Doubly skiplist for reverse scan (#116)

Signed-off-by: Little-Wallace liuwei@pingcap.com When the iterator read keys in reverse order, each Prev() function cost O(log n) times. So I add prev pointer for every node in skiplist to improve the Prev() function.
tikv · Sep 19, 2019 · eb7c79f · eb7c79f
1 parent c142c93
commit eb7c79f
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Showing 6 changed files with 1,207 additions and 46 deletions.
diff --git a/db/c.cc b/db/c.cc
@@ -2635,6 +2635,12 @@ void rocksdb_options_set_memtable_huge_page_size(rocksdb_options_t* opt,
   opt->rep.memtable_huge_page_size = v;
 }
 
+void rocksdb_options_set_doubly_skip_list_rep(rocksdb_options_t *opt) {
+  rocksdb::MemTableRepFactory* factory = new rocksdb::DoublySkipListFactory();
+  opt->rep.memtable_factory.reset(factory);
+}
+
+
 void rocksdb_options_set_hash_skip_list_rep(
     rocksdb_options_t *opt, size_t bucket_count,
     int32_t skiplist_height, int32_t skiplist_branching_factor) {

diff --git a/include/rocksdb/c.h b/include/rocksdb/c.h
@@ -966,6 +966,7 @@ extern ROCKSDB_LIBRARY_API void rocksdb_options_set_hash_skip_list_rep(
     rocksdb_options_t*, size_t, int32_t, int32_t);
 extern ROCKSDB_LIBRARY_API void rocksdb_options_set_hash_link_list_rep(
     rocksdb_options_t*, size_t);
+extern ROCKSDB_LIBRARY_API void rocksdb_options_set_doubly_skip_list_rep(rocksdb_options_t *opt);
 extern ROCKSDB_LIBRARY_API void rocksdb_options_set_plain_table_factory(
     rocksdb_options_t*, uint32_t, int, double, size_t);
 

diff --git a/include/rocksdb/memtablerep.h b/include/rocksdb/memtablerep.h
@@ -300,7 +300,26 @@ class SkipListFactory : public MemTableRepFactory {
 
   bool CanHandleDuplicatedKey() const override { return true; }
 
- private:
+private:
+  const size_t lookahead_;
+};
+
+// This uses a doubly skip list to store keys, which is similar to skip list, but optimize for prev seek.
+class DoublySkipListFactory : public MemTableRepFactory {
+ public:
+  explicit DoublySkipListFactory(size_t lookahead = 0) : lookahead_(lookahead) {}
+
+  using MemTableRepFactory::CreateMemTableRep;
+  virtual MemTableRep* CreateMemTableRep(const MemTableRep::KeyComparator&,
+                                 Allocator*, const SliceTransform*,
+                                 Logger* logger) override;
+  const char* Name() const override { return "DoublySkipListFactory"; }
+
+  bool IsInsertConcurrentlySupported() const override { return true; }
+
+  bool CanHandleDuplicatedKey() const override { return true; }
+
+private:
   const size_t lookahead_;
 };