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, 117 insertions, 32 deletions

diff --git a/erts/emulator/beam/erl_db.c b/erts/emulator/beam/erl_db.c
index 0a50af4d1a..c0f5c506f4 100644
--- a/erts/emulator/beam/erl_db.c
+++ b/erts/emulator/beam/erl_db.c
@@ -42,6 +42,7 @@
 #include "bif.h"
 #include "big.h"
 #include "erl_binary.h"
+#include "bif.h"
 
 
 erts_atomic_t erts_ets_misc_mem_size;
@@ -64,6 +65,11 @@ do {                                                                     \
     }                                                                    \
 }while(0)
 
+#define DB_GET_APPROX_NITEMS(DB)                                        \
+    erts_flxctr_read_approx(&(DB)->common.counters, ERTS_DB_TABLE_NITEMS_COUNTER_ID)
+#define DB_GET_APPROX_MEM_CONSUMED(DB)                                  \
+    erts_flxctr_read_approx(&(DB)->common.counters, ERTS_DB_TABLE_MEM_COUNTER_ID)
+
 static BIF_RETTYPE db_bif_fail(Process* p, Uint freason,
                                Uint bif_ix, Export* bif_exp)
 {
@@ -398,8 +404,9 @@ static void
 free_dbtable(void *vtb)
 {
     DbTable *tb = (DbTable *) vtb;
-
-    ASSERT(erts_atomic_read_nob(&tb->common.memory_size) == sizeof(DbTable));
+    ASSERT(erts_flxctr_is_snapshot_ongoing(&tb->common.counters) ||
+           sizeof(DbTable) == erts_flxctr_read_approx(&tb->common.counters,
+                                                      ERTS_DB_TABLE_MEM_COUNTER_ID));
 
     erts_rwmtx_destroy(&tb->common.rwlock);
     erts_mtx_destroy(&tb->common.fixlock);
@@ -408,7 +415,8 @@ free_dbtable(void *vtb)
     if (tb->common.btid)
         erts_bin_release(tb->common.btid);
 
-    erts_db_free(ERTS_ALC_T_DB_TABLE, tb, (void *) tb, sizeof(DbTable));
+    erts_flxctr_destroy(&tb->common.counters, ERTS_ALC_T_DB_TABLE);
+    erts_free(ERTS_ALC_T_DB_TABLE, tb);
 }
 
 static void schedule_free_dbtable(DbTable* tb)
@@ -1731,12 +1739,16 @@ BIF_RETTYPE ets_new_2(BIF_ALIST_2)
      */
     {
         DbTable init_tb;
-
-	erts_atomic_init_nob(&init_tb.common.memory_size, 0);
+        erts_flxctr_init(&init_tb.common.counters, 0, 2, ERTS_ALC_T_DB_TABLE);
 	tb = (DbTable*) erts_db_alloc(ERTS_ALC_T_DB_TABLE,
 				      &init_tb, sizeof(DbTable));
-	erts_atomic_init_nob(&tb->common.memory_size,
-				 erts_atomic_read_nob(&init_tb.common.memory_size));
+        erts_flxctr_init(&tb->common.counters,
+                         status & DB_CA_ORDERED_SET,
+                         2,
+                         ERTS_ALC_T_DB_TABLE);
+        erts_flxctr_add(&tb->common.counters,
+                        ERTS_DB_TABLE_MEM_COUNTER_ID,
+                        DB_GET_APPROX_MEM_CONSUMED(&init_tb));
     }
 
     tb->common.meth = meth;
@@ -1750,8 +1762,6 @@ BIF_RETTYPE ets_new_2(BIF_ALIST_2)
     tb->common.owner = BIF_P->common.id;
     set_heir(BIF_P, tb, heir, heir_data);
 
