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, 14 insertions, 8 deletions

diff --git a/erts/emulator/beam/erl_cpu_topology.h b/erts/emulator/beam/erl_cpu_topology.h
index 88bcad79ab..4a428d7972 100644
--- a/erts/emulator/beam/erl_cpu_topology.h
+++ b/erts/emulator/beam/erl_cpu_topology.h
@@ -27,14 +27,19 @@
 #ifndef ERL_CPU_TOPOLOGY_H__
 #define ERL_CPU_TOPOLOGY_H__
 
-void erts_pre_early_init_cpu_topology(int *max_rg_p,
-				      int *conf_p,
-				      int *onln_p,
-				      int *avail_p);
-void erts_early_init_cpu_topology(int no_schedulers,
-				  int *max_main_threads_p,
-				  int max_reader_groups,
-				  int *reader_groups_p);
+void
+erts_pre_early_init_cpu_topology(int *max_dcg_p,
+                                 int *max_rg_p,
+				 int *conf_p,
+				 int *onln_p,
+				 int *avail_p);
+void
+erts_early_init_cpu_topology(int no_schedulers,
+			     int *max_main_threads_p,
+			     int max_reader_groups,
+			     int *reader_groups_p,
+                             int max_decentralized_counter_groups,
+                             int *decentralized_counter_groups_p);
 void erts_init_cpu_topology(void);
 
 
@@ -70,6 +75,7 @@ Eterm erts_bind_schedulers(Process *c_p, Eterm how);
 Eterm erts_get_schedulers_binds(Process *c_p);
 
 Eterm erts_get_reader_groups_map(Process *c_p);
+Eterm erts_get_decentralized_counter_groups_map(Process *c_p);
 
 Eterm erts_set_cpu_topology(Process *c_p, Eterm term);
 Eterm erts_get_cpu_topology_term(Process *c_p, Eterm which);