Add a small cache to SST file

so that a HEAD which folllows a HEAD (e.g. when a GET follows a HEAD) has a chance of avoiding the binary_to_term CPU load

src/leveled_log.erl

@@ -226,10 +226,11 @@
     {"SST12",
         {info, "SST Timings for sample_count=~w"
                 ++ " at timing points index_query_time=~w"
-                ++ " lookup_cache_time=~w slot_index_time=~w slot_fetch_time=~w"
+                ++ " lookup_cache_time=~w slot_index_time=~w "
+                ++ " fetch_cache_time=~w slot_fetch_time=~w"
                 ++ " noncached_block_fetch_time=~w"
                 ++ " exiting at points slot_index=~w"
-                ++ " slot_fetch=~w noncached_block_fetch=~w"}},
+                ++ " fetch_cache=~w slot_fetch=~w noncached_block_fetch=~w"}},
     
     
     {"I0001",

src/leveled_sst.erl

@@ -78,6 +78,7 @@
 -define(TREE_SIZE, 4).
 -define(TIMING_SAMPLECOUNTDOWN, 10000).
 -define(TIMING_SAMPLESIZE, 100).
+-define(CACHE_SIZE, 32).
 
 -include_lib("eunit/include/eunit.hrl").
 
@@ -142,17 +143,20 @@
                     blockindex_cache,
                     compression_method = native :: press_methods(),
                     timings = no_timing :: sst_timings(),
-                    timings_countdown = 0 :: integer()}).
+                    timings_countdown = 0 :: integer(),
+                    fetch_cache = array:new([{size, ?CACHE_SIZE}])}).
 
 -record(sst_timings, 
                 {sample_count = 0 :: integer(),
                     index_query_time = 0 :: integer(),
                     lookup_cache_time = 0 :: integer(),
                     slot_index_time = 0 :: integer(),
+                    fetch_cache_time = 0 :: integer(),
                     slot_fetch_time = 0 :: integer(),
                     noncached_block_time = 0 :: integer(),
                     lookup_cache_count = 0 :: integer(),
                     slot_index_count = 0 :: integer(),
+                    fetch_cache_count = 0 :: integer(),
                     slot_fetch_count = 0 :: integer(),
                     noncached_block_count = 0 :: integer()}).
 
@@ -672,6 +676,17 @@ fetch(LedgerKey, Hash, State, Timings0) ->
                 _ ->
                     {SW3, Timings3} =
                         update_timings(SW2, Timings2, slot_index, true),
+                    FetchCache = State#state.fetch_cache,
+                    CacheHash = cache_hash(Hash),
+                    case array:get(CacheHash, FetchCache) of 
+                        {LedgerKey, V} ->
+                            {_SW4, Timings4} = 
+                                update_timings(SW3, 
+                                                Timings3, 
+                                                fetch_cache, 
+                                                false),
+                            {{LedgerKey, V}, State, Timings4};
+                        _ ->
                             StartPos = Slot#slot_index_value.start_position,
                             Result = 
                                 check_blocks(PosList,
@@ -681,9 +696,17 @@ fetch(LedgerKey, Hash, State, Timings0) ->
                                                 LedgerKey, 
                                                 PressMethod,
                                                 not_present),
+                            FetchCache0 = 
+                                array:set(CacheHash, Result, FetchCache),
                             {_SW4, Timings4} = 
-                        update_timings(SW3, Timings3, slot_fetch, false),
+                                update_timings(SW3, 
-                    {Result, State, Timings4}
+                                                Timings3, 
+                                                slot_fetch, 
+                                                false),
+                            {Result, 
+                                State#state{fetch_cache = FetchCache0}, 
+                                Timings4}
+                    end
             end 
     end.
 
@@ -1471,6 +1494,11 @@ extra_hash({SegHash, _ExtraHash}) when is_integer(SegHash) ->
 extra_hash(NotHash) ->
     NotHash.
 
+cache_hash({_SegHash, ExtraHash}) when is_integer(ExtraHash) ->
+    ExtraHash band (?CACHE_SIZE - 1);
+cache_hash(_NotHash) ->
+    0.
+
 tune_hash(SegHash) ->
     SegHash band 32767.
 
@@ -1810,9 +1838,11 @@ log_timings(Timings) ->
                                 Timings#sst_timings.index_query_time,
                                 Timings#sst_timings.lookup_cache_time,
                                 Timings#sst_timings.slot_index_time,
+                                Timings#sst_timings.fetch_cache_time,
                                 Timings#sst_timings.slot_fetch_time,
                                 Timings#sst_timings.noncached_block_time,
                                 Timings#sst_timings.slot_index_count,
+                                Timings#sst_timings.fetch_cache_count,
                                 Timings#sst_timings.slot_fetch_count,
                                 Timings#sst_timings.noncached_block_count]).
 
@@ -1832,6 +1862,9 @@ update_timings(SW, Timings, Stage, Continue) ->
             slot_index ->
                 SIT = Timings#sst_timings.slot_index_time,
                 Timings#sst_timings{slot_index_time = SIT + Timer};
+            fetch_cache ->
+                FCT = Timings#sst_timings.fetch_cache_time,
+                Timings#sst_timings{fetch_cache_time = FCT + Timer};
             slot_fetch ->
                 SFT = Timings#sst_timings.slot_fetch_time,
                 Timings#sst_timings{slot_fetch_time = SFT + Timer};
@@ -1848,6 +1881,9 @@ update_timings(SW, Timings, Stage, Continue) ->
                     slot_index ->
                         SIC = Timings#sst_timings.slot_index_count,
                         Timings0#sst_timings{slot_index_count = SIC + 1};
+                    fetch_cache ->
+                        FCC = Timings#sst_timings.fetch_cache_count,
+                        Timings0#sst_timings{fetch_cache_count = FCC + 1};
                     slot_fetch ->
                         SFC = Timings#sst_timings.slot_fetch_count,
                         Timings0#sst_timings{slot_fetch_count = SFC + 1};