-    erts_atomic_init_nob(&tb->common.nitems, 0);
-
     tb->common.fixing_procs = NULL;
     tb->common.compress = is_compressed;
 #ifdef ETS_DBG_FORCE_TRAP
@@ -2128,19 +2138,18 @@ BIF_RETTYPE ets_internal_delete_all_2(BIF_ALIST_2)
 {
     SWord initial_reds = ERTS_BIF_REDS_LEFT(BIF_P);
     SWord reds = initial_reds;
-    Eterm nitems;
+    Eterm nitems_holder = THE_NON_VALUE;
     DbTable* tb;
-
     CHECK_TABLES();
 
     DB_BIF_GET_TABLE(tb, DB_WRITE, LCK_WRITE, BIF_ets_internal_delete_all_2);
 
     if (BIF_ARG_2 == am_undefined) {
-        nitems = erts_make_integer(erts_atomic_read_nob(&tb->common.nitems),
-                                   BIF_P);
-
-        reds = tb->common.meth->db_delete_all_objects(BIF_P, tb, reds);
-
+        reds = tb->common.meth->db_delete_all_objects(BIF_P,
+                                                      tb,
+                                                      reds,
+                                                      &nitems_holder);
+        ASSERT(nitems_holder != THE_NON_VALUE);
         ASSERT(!(tb->common.status & DB_BUSY));
 
         if (reds < 0) {
@@ -2159,7 +2168,7 @@ BIF_RETTYPE ets_internal_delete_all_2(BIF_ALIST_2)
             db_unlock(tb, LCK_WRITE);
             BUMP_ALL_REDS(BIF_P);
             BIF_TRAP2(bif_export[BIF_ets_internal_delete_all_2], BIF_P,
-                      BIF_ARG_1, nitems);
+                      BIF_ARG_1, nitems_holder);
         }
         else {
             /* Done, no trapping needed */
@@ -2169,15 +2178,19 @@ BIF_RETTYPE ets_internal_delete_all_2(BIF_ALIST_2)
     }
     else {
         /*
-         * The table lookup succeeded and second argument is nitems
+         * The table lookup succeeded and second argument is nitems_holder
          * and not 'undefined', which means we have trapped at least once
          * and are now done.
          */
-        nitems = BIF_ARG_2;
+        nitems_holder = BIF_ARG_2;
     }
-
     db_unlock(tb, LCK_WRITE);
+    {
+    Eterm nitems =
+        tb->common.meth->db_delete_all_objects_get_nitems_from_holder(BIF_P,
+                                                                      nitems_holder);
     BIF_RET(nitems);
+    }
 }
 
 static void delete_all_objects_continue(Process* p, DbTable* tb)
@@ -2190,7 +2203,7 @@ static void delete_all_objects_continue(Process* p, DbTable* tb)
     if ((tb->common.status & (DB_DELETE|DB_BUSY)) != DB_BUSY)
         return;
 
-    reds = tb->common.meth->db_delete_all_objects(p, tb, reds);
+    reds = tb->common.meth->db_delete_all_objects(p, tb, reds, NULL);
     
