Decentralized counters for ETS ordered_set with write_concurrency

Previously, all ETS tables used centralized counter variables to keep
track of the number of items stored and the amount of memory
consumed. These counters can cause scalability problems (especially on
big NUMA systems). This commit adds an implementation of a
decentralized counter and modifies the implementation of ETS so that
ETS tables of type ordered_set with write_concurrency enabled use the
decentralized counter. [Experiments][1] indicate that this change
substantially improves the scalability of ETS ordered_set tables with
write_concurrency enabled in scenarios with frequent `ets:insert/2`
and `ets:delete/2` calls.

The new counter is implemented in the module erts_flxctr
(`erts_flxctr.h` and `erts_flxctr.c`). The module has the suffix
flxctr as it contains the implementation of a flexible counter (i.e.,
counter instances can be configured to be either centralized or
decentralized). Counters that are configured to be centralized are
implemented with a single counter variable which is modified with
atomic operations. Decentralized counters are spread over several
cache lines (how many can be configured with the parameter
`+dcg`). The scheduler threads are mapped to cache lines so that there
is no single point of contention when decentralized counters are
updated. The thread progress functionality of the Erlang VM is
utilized to implement support for linearizable snapshots of
decentralized counters. The snapshot functionality is used by the
`ets:info/1` and `ets:info/2` functions.

[1]: http://winsh.me/ets_catree_benchmark/flxctr_res.html

1 files changed, 57 insertions, 3 deletions

diff --git a/erts/emulator/beam/erl_db_catree.c b/erts/emulator/beam/erl_db_catree.c
index 0402c6b7b4..962fe4c4f8 100644
--- a/erts/emulator/beam/erl_db_catree.c
+++ b/erts/emulator/beam/erl_db_catree.c
@@ -149,7 +149,12 @@ static SWord db_free_table_continue_catree(DbTable *tbl, SWord);
 static void db_foreach_offheap_catree(DbTable *,
                                       void (*)(ErlOffHeap *, void *),
                                       void *);
-static SWord db_delete_all_objects_catree(Process* p, DbTable* tbl, SWord reds);
+static SWord db_delete_all_objects_catree(Process* p,
+                                          DbTable* tbl,
+                                          SWord reds,
+                                          Eterm* nitems_holder_wb);
+static Eterm db_delete_all_objects_get_nitems_from_holder_catree(Process* p,
+                                                                 Eterm nitems_holder);
 static int
 db_lookup_dbterm_catree(Process *, DbTable *, Eterm key, Eterm obj,
                         DbUpdateHandle*);
@@ -191,6 +196,7 @@ DbTableMethod db_catree =
     db_select_replace_continue_catree,
     db_take_catree,
     db_delete_all_objects_catree,
+    db_delete_all_objects_get_nitems_from_holder_catree,
     db_free_table_catree,
     db_free_table_continue_catree,
     db_print_catree,
@@ -1357,6 +1363,8 @@ static SWord do_free_base_node_cont(DbTableCATree *tb, SWord num_left)
                 PUSH_NODE(&tb->free_stack_elems, root);
                 root = p;
             } else {
+                DEC_NITEMS((DbTable*)tb);
+                tb->nr_of_deleted_items++;
                 free_term((DbTable*)tb, root);
                 if (--num_left >= 0) {
                     break;
@@ -1397,6 +1405,7 @@ int db_create_catree(Process *p, DbTable *tbl)
     root = create_base_node(tb, NULL);
     tb->deletion = 0;
     tb->base_nodes_to_free_list = NULL;
+    tb->nr_of_deleted_items = 0;
     erts_atomic_init_relb(&(tb->root), (erts_aint_t)root);
     return DB_ERROR_NONE;
 }
@@ -2050,6 +2059,7 @@ static SWord db_free_table_continue_catree(DbTable *tbl, SWord reds)
         PUSH_NODE(&tb->free_stack_rnodes, GET_ROOT(tb));
         tb->is_routing_nodes_freed = 0;
         tb->base_nodes_to_free_list = NULL;
+        tb->nr_of_deleted_items = 0;
     }
     if ( ! tb->is_routing_nodes_freed ) {
         reds = do_free_routing_nodes_catree_cont(tb, reds);
@@ -2079,13 +2089,57 @@ static SWord db_free_table_continue_catree(DbTable *tbl, SWord reds)
     return 1;
 }
 
-static SWord db_delete_all_objects_catree(Process* p, DbTable* tbl, SWord reds)
+static
+int db_catree_nr_of_items_deleted_wb_dtor(Binary *context_bin) {
+    (void)context_bin;
+    return 1;
+}
+
+typedef struct {
+    Uint nr_of_deleted_items;
+} DbCATreeNrOfItemsDeletedWb;
+
+static Eterm
+create_and_install_num_of_deleted_items_wb_bin(Process *p, DbTableCATree *tb)
+{
+    Binary* bin =
+        erts_create_magic_binary(sizeof(DbCATreeNrOfItemsDeletedWb),
+                                 db_catree_nr_of_items_deleted_wb_dtor);
+    DbCATreeNrOfItemsDeletedWb* data = ERTS_MAGIC_BIN_DATA(bin);
+    Eterm* hp = HAlloc(p, ERTS_MAGIC_REF_THING_SIZE);
+    Eterm mref = erts_mk_magic_ref(&hp, &MSO(p), bin);
+    data->nr_of_deleted_items = 0;
+    tb->nr_of_deleted_items_wb = bin;
+    erts_refc_inctest(&bin->intern.refc, 2);   
+    return mref;
+}
+
+static Eterm db_delete_all_objects_get_nitems_from_holder_catree(Process* p,
+                                                                 Eterm mref)
 {
+    Binary* bin = erts_magic_ref2bin(mref);
+    DbCATreeNrOfItemsDeletedWb* data = ERTS_MAGIC_BIN_DATA(bin);
+    return erts_make_integer(data->nr_of_deleted_items, p);
+}
+
+static SWord db_delete_all_objects_catree(Process* p,
+                                          DbTable* tbl,
+                                          SWord reds,
+                                          Eterm* nitems_holder_wb)
+{
+    DbTableCATree *tb = &tbl->catree;
+    DbCATreeNrOfItemsDeletedWb* data;
+    if (!tb->deletion) {
+        *nitems_holder_wb =
+            create_and_install_num_of_deleted_items_wb_bin(p, tb);
+    }
     reds = db_free_table_continue_catree(tbl, reds);
     if (reds < 0)
         return reds;
+    data = ERTS_MAGIC_BIN_DATA(tb->nr_of_deleted_items_wb);
+    data->nr_of_deleted_items = tb->nr_of_deleted_items;
+    erts_bin_release(tb->nr_of_deleted_items_wb);
     db_create_catree(p, tbl);
-    erts_atomic_set_nob(&tbl->catree.common.nitems, 0);
     return reds;
 }