Make arena block size configurable

Summary:
Add an option for arena block size, default value 4096 bytes. Arena will allocate blocks with such size.

I am not sure about passing parameter to skiplist in the new virtualized framework, though I talked to Jim a bit. So add Jim as reviewer.

Test Plan:
new unit test, I am running db_test.

For passing paramter from configured option to Arena, I tried tests like:

  TEST(DBTest, Arena_Option) {
  std::string dbname = test::TmpDir() + "/db_arena_option_test";
  DestroyDB(dbname, Options());

  DB* db = nullptr;
  Options opts;
  opts.create_if_missing = true;
  opts.arena_block_size = 1000000; // tested 99, 999999
  Status s = DB::Open(opts, dbname, &db);
  db->Put(WriteOptions(), "a", "123");
  }

and printed some debug info. The results look good. Any suggestion for such a unit-test?

Reviewers: haobo, dhruba, emayanke, jpaton

Reviewed By: dhruba

CC: leveldb, zshao

Differential Revision: https://reviews.facebook.net/D11799

include/leveldb/arena.h

@@ -0,0 +1,38 @@
+// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+//
+// Arena class defines memory allocation methods. It's used by memtable and
+// skiplist.
+
+#ifndef STORAGE_LEVELDB_INCLUDE_ARENA_H_
+#define STORAGE_LEVELDB_INCLUDE_ARENA_H_
+
+namespace leveldb {
+
+class Arena {
+ public:
+  Arena() {};
+  virtual ~Arena() {};
+
+  // Return a pointer to a newly allocated memory block of "bytes" bytes.
+  virtual char* Allocate(size_t bytes) = 0;
+
+  // Allocate memory with the normal alignment guarantees provided by malloc.
+  virtual char* AllocateAligned(size_t bytes) = 0;
+
+  // Returns an estimate of the total memory used by arena.
+  virtual const size_t ApproximateMemoryUsage() = 0;
+
+  // Returns the total number of bytes in all blocks allocated so far.
+  virtual const size_t MemoryAllocatedBytes() = 0;
+
+ private:
+  // No copying allowed
+  Arena(const Arena&);
+  void operator=(const Arena&);
+};
+
+}  // namespace leveldb
+
+#endif  // STORAGE_LEVELDB_INCLUDE_ARENA_H_

include/leveldb/memtablerep.h

@@ -13,6 +13,7 @@
 #define STORAGE_LEVELDB_DB_TABLE_H_
 
 #include <memory>
+#include "leveldb/arena.h"
 
 namespace leveldb {
 
@@ -35,10 +36,6 @@ class MemTableRep {
   // Returns true iff an entry that compares equal to key is in the collection.
   virtual bool Contains(const char* key) const = 0;
 
-  // Returns an estimate of the number of bytes of data in use by this
-  // data structure.
-  virtual size_t ApproximateMemoryUsage() = 0;
-
   virtual ~MemTableRep() { }
 
   // Iteration over the contents of a skip collection
@@ -83,7 +80,7 @@ class MemTableRepFactory {
  public:
   virtual ~MemTableRepFactory() { };
   virtual std::shared_ptr<MemTableRep> CreateMemTableRep(
-    MemTableRep::KeyComparator&) = 0;
+    MemTableRep::KeyComparator&, Arena* arena) = 0;
 };
 
 }

include/leveldb/options.h

@@ -375,6 +375,13 @@ struct Options {
   // Number of shards used for table cache.
   int table_cache_numshardbits;
 
+  // size of one block in arena memory allocation.
+  // If <= 0, a proper value is automatically calculated (usually 1/10 of
+  // writer_buffer_size).
+  //
+  // Default: 0
+  size_t arena_block_size;
+
   // Create an Options object with default values for all fields.
   Options();