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Subject [couchdb] 02/02: Opimize writing btree nodes
Date Fri, 12 May 2017 16:05:58 GMT
This is an automated email from the ASF dual-hosted git repository.

davisp pushed a commit to branch COUCHDB-3298-optimize-writing-btree-nodes
in repository

commit fa38bd89c78c13a017e0625df252437ac5ebab74
Author: Paul J. Davis <>
AuthorDate: Wed May 3 12:27:08 2017 -0500

    Opimize writing btree nodes
    As it turns out, the original change in COUCHDB-3298 ends up hurting
    disk usage when a view emits large amounts of data (i.e., more than
    half of the btree chunk size). The cause for this is that instead of
    writing single element nodes it would instead prefer to write kv nodes
    with three elements. While normally we might prefer this in memory, it
    turns out that our append only storage this causes a significantly more
    amount of trash on disk.
    We can show this with a few trivial examples. Imagine we write KV's a
    through f. The two following patterns show the nodes as we write each
    new kv.
        Before 3298:
        [a, b]
        [a, b]', [c]
        [a, b]', [c, d]
        [a, b]', [c, d]', [e]
        [a, b]', [c, d]', [e, f]
        After 3298:
        [a, b]
        [a, b, c]
        [a, b]', [c, d]
        [a, b]', [c, d, e]
        [a, b]', [c, d]', [e, f]
    The thing to realize here is which of these nodes end up as garbage. In
    the first example we end up with [a], [a, b], [c], [c, d], and [e] nodes
    that have been orphaned. Where as in the second case we end up with
    [a], [a, b], [a, b, c], [c, d], [c, d, e] as nodes that have been
    orphaned. A quick aside, the reason that [a, b] and [c, d] are orphaned
    is due to how a btree update works. For instance, when adding c, we read
    [a, b] into memory, append c, and then during our node write we call
    chunkify which gives us back [a, b], [c] which leads us to writing [a,
    b] a second time.
    The main benefit of this patch is to realize when its possible to reuse
    a node that already exists on disk. It achieves this by looking at the
    list of key/values when writing new nodes and comparing it to the old
    list of key/values for the node read from disk. By checking to see if
    the old list exists unchanged in the new list we can just reuse the old
    node. Node reuse is limited to when the old node is larger than 50% of
    the chunk threshold to maintain the B+Tree properties.
    The disk usage improvements this gives can also be quite dramatic. In
    the case above when we have ordered keys with large values (> 50% of the
    btree chunk size) we find upwards of 50% less disk usage. Random keys
    also benefit as well though to a lesser extent depending on disk size
    (as they will often be in the middle of an existing node which prevents
    our optimization).
 src/couch/src/couch_btree.erl | 64 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 63 insertions(+), 1 deletion(-)

diff --git a/src/couch/src/couch_btree.erl b/src/couch/src/couch_btree.erl
index d61daf1..ea224b1 100644
--- a/src/couch/src/couch_btree.erl
+++ b/src/couch/src/couch_btree.erl
@@ -19,6 +19,8 @@
+-define(FILL_RATIO, 0.5).
 extract(#btree{extract_kv=undefined}, Value) ->
 extract(#btree{extract_kv=Extract}, Value) ->
@@ -398,7 +400,14 @@ modify_node(Bt, RootPointerInfo, Actions, QueryOutput) ->
         {LastKey, _LastValue} = element(tuple_size(NodeTuple), NodeTuple),
         {ok, [{LastKey, RootPointerInfo}], QueryOutput2};
     _Else2 ->
-        {ok, ResultList} = write_node(Bt, NodeType, NewNodeList),
+        {ok, ResultList} = case RootPointerInfo of
+        nil ->
+            write_node(Bt, NodeType, NewNodeList);
+        _ ->
+            {LastKey, _LastValue} = element(tuple_size(NodeTuple), NodeTuple),
+            OldNode = {LastKey, RootPointerInfo},
+            write_node(Bt, OldNode, NodeType, NodeList, NewNodeList)
+        end,
         {ok, ResultList, QueryOutput2}
@@ -442,6 +451,59 @@ write_node(#btree{fd = Fd, compression = Comp} = Bt, NodeType, NodeList)
     {ok, ResultList}.
+write_node(Bt, _OldNode, NodeType, [], NewList) ->
+    write_node(Bt, NodeType, NewList);
+write_node(Bt, _OldNode, NodeType, [_], NewList) ->
+    write_node(Bt, NodeType, NewList);
+write_node(Bt, OldNode, NodeType, OldList, NewList) ->
+    case can_reuse_old_node(OldList, NewList) of
+        {true, Prefix, Suffix} ->
+            {ok, PrefixKVs} = case Prefix of
+                [] -> {ok, []};
+                _ -> write_node(Bt, NodeType, Prefix)
+            end,
+            {ok, SuffixKVs} = case Suffix of
+                [] -> {ok, []};
+                _ -> write_node(Bt, NodeType, Suffix)
+            end,
+            Result = PrefixKVs ++ [OldNode] ++ SuffixKVs,
+            {ok, Result};
+        false ->
+            write_node(Bt, NodeType, NewList)
+    end.
+can_reuse_old_node(OldList, NewList) ->
+    {Prefix, RestNewList} = remove_prefix_kvs(hd(OldList), NewList),
+    case old_list_is_prefix(OldList, RestNewList, 0) of
+        {true, Size, Suffix} ->
+            ReuseThreshold = get_chunk_size() * ?FILL_RATIO,
+            if Size < ReuseThreshold -> false; true ->
+                {true, Prefix, Suffix}
+            end;
+        false ->
+            false
+    end.
+remove_prefix_kvs(KV1, [KV2 | Rest]) when KV2 < KV1 ->
+    {Prefix, RestNewList} = remove_prefix_kvs(KV1, Rest),
+    {[KV2 | Prefix], RestNewList};
+remove_prefix_kvs(_, RestNewList) ->
+    {[], RestNewList}.
+% No more KV's in the old node so its a prefix
+old_list_is_prefix([], Suffix, Size) ->
+    {true, Size, Suffix};
+% Some KV's have been removed from the old node
+old_list_is_prefix(_OldList, [], _Size) ->
+    false;
+% KV is equal in both old and new node so continue
+old_list_is_prefix([KV | Rest1], [KV | Rest2], Acc) ->
+    old_list_is_prefix(Rest1, Rest2, ?term_size(KV) + Acc);
+% KV mismatch between old and new node so not a prefix
+old_list_is_prefix(_OldList, _NewList, _Acc) ->
+    false.
 modify_kpnode(Bt, {}, _LowerBound, Actions, [], QueryOutput) ->
     modify_node(Bt, nil, Actions, QueryOutput);
 modify_kpnode(_Bt, NodeTuple, LowerBound, [], ResultNode, QueryOutput) ->

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