optimise resource lookup

This is worth ~2% in "MCM -a" when running with two Erlang schedulers,
and drops rabbit_misc:dirty_read/1 from #21 to #49 in the fprof
analysis of the publisher channel.

2 files changed, 10 insertions, 8 deletions

diff --git a/src/rabbit_misc.erl b/src/rabbit_misc.erl
index b1cf45e7fc..13a553f124 100644
--- a/src/rabbit_misc.erl
+++ b/src/rabbit_misc.erl
@@ -257,8 +257,15 @@ val({Type, Value}) ->
           end,
     lists:flatten(io_lib:format(Fmt, [Value, Type])).
 
-dirty_read(ReadSpec) ->
-    case mnesia:dirty_read(ReadSpec) of
+%% Normally we'd call mnesia:dirty_read/1 here, but that is quite
+%% expensive due to general mnesia overheads (figuring out table types
+%% and locations, etc). We get away with bypassing these because we
+%% know that the tables we are looking at here
+%% - are not the schema table
+%% - have a local ram copy
+%% - do not have any indices
+dirty_read({Table, Key}) ->
+    case ets:lookup(Table, Key) of
         [Result] -> {ok, Result};
         []       -> {error, not_found}
     end.
diff --git a/src/rabbit_router.erl b/src/rabbit_router.erl
index 36fdb203ce..e9c4479a63 100644
--- a/src/rabbit_router.erl
+++ b/src/rabbit_router.erl
@@ -108,12 +108,7 @@ check_delivery(_   , true, {_    , []}) -> {not_delivered, []};
 check_delivery(_   , _   , {_    , Qs}) -> {routed, Qs}.
 
 %% Normally we'd call mnesia:dirty_read/1 here, but that is quite
-%% expensive due to general mnesia overheads (figuring out table types
-%% and locations, etc). We get away with bypassing these because we
-%% know that the table
-%% - is not the schema table
-%% - has a local ram copy
-%% - does not have any indices
+%% expensive for the reasons explained in rabbit_misc:dirty_read/1.
 lookup_qpids(QNames) ->
     lists:foldl(fun (QName, QPids) ->
                         case ets:lookup(rabbit_queue, QName) of