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, 27 insertions, 3 deletions

diff --git a/erts/emulator/beam/erl_init.c b/erts/emulator/beam/erl_init.c
index 12750b9aa6..547e4064a2 100644
--- a/erts/emulator/beam/erl_init.c
+++ b/erts/emulator/beam/erl_init.c
@@ -593,6 +593,7 @@ void erts_usage(void)
     erts_fprintf(stderr, "               no_time_warp|single_time_warp|multi_time_warp\n");
     erts_fprintf(stderr, "-d             don't write a crash dump for internally detected errors\n");
     erts_fprintf(stderr, "               (halt(String) will still produce a crash dump)\n");
+    erts_fprintf(stderr, "-dcg           set the limit for the number of decentralized counter groups\n");
     erts_fprintf(stderr, "-fn[u|a|l]     Control how filenames are interpreted\n");
     erts_fprintf(stderr, "-hms size      set minimum heap size in words (default %d)\n",
 	       H_DEFAULT_SIZE);
@@ -785,6 +786,8 @@ early_init(int *argc, char **argv) /*
     int dirty_io_scheds;
     int max_reader_groups;
     int reader_groups;
+    int max_decentralized_counter_groups;
+    int decentralized_counter_groups;
     char envbuf[21]; /* enough for any 64-bit integer */
     size_t envbufsz;
 
@@ -804,7 +807,8 @@ early_init(int *argc, char **argv) /*
 
     erts_initialized = 0;
 
-    erts_pre_early_init_cpu_topology(&max_reader_groups,
+    erts_pre_early_init_cpu_topology(&max_decentralized_counter_groups,
+                                     &max_reader_groups,
 				     &ncpu,
 				     &ncpuonln,
 				     &ncpuavail);
@@ -865,6 +869,24 @@ early_init(int *argc, char **argv) /*
 	    }
 	    if (argv[i][0] == '-') {
 		switch (argv[i][1]) {
+		case 'd': {
+		    char *sub_param = argv[i]+2;
+		    if (has_prefix("cg", sub_param)) {
+			char *arg = get_arg(sub_param+2, argv[i+1], &i);
+			if (sscanf(arg, "%d", &max_decentralized_counter_groups) != 1) {
+			    erts_fprintf(stderr,
+					 "bad decentralized counter groups limit: %s\n", arg);
+			    erts_usage();
+			}
+			if (max_decentralized_counter_groups < 0) {
+			    erts_fprintf(stderr,
+					 "bad decentralized counter groups limit: %d\n",
+					 max_decentralized_counter_groups);
+			    erts_usage();
+			}
+		    }
+		    break;
+		}
 		case 'r': {
 		    char *sub_param = argv[i]+2;
 		    if (has_prefix("g", sub_param)) {
@@ -1186,8 +1208,10 @@ early_init(int *argc, char **argv) /*
     erts_early_init_cpu_topology(no_schedulers,
 				 &max_main_threads,
 				 max_reader_groups,
-				 &reader_groups);
-
+				 &reader_groups,
+                                 max_decentralized_counter_groups,
+                                 &decentralized_counter_groups);
+    erts_flxctr_setup(decentralized_counter_groups);
     {
 	erts_thr_late_init_data_t elid = ERTS_THR_LATE_INIT_DATA_DEF_INITER;
 	elid.mem.std.alloc = ethr_std_alloc;