davisp commented on a change in pull request #610: Optimize ddoc cache URL: https://github.com/apache/couchdb/pull/610#discussion_r126492643
########## File path: src/ddoc_cache/src/ddoc_cache_entry.erl ########## @@ -0,0 +1,332 @@ +% Licensed under the Apache License, Version 2.0 (the "License"); you may not +% use this file except in compliance with the License. You may obtain a copy of +% the License at +% +% http://www.apache.org/licenses/LICENSE-2.0 +% +% Unless required by applicable law or agreed to in writing, software +% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT +% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the +% License for the specific language governing permissions and limitations under +% the License. + +-module(ddoc_cache_entry). +-behaviour(gen_server). + + +-export([ + dbname/1, + ddocid/1, + recover/1, + insert/2, + + start_link/2, + shutdown/1, + open/2, + accessed/1, + refresh/1 +]). + +-export([ + init/1, + terminate/2, + handle_call/3, + handle_cast/2, + handle_info/2, + code_change/3 +]). + +-export([ + do_open/1 +]). + + +-include("ddoc_cache.hrl"). + + +-record(st, { + key, + val, + opener, + waiters, + ts, + accessed +}). + + +dbname({Mod, Arg}) -> + Mod:dbname(Arg). + + +ddocid({Mod, Arg}) -> + Mod:ddocid(Arg). + + +recover({Mod, Arg}) -> + Mod:recover(Arg). + + +insert({Mod, Arg}, Value) -> + Mod:insert(Arg, Value). + + +start_link(Key, Default) -> + Pid = proc_lib:spawn_link(?MODULE, init, [{Key, Default}]), + {ok, Pid}. + + +shutdown(Pid) -> + ok = gen_server:call(Pid, shutdown). + + +open(Pid, Key) -> + try + Resp = gen_server:call(Pid, open), + case Resp of + {open_ok, Val} -> + Val; + {open_error, {T, R, S}} -> + erlang:raise(T, R, S) + end + catch exit:_ -> + % Its possible that this process was evicted just + % before we tried talking to it. Just fallback + % to a standard recovery + recover(Key) + end. + + +accessed(Pid) -> + gen_server:cast(Pid, accessed). + + +refresh(Pid) -> + gen_server:cast(Pid, force_refresh). + + +init({Key, undefined}) -> + true = ets:update_element(?CACHE, Key, {#entry.pid, self()}), + St = #st{ + key = Key, + opener = spawn_opener(Key), + waiters = [], + accessed = 1 + }, + ?EVENT(started, Key), + gen_server:enter_loop(?MODULE, [], St); + +init({Key, Default}) -> + Updates = [ + {#entry.val, Default}, + {#entry.pid, self()} + ], + NewTs = os:timestamp(), + true = ets:update_element(?CACHE, Key, Updates), + true = ets:insert(?LRU, {{NewTs, Key, self()}}), + St = #st{ + key = Key, + val = {open_ok, {ok, Default}}, + opener = start_timer(), + waiters = undefined, + ts = NewTs, + accessed = 1 + }, + ?EVENT(default_started, Key), + gen_server:enter_loop(?MODULE, [], St). + + +terminate(_Reason, St) -> + #st{ + key = Key, + opener = Pid, + ts = Ts + } = St, + % We may have already deleted our cache entry + % during shutdown + Pattern = #entry{key = Key, pid = self(), _ = '_'}, + CacheMSpec = [{Pattern, [], [true]}], + true = ets:select_delete(?CACHE, CacheMSpec) < 2, + % We may have already deleted our LRU entry + % during shutdown + if Ts == undefined -> ok; true -> + LruMSpec = [{{{Ts, Key, self()}}, [], [true]}], + true = ets:select_delete(?LRU, LruMSpec) < 2 + end, + % Blow away any current opener if it exists + if not is_pid(Pid) -> ok; true -> + catch exit(Pid, kill) + end, + ok. + + +handle_call(open, From, #st{val = undefined} = St) -> + NewSt = St#st{ + waiters = [From | St#st.waiters] + }, + {noreply, NewSt}; + +handle_call(open, _From, St) -> + {reply, St#st.val, St}; + +handle_call(shutdown, _From, St) -> + remove_from_cache(St), + {stop, normal, ok, St}; + +handle_call(Msg, _From, St) -> + {stop, {bad_call, Msg}, {bad_call, Msg}, St}. + + +handle_cast(accessed, St) -> + ?EVENT(accessed, St#st.key), + drain_accessed(), + NewSt = St#st{ + accessed = St#st.accessed + 1 + }, + {noreply, update_lru(NewSt)}; + +handle_cast(force_refresh, St) -> + % If we had frequent design document updates + % they could end up racing accessed events and + % end up prematurely evicting this entry from + % cache. To prevent this we just make sure that + % accessed is set to at least 1 before we + % execute a refresh. + NewSt = if St#st.accessed > 0 -> St; true -> + St#st{accessed = 1} + end, + handle_cast(refresh, NewSt); + +handle_cast(refresh, #st{accessed = 0} = St) -> + {stop, normal, St}; + +handle_cast(refresh, #st{opener = Ref} = St) when is_reference(Ref) -> + #st{ + key = Key + } = St, + erlang:cancel_timer(Ref), + NewSt = St#st{ + opener = spawn_opener(Key), + accessed = 0 + }, + {noreply, NewSt}; + +handle_cast(refresh, #st{opener = Pid} = St) when is_pid(Pid) -> + catch exit(Pid, kill), + receive + {'DOWN', _, _, Pid, _} -> ok + end, + NewSt = St#st{ + opener = spawn_opener(St#st.key), + accessed = 0 + }, + {noreply, NewSt}; + +handle_cast(Msg, St) -> + {stop, {bad_cast, Msg}, St}. + + +handle_info({'DOWN', _, _, Pid, Resp}, #st{key = Key, opener = Pid} = St) -> + case Resp of + {open_ok, Key, {ok, Val}} -> + update_cache(St, Val), + NewSt1 = St#st{ + val = {open_ok, {ok, Val}}, + opener = start_timer(), + waiters = undefined + }, + NewSt2 = update_lru(NewSt1), + if not is_list(St#st.waiters) -> ok; true -> + respond(St#st.waiters, {open_ok, {ok, Val}}) + end, + {noreply, NewSt2}; + {Status, Key, Other} -> + NewSt = St#st{ + val = {Status, Other}, + opener = undefined, + waiters = undefined + }, + remove_from_cache(NewSt), + if not is_list(St#st.waiters) -> ok; true -> + respond(St#st.waiters, {Status, Other}) + end, + {stop, normal, NewSt} + end; + +handle_info(Msg, St) -> + {stop, {bad_info, Msg}, St}. + + +code_change(_, St, _) -> + {ok, St}. + + +spawn_opener(Key) -> + {Pid, _} = erlang:spawn_monitor(?MODULE, do_open, [Key]), + Pid. + + +start_timer() -> + TimeOut = config:get_integer( + "ddoc_cache", "refresh_timeout", ?REFRESH_TIMEOUT), + erlang:send_after(TimeOut, self(), {'$gen_cast', refresh}). + + +do_open(Key) -> + try recover(Key) of + Resp -> + erlang:exit({open_ok, Key, Resp}) + catch T:R -> + S = erlang:get_stacktrace(), + erlang:exit({open_error, Key, {T, R, S}}) + end. + + +update_lru(#st{key = Key, ts = Ts} = St) -> + if Ts == undefined -> ok; true -> + MSpec = [{{{Ts, Key, self()}}, [], [true]}], + 1 = ets:select_delete(?LRU, MSpec) + end, + NewTs = os:timestamp(), + true = ets:insert(?LRU, {{NewTs, Key, self()}}), + St#st{ts = NewTs}. + + +update_cache(#st{val = undefined} = St, Val) -> + true = ets:update_element(?CACHE, St#st.key, {#entry.val, Val}), + ?EVENT(inserted, St#st.key); + +update_cache(#st{val = V1} = _St, V2) when {open_ok, {ok, V2}} == V1 -> + ?EVENT(update_noop, _St#st.key); + +update_cache(St, Val) -> + true = ets:update_element(?CACHE, St#st.key, {#entry.val, Val}), + ?EVENT(updated, {St#st.key, Val}). + + +remove_from_cache(St) -> + #st{ + key = Key, + ts = Ts + } = St, + Pattern = #entry{key = Key, pid = self(), _ = '_'}, + CacheMSpec = [{Pattern, [], [true]}], + 1 = ets:select_delete(?CACHE, CacheMSpec), + if Ts == undefined -> ok; true -> + LruMSpec = [{{{Ts, Key, self()}}, [], [true]}], + 1 = ets:select_delete(?LRU, LruMSpec) Review comment: Replying to all three of your ets comments in one place, the motivation for select_delete was to assert that we're deleting *exactly* one entry in each of these places. My recollection when I wrote this was that a select that matches keys exactly is either O(1) or O(log N) for set/ordered_set respectively. I'll write a small test script today to try and prove that's the case because I can't find it specifically written out in the docs for select. My reasoning was based on the docs for match: If the key is specified in the pattern, the match is very efficient. If the key is not specified, that is, if it is a variable or an underscore, the entire table must be searched. However there's no similar comment for select now that I go back and read so I'll do the experiment to see if it is indeed devolving to an O(N) operation. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on GitHub and use the URL above to go to the specific comment. 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