     if (reds < 0) {
         BUMP_ALL_REDS(p);
@@ -3277,13 +3290,29 @@ BIF_RETTYPE ets_info_1(BIF_ALIST_1)
     int i;
     Eterm* hp;
     Uint freason;
+    Sint size = -1;
+    Sint memory = -1;
+    Eterm table;
+    int is_ctrs_read_result_set = 0;
     /*Process* rp = NULL;*/
     /* If/when we implement lockless private tables:
     Eterm owner;
     */
-
-    if ((tb = db_get_table(BIF_P, BIF_ARG_1, DB_INFO, LCK_READ, &freason)) == NULL) {
-	if (freason == BADARG && (is_atom(BIF_ARG_1) || is_ref(BIF_ARG_1)))
+    if(is_tuple(BIF_ARG_1) &&
+       is_tuple_arity(BIF_ARG_1, 2) &&
+       erts_flxctr_is_snapshot_result(tuple_val(BIF_ARG_1)[1])) {
+        Eterm counter_read_result  = tuple_val(BIF_ARG_1)[1];
+        table = tuple_val(BIF_ARG_1)[2];
+        size = erts_flxctr_get_snapshot_result_after_trap(counter_read_result,
+                                                          ERTS_DB_TABLE_NITEMS_COUNTER_ID);
+        memory = erts_flxctr_get_snapshot_result_after_trap(counter_read_result,
+                                                            ERTS_DB_TABLE_MEM_COUNTER_ID);
+        is_ctrs_read_result_set = 1;
+    } else {
+        table = BIF_ARG_1;
+    }
+    if ((tb = db_get_table(BIF_P, table, DB_INFO, LCK_READ, &freason)) == NULL) {
+	if (freason == BADARG && (is_atom(table) || is_ref(table)))
 	    BIF_RET(am_undefined);
 	else
             return db_bif_fail(BIF_P, freason, BIF_ets_info_1, NULL);
@@ -3314,9 +3343,35 @@ BIF_RETTYPE ets_info_1(BIF_ALIST_1)
 	    BIF_ERROR(BIF_P, BADARG);
 	}
     }*/
+
+    if (!is_ctrs_read_result_set) {
+        ErtsFlxCtrSnapshotResult res =
+            erts_flxctr_snapshot(&tb->common.counters, ERTS_ALC_T_DB_TABLE, BIF_P);
+        if (ERTS_FLXCTR_GET_RESULT_AFTER_TRAP == res.type) {
+            Eterm tuple;
+            db_unlock(tb, LCK_READ);
+            hp = HAlloc(BIF_P, 3);
+            tuple = TUPLE2(hp, res.trap_resume_state, table);
+            BIF_TRAP1(bif_export[BIF_ets_info_1], BIF_P, tuple);
+        } else if (res.type == ERTS_FLXCTR_TRY_AGAIN_AFTER_TRAP) {
+            db_unlock(tb, LCK_READ);
+            BIF_TRAP1(bif_export[BIF_ets_info_1], BIF_P, table);
+        } else {
+            size = res.result[ERTS_DB_TABLE_NITEMS_COUNTER_ID];
+            memory = res.result[ERTS_DB_TABLE_MEM_COUNTER_ID];
+            is_ctrs_read_result_set = 1;
+        }
+    }
     for (i = 0; i < sizeof(fields)/sizeof(Eterm); i++) {
-	results[i] = table_info(BIF_P, tb, fields[i]);
-	ASSERT(is_value(results[i]));
+        if (is_ctrs_read_result_set && am_size == fields[i]) {
+            results[i] = erts_make_integer(size, BIF_P);
+        } else if (is_ctrs_read_result_set && am_memory == fields[i]) {
+            Sint words = (Sint) ((memory + sizeof(Sint) - 1) / sizeof(Sint));
+            results[i] = erts_make_integer(words, BIF_P);
+        } else {
+            results[i] = table_info(BIF_P, tb, fields[i]);
+            ASSERT(is_value(results[i]));
+        }
     }
     db_unlock(tb, LCK_READ);
 
