From c8964e955c368d6414213eec7f762aa43c1f160d Mon Sep 17 00:00:00 2001
From: Kresten Krab Thorup <krab@trifork.com>
Date: Tue, 1 May 2012 16:27:06 +0200
Subject: [PATCH] Config option {merge_strategy, fast|predictable}

Both options have same log2(N) upper bound on
latencies, but `fast' fluctuates.
---
 README.md           | 13 ++++++++++---
 src/hanoi_level.erl | 14 +++++++++++---
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index 99c1dee..3e16bd8 100644
--- a/README.md
+++ b/README.md
@@ -38,9 +38,16 @@ Put these values in your `app.config` in the `hanoi` section
           {data_root, "./data/hanoi"},
           {compress, none | snappy | gzip},
           {sync_strategy, none | sync | {seconds, N}},
-          {page_size, 8192}
-          {write_buffer_size, 524288}  % 512kB
-          {read_buffer_size, 524288}  % 512kB
+          {page_size, 8192},
+          {write_buffer_size, 524288},  % 512kB
+          {read_buffer_size, 524288},  % 512kB
+
+          %%
+          %% The merge strategy is one of `fast' or `predictable'.
+          %% Both have same log2(N) worst case, but `fast' is
+          %% sometimes faster; yielding latency fluctuations.
+          %%
+          {merge_strategy, fast | predictable}
          ]},
 ```
 
diff --git a/src/hanoi_level.erl b/src/hanoi_level.erl
index 35bab1d..a146db3 100644
--- a/src/hanoi_level.erl
+++ b/src/hanoi_level.erl
@@ -514,7 +514,7 @@ main_loop(State = #state{ next=Next }) ->
                         {ok, PID} = ?MODULE:open(State#state.dir, State#state.level + 1, undefined,
                                                  State#state.opts, State#state.owner ),
                         State#state.owner ! { bottom_level, State#state.level + 1 },
-                        State#state{ next=PID };
+                        State#state{ next=PID, max_level= State#state.level+1 };
                    true ->
                         State
                 end,
@@ -582,10 +582,18 @@ do_step(StepFrom, PreviousWork, State) ->
        true ->
             WorkLeftHere = 0
     end,
+    WorkUnit      = ?BTREE_SIZE(?TOP_LEVEL),
     MaxLevel      = max(State#state.max_level, State#state.level),
-    TotalWork     = (MaxLevel-?TOP_LEVEL+1) * ?BTREE_SIZE(?TOP_LEVEL),
+    TotalWork     = (MaxLevel-?TOP_LEVEL+1) * WorkUnit,
     WorkUnitsLeft = max(0, TotalWork-PreviousWork),
-    WorkToDoHere  = min(WorkLeftHere, WorkUnitsLeft),
+
+    case hanoi:get_opt( merge_strategy, State#state.opts, predictable) of
+        fast ->
+            WorkToDoHere = min(WorkLeftHere, WorkUnitsLeft);
+        predictable ->
+            WorkToDoHere = min(WorkLeftHere, WorkUnit)
+    end,
+
     WorkIncludingHere  = PreviousWork + WorkToDoHere,
 
     ?log("do_step prev:~p, do:~p of ~p ~n", [PreviousWork, WorkToDoHere, WorkLeftHere]),