nickva commented on a change in pull request #610: Optimize ddoc cache URL: https://github.com/apache/couchdb/pull/610#discussion_r126274885
########## 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: This looks like it could be a direct element delete by key, an O(log n) operation. I am not 100% sure but `select_delete` might be O(n) unless it is clever enough to see that the key is specified exactly. So it would be: ``` true = ets:delete(?LRU, {Ts, Key, self()}). ``` ---------------------------------------------------------------- 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. For queries about this service, please contact Infrastructure at: us...@infra.apache.org With regards, Apache Git Services
