Two memory management helpers

Two helpers for memory management:

1 - a scan over the cdb file may lead to a lot of binary references being made.  So force a GC fater the scan.

2 - the penciller files contain slots that will be frequently read - so advice the page cache to pre-load them on startup.

This is in response to unexpected memory mangement issues in a potentially non-conventional setup - where the erlang VM held a lot of memory (that could be GC'd , in preference to the page cache - and consequently disk I/O and request latency were higher than expected.

src/leveled_cdb.erl

@@ -1301,6 +1301,7 @@ scan_over_file(Handle, Position, FilterFun, Output, LastKey) ->
             end,
             % Bring file back to that position
             {ok, Position} = file:position(Handle, {bof, Position}),
+            garbage_collect(),
             {eof, Output};
         {Key, ValueAsBin, KeyLength, ValueLength} ->
             NewPosition = case Key of
@@ -1316,10 +1317,12 @@ scan_over_file(Handle, Position, FilterFun, Output, LastKey) ->
                             Output,
                             fun extract_valueandsize/1) of
                 {stop, UpdOutput} ->
+                    garbage_collect(),
                     {Position, UpdOutput};
                 {loop, UpdOutput} ->
                     case NewPosition of
                         eof ->
+                            garbage_collect(),
                             {eof, UpdOutput};
                         _ ->
                             scan_over_file(Handle,

src/leveled_sst.erl

@@ -1277,6 +1277,7 @@ open_reader(Filename) ->
     <<FileVersion:8/integer, 
         SlotsLength:32/integer, 
         SummaryLength:32/integer>> = Lengths,
+    ok = file:advise(Handle, 9, SlotsLength, will_need),
     {ok, SummaryBin} = file:pread(Handle, SlotsLength + 9, SummaryLength),
     {Handle, FileVersion, SummaryBin}.