Compaction: Add snooze_period_ms for finer tuning

We are experiencing an issue similar to #1579, partly because we have
dozens of thousands of databases.

On our servers:
- `snooze_period = 0` leads to a significant CPU load,
- `snooze_period = 1` is too long for compaction to finish within 20
  hours (after 20 hours, compaction is stopped to allow other
  CPU-intensive processes to run, and when compaction restarts, it does
  not pick up where it left 4 hours earlier -- by the way proposal at
  #1775 would be really great to fix that!)

This commit introduces a new option `snooze_period_ms` (measured in
milliseconds), and deprecates `snooze_period` while still supporting it
for obvious legacy reasons.

rel/overlay/etc/default.ini

@@ -435,8 +435,9 @@ min_file_size = 131072
 ; databases, the compaction_daemon can create significant CPU load when
 ; checking whether databases and view indexes need compacting. The
 ; snooze_period setting ensures a smoother CPU load. Defaults to
-; 3 seconds wait.
-; snooze_period = 3
+; 3000 milliseconds wait. Note that this option was formerly called
+; snooze_period, measured in seconds (it is currently still supported).
+; snooze_period_ms = 3000
 
 [compactions]
 ; List of compaction rules for the compaction daemon.

src/couch/src/couch_compaction_daemon.erl

@@ -125,8 +125,16 @@ handle_config_terminate(_, stop, _) ->
 handle_config_terminate(_Server, _Reason, _State) ->
     erlang:send_after(?RELISTEN_DELAY, whereis(?MODULE), restart_config_listener).
 
+get_snooze_period() ->
+    % The snooze_period_ms option should be used, but snooze_period is supported
+    % for legacy reasons.
+    Default = config:get_integer("compaction_daemon", "snooze_period", 3),
+    case config:get_integer("compaction_daemon", "snooze_period_ms", -1) of
+        -1 -> Default * 1000;
+        SnoozePeriod -> SnoozePeriod
+    end.
+
 compact_loop(Parent) ->
-    SnoozePeriod = config:get_integer("compaction_daemon", "snooze_period", 3),
     {ok, _} = couch_server:all_databases(
         fun(DbName, Acc) ->
             case ets:info(?CONFIG_ETS, size) =:= 0 of
@@ -140,7 +148,7 @@ compact_loop(Parent) ->
                     case check_period(Config) of
                     true ->
                         maybe_compact_db(Parent, DbName, Config),
-                        ok = timer:sleep(SnoozePeriod * 1000);
+                        ok = timer:sleep(get_snooze_period());
                     false ->
                         ok
                     end
@@ -231,8 +239,7 @@ maybe_compact_views(DbName, [DDocName | Rest], Config) ->
         timeout ->
             ok
         end,
-        SnoozePeriod = config:get_integer("compaction_daemon", "snooze_period", 3),
-        ok = timer:sleep(SnoozePeriod * 1000);
+        ok = timer:sleep(get_snooze_period());
     false ->
         ok
     end.
@@ -597,4 +604,60 @@ abs_path2_test() ->
     ?assertEqual({ok, "/a/b/"}, abs_path2("/a/b")),
     ok.
 
+get_snooze_period_test_() ->
+    {
+        foreach,
+        fun() ->
+            meck:new(config, [passthrough])
+        end,
+        fun(_) ->
+            meck:unload()
+        end,
+        [
+            {"should return default value without config attributes",
+            fun should_default_without_config/0},
+            {"should respect old config attribute",
+            fun should_respect_old_config/0},
+            {"should respect old config set to zero",
+            fun should_respect_old_config_zero/0},
+            {"should respect new config attribute",
+            fun should_respect_new_config/0},
+            {"should respect new config set to zero",
+            fun should_respect_new_config_zero/0}
+        ]
+    }.
+
+should_default_without_config() ->
+    ?assertEqual(3000, get_snooze_period()).
+
+should_respect_old_config() ->
+    meck:expect(config, get_integer, fun
+        ("compaction_daemon", "snooze_period", _) -> 1;
+        (_, _, Default) -> Default
+    end),
+    ?assertEqual(1000, get_snooze_period()).
+
+should_respect_old_config_zero() ->
+    meck:expect(config, get_integer, fun
+        ("compaction_daemon", "snooze_period", _) -> 0;
+        (_, _, Default) -> Default
+    end),
+    ?assertEqual(0, get_snooze_period()).
+
+should_respect_new_config() ->
+    meck:expect(config, get_integer, fun
+        ("compaction_daemon", "snooze_period", _) -> 1;
+        ("compaction_daemon", "snooze_period_ms", _) -> 300;
+        (_, _, Default) -> Default
+    end),
+    ?assertEqual(300, get_snooze_period()).
+
+should_respect_new_config_zero() ->
+    meck:expect(config, get_integer, fun
+        ("compaction_daemon", "snooze_period", _) -> 1;
+        ("compaction_daemon", "snooze_period_ms", _) -> 0;
+        (_, _, Default) -> Default
+    end),
+    ?assertEqual(0, get_snooze_period()).
+
 -endif.

src/couch/test/couchdb_compaction_daemon_tests.erl

@@ -24,7 +24,7 @@
 start() ->
     Ctx = test_util:start_couch(),
     ok = config:set("compaction_daemon", "check_interval", "3", false),
-    ok = config:set("compaction_daemon", "snooze_period", "0", false),
+    ok = config:set("compaction_daemon", "snooze_period_ms", "0", false),
     ok = config:set("compaction_daemon", "min_file_size", "100000", false),
     ok = config:delete("compactions", "_default", false),
     ok = meck:new(?MODS_TO_MOCK, [passthrough]),