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, 2 insertions, 0 deletions

diff --git a/erts/emulator/beam/erl_process.h b/erts/emulator/beam/erl_process.h
index 711b73417d..6118c671ee 100644
--- a/erts/emulator/beam/erl_process.h
+++ b/erts/emulator/beam/erl_process.h
@@ -641,6 +641,7 @@ struct ErtsSchedulerData_ {
     ErtsSchedType type;
     Uint no;			/* Scheduler number for normal schedulers */
     Uint dirty_no;  /* Scheduler number for dirty schedulers */
+    int flxctr_slot_no; /* slot nr when a flxctr is used */
     struct enif_environment_t *current_nif;
     Process *dirty_shadow_process;
     Port *current_port;
@@ -1847,6 +1848,7 @@ int erts_resume_processes(ErtsProcList *);
 void erts_deep_process_dump(fmtfn_t, void *);
 
 Eterm erts_get_reader_groups_map(Process *c_p);
+Eterm erts_get_decentralized_counter_groups_map(Process *c_p);
 Eterm erts_debug_reader_groups_map(Process *c_p, int groups);
 
 Uint erts_debug_nbalance(void);