@@ -3344,14 +3399,43 @@ BIF_RETTYPE ets_info_2(BIF_ALIST_2)
     DbTable* tb;
     Eterm ret = THE_NON_VALUE;
     Uint freason;
-
+    if (erts_flxctr_is_snapshot_result(BIF_ARG_1)) {
+        Sint res;
+        if (am_memory == BIF_ARG_2) {
+            res = erts_flxctr_get_snapshot_result_after_trap(BIF_ARG_1,
+                                                             ERTS_DB_TABLE_MEM_COUNTER_ID);
+            res = (Sint) ((res + sizeof(Sint) - 1) / sizeof(Sint));
+        } else {
+            res = erts_flxctr_get_snapshot_result_after_trap(BIF_ARG_1,
+                                                             ERTS_DB_TABLE_NITEMS_COUNTER_ID);
+        }
+        BIF_RET(erts_make_integer(res, BIF_P));
+    }
     if ((tb = db_get_table(BIF_P, BIF_ARG_1, DB_INFO, LCK_READ, &freason)) == NULL) {
 	if (freason == BADARG && (is_atom(BIF_ARG_1) || is_ref(BIF_ARG_1)))
 	    BIF_RET(am_undefined);
         else
             return db_bif_fail(BIF_P, freason, BIF_ets_info_2, NULL);
     }
-    ret = table_info(BIF_P, tb, BIF_ARG_2);
+    if (BIF_ARG_2 == am_size || BIF_ARG_2 == am_memory) {
+        ErtsFlxCtrSnapshotResult res =
+            erts_flxctr_snapshot(&tb->common.counters, ERTS_ALC_T_DB_TABLE, BIF_P);
+        if (ERTS_FLXCTR_GET_RESULT_AFTER_TRAP == res.type) {
+            db_unlock(tb, LCK_READ);
+            BIF_TRAP2(bif_export[BIF_ets_info_2], BIF_P, res.trap_resume_state, BIF_ARG_2);
+        } else if (res.type == ERTS_FLXCTR_TRY_AGAIN_AFTER_TRAP) {
+            db_unlock(tb, LCK_READ);
+            BIF_TRAP2(bif_export[BIF_ets_info_2], BIF_P, BIF_ARG_1, BIF_ARG_2);
+        } else if (BIF_ARG_2 == am_size) {
+            ret = erts_make_integer(res.result[ERTS_DB_TABLE_NITEMS_COUNTER_ID], BIF_P);
+        } else { /* BIF_ARG_2 == am_memory */
+            Sint r = res.result[ERTS_DB_TABLE_MEM_COUNTER_ID];
+            r = (Sint) ((r + sizeof(Sint) - 1) / sizeof(Sint));
+            ret = erts_make_integer(r, BIF_P);
+        }
+    } else {
+        ret = table_info(BIF_P, tb, BIF_ARG_2);
+    }
     db_unlock(tb, LCK_READ);
     if (is_non_value(ret)) {
 	BIF_ERROR(BIF_P, BADARG);
@@ -4121,7 +4205,8 @@ static Eterm table_info(Process* p, DbTable* tb, Eterm What)
     int use_monotonic;
 
     if (What == am_size) {
-	ret = make_small(erts_atomic_read_nob(&tb->common.nitems));
+        Uint size = (Uint) (DB_GET_APPROX_NITEMS(tb));
+        ret = erts_make_integer(size, p);
     } else if (What == am_type) {
 	if (tb->common.status & DB_SET)  {
 	    ret = am_set;
@@ -4136,7 +4221,7 @@ static Eterm table_info(Process* p, DbTable* tb, Eterm What)
 	    ret = am_bag;
 	}
     } else if (What == am_memory) {
-	Uint words = (Uint) ((erts_atomic_read_nob(&tb->common.memory_size)
+	Uint words = (Uint) ((DB_GET_APPROX_MEM_CONSUMED(tb)
 			      + sizeof(Uint)
 			      - 1)
 			     / sizeof(Uint));
@@ -4294,9 +4379,9 @@ static void print_table(fmtfn_t to, void *to_arg, int show,  DbTable* tb)
 
     tb->common.meth->db_print(to, to_arg, show, tb);
 
-    erts_print(to, to_arg, "Objects: %d\n", (int)erts_atomic_read_nob(&tb->common.nitems));
+    erts_print(to, to_arg, "Objects: %d\n", (int)DB_GET_APPROX_NITEMS(tb));
     erts_print(to, to_arg, "Words: %bpu\n",
-	       (Uint) ((erts_atomic_read_nob(&tb->common.memory_size)
+	       (Uint) ((DB_GET_APPROX_MEM_CONSUMED(tb)
 			+ sizeof(Uint)
 			- 1)
 		       / sizeof(Uint)));