.
-       [heap_size / 150] is really [heap_size * (2/3) / 100] (but faster). */
-    Bsize_wsize (Caml_state->stat_heap_wsz) / 150 * caml_custom_major_ratio;
-  mlsize_t max_minor =
-    Bsize_wsize (Caml_state->minor_heap_wsz) / 100 * caml_custom_minor_ratio;
-  value v = alloc_custom_gen (ops, bsz, mem, max_major, mem_minor, max_minor);
+  value v = alloc_custom_gen (ops, bsz, mem, 0, get_max_minor());
   caml_memprof_track_custom(v, mem);
   return v;
 }

runtime/memory.c

@@ -21,6 +21,7 @@
 #include <stddef.h>
 #include "caml/address_class.h"
 #include "caml/config.h"
+#include "caml/custom.h"
 #include "caml/fail.h"
 #include "caml/freelist.h"
 #include "caml/gc.h"
@@ -573,10 +574,16 @@ CAMLexport void caml_free_dependent_memory (mlsize_t nbytes)
    this time.
    Note that only [res/max] is relevant.  The units (and kind of
    resource) can change between calls to [caml_adjust_gc_speed].
+
+   If [max] = 0, then we use a number proportional to the major heap
+   size and [caml_custom_major_ratio]. In this case, [mem] should
+   be a number of bytes and the trade-off between GC work and space
+   overhead is under the control of the user through
+   [caml_custom_major_ratio].
 */
 CAMLexport void caml_adjust_gc_speed (mlsize_t res, mlsize_t max)
 {
-  if (max == 0) max = 1;
+  if (max == 0) max = caml_custom_get_max_major ();
   if (res > max) res = max;
   caml_extra_heap_resources += (double) res / (double) max;
   if (caml_extra_heap_resources > 1.0){
@@ -586,6 +593,19 @@ CAMLexport void caml_adjust_gc_speed (mlsize_t res, mlsize_t max)
   }
 }
 
+/* This function is analogous to [caml_adjust_gc_speed]. When the
+   accumulated sum of [res/max] values reaches 1, a minor GC is
+   triggered.
+*/
+CAMLexport void caml_adjust_minor_gc_speed (mlsize_t res, mlsize_t max)
+{
+  if (max == 0) max = 1;
+  Caml_state->extra_heap_resources_minor += (double) res / (double) max;
+  if (Caml_state->extra_heap_resources_minor > 1.0) {
+    caml_minor_collection ();
+  }
+}
+
 /* You must use [caml_initialize] to store the initial value in a field of
    a shared block, unless you are sure the value is not a young block.
    A block value [v] is a shared block if and only if [Is_in_heap (v)]

stdlib/gc.mli

@@ -232,13 +232,11 @@ type control =
 
     custom_minor_max_size : int;
     (** Maximum amount of out-of-heap memory for each custom value
-        allocated in the minor heap. When a custom value is allocated
-        on the minor heap and holds more than this many bytes, only
-        this value is counted against [custom_minor_ratio] and the
-        rest is directly counted against [custom_major_ratio].
+        allocated in the minor heap. Custom values that hold more
+        than this many bytes are allocated on the major heap.
         Note: this only applies to values allocated with
         [caml_alloc_custom_mem] (e.g. bigarrays).
-        Default: 8192 bytes.
+        Default: 70000 bytes.
         @since 4.08.0 *)
   }
 (** The GC parameters are given as a [control] record.  Note that