From 60e29f2ff084e35e5e72164ced44857389fbd805 Mon Sep 17 00:00:00 2001
From: Martin Sumner <martin.sumner@adaptip.co.uk>
Date: Fri, 6 Mar 2020 11:29:25 +0000
Subject: [PATCH] (slightly) less random reads on journal compaction

---
 src/leveled_cdb.erl    | 54 ++++++++++++++++++++++++------------------
 src/leveled_iclerk.erl |  4 ++++
 src/leveled_log.erl    |  6 ++++-
 3 files changed, 40 insertions(+), 24 deletions(-)

diff --git a/src/leveled_cdb.erl b/src/leveled_cdb.erl
index 49418ff..f40a17b 100644
--- a/src/leveled_cdb.erl
+++ b/src/leveled_cdb.erl
@@ -130,6 +130,7 @@
 -define(DELETE_TIMEOUT, 10000).
 -define(TIMING_SAMPLECOUNTDOWN, 5000).
 -define(TIMING_SAMPLESIZE, 100).
+-define(GETPOS_FACTOR, 8).
 -define(MAX_OBJECT_SIZE, 1000000000). 
     % 1GB but really should be much smaller than this
 
@@ -266,29 +267,36 @@ cdb_getpositions(Pid, SampleSize) ->
     % requests waiting for this to complete, loop over each of the 256 indexes
     % outside of the FSM processing loop - to allow for other messages to be
     % interleaved
-    case SampleSize of
-        all ->
-            FoldFun = 
-                fun(Index, Acc) ->
-                    cdb_getpositions_fromidx(Pid, all, Index, Acc)
-                end,
-            IdxList = lists:seq(0, 255),
-            lists:foldl(FoldFun, [], IdxList);
-        S0 ->
-            FoldFun = 
-                fun({_R, Index}, Acc) ->
-                    case length(Acc) of
-                        S0 ->
-                            Acc;
-                        L when L < S0 ->
-                            cdb_getpositions_fromidx(Pid, S0, Index, Acc)
-                    end
-                end,
-            RandFun = fun(X) -> {leveled_rand:uniform(), X} end,
-            SeededL = lists:map(RandFun, lists:seq(0, 255)),
-            SortedL = lists:keysort(1, SeededL),
-            lists:foldl(FoldFun, [], SortedL)
-    end.
+    SW = os:timestamp(),
+    PosList =  
+        case SampleSize of
+            all ->
+                FoldFun = 
+                    fun(Index, Acc) ->
+                        cdb_getpositions_fromidx(Pid, all, Index, Acc)
+                    end,
+                IdxList = lists:seq(0, 255),
+                lists:foldl(FoldFun, [], IdxList);
+            S0 ->
+                FC = ?GETPOS_FACTOR * S0,
+                FoldFun = 
+                    fun({_R, Index}, Acc) ->
+                        case length(Acc) of
+                            FC ->
+                                Acc;
+                            L when L < FC ->
+                                cdb_getpositions_fromidx(Pid, FC, Index, Acc)
+                        end
+                    end,
+                RandFun = fun(X) -> {leveled_rand:uniform(), X} end,
+                SeededL = lists:map(RandFun, lists:seq(0, 255)),
+                SortedL = lists:keysort(1, SeededL),
+                PosList0 = lists:foldl(FoldFun, [], SortedL),
+                P1 = leveled_rand:uniform(max(1, length(PosList0) - S0)),
+                lists:sublist(lists:sort(PosList0), P1, S0)
+        end,
+    leveled_log:log_timer("CDB22", [length(PosList)], SW),
+    PosList.
 
 cdb_getpositions_fromidx(Pid, SampleSize, Index, Acc) ->
     gen_fsm:sync_send_event(Pid,
diff --git a/src/leveled_iclerk.erl b/src/leveled_iclerk.erl
index fb8a088..21ed1ba 100644
--- a/src/leveled_iclerk.erl
+++ b/src/leveled_iclerk.erl
@@ -508,6 +508,10 @@ schedule_compaction(CompactionHours, RunsPerDay, CurrentTS) ->
 check_single_file(CDB, FilterFun, FilterServer, MaxSQN, SampleSize, BatchSize) ->
     FN = leveled_cdb:cdb_filename(CDB),
     PositionList = leveled_cdb:cdb_getpositions(CDB, SampleSize),
+    AvgJump =
+        (lists:last(PositionList) - lists:nth(1, PositionList))
+            div length(PositionList),
+    leveled_log:log("IC014", [AvgJump]),
     KeySizeList = fetch_inbatches(PositionList, BatchSize, CDB, []),
     Score = 
         size_comparison_score(KeySizeList, FilterFun, FilterServer, MaxSQN),
diff --git a/src/leveled_log.erl b/src/leveled_log.erl
index 27344dc..cfc2791 100644
--- a/src/leveled_log.erl
+++ b/src/leveled_log.erl
@@ -356,6 +356,8 @@
     {"IC013",
         {warn, "File with name ~s to be ignored in manifest as scanning for "
                 ++ "first key returned empty - maybe corrupted"}},
+    {"IC014",
+        {info, "Fetching position list with average byte jump ~p"}},
 
     {"CDB01",
         {info, "Opening file for writing with filename ~s"}},
@@ -404,7 +406,9 @@
     {"CDB20",
         {warn, "Error ~w caught when safe reading a file to length ~w"}},
     {"CDB21",
-        {warn, "File ~s to be deleted but already gone"}}
+        {warn, "File ~s to be deleted but already gone"}},
+    {"CDB22",
+        {info, "Positions ~w fetch"}}
         
     ]).