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

util/options.cc

@@ -61,6 +61,7 @@ Options::Options()
       max_manifest_file_size(std::numeric_limits<uint64_t>::max()),
       no_block_cache(false),
       table_cache_numshardbits(4),
+      arena_block_size(0),
       disable_auto_compactions(false),
       WAL_ttl_seconds(0),
       manifest_preallocation_size(4 * 1024 * 1024),
@@ -174,6 +175,8 @@ Options::Dump(Logger* log) const
         no_block_cache);
     Log(log,"               Options.table_cache_numshardbits: %d",
         table_cache_numshardbits);
+    Log(log,"                       Options.arena_block_size: %ld",
+        arena_block_size);
     Log(log,"    Options.delete_obsolete_files_period_micros: %ld",
         delete_obsolete_files_period_micros);
     Log(log,"             Options.max_background_compactions: %d",