Add arena_block_size flag to db_bench (#7654)

Summary:
db_bench currently does not allow overriding the default `arena_block_size `calculation ([memtable size/8](https://github.com/facebook/rocksdb/blob/master/db/column_family.cc#L216)). For memtables whose size is in gigabytes, the `arena_block_size` defaults to hundreds of megabytes (affecting performance).

Exposing this option in db_bench would allow us to test the workloads with various `arena_block_size` values.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7654

Reviewed By: jay-zhuang

Differential Revision: D24996812

Pulled By: ajkr

fbshipit-source-id: a5e3d2c83d9f89e1bb8382f2e8dd476c79e33bef

tools/db_bench_tool.cc

@@ -323,6 +323,9 @@ DEFINE_int64(db_write_buffer_size,
 DEFINE_bool(cost_write_buffer_to_cache, false,
             "The usage of memtable is costed to the block cache");
 
+DEFINE_int64(arena_block_size, ROCKSDB_NAMESPACE::Options().arena_block_size,
+             "The size, in bytes, of one block in arena memory allocation.");
+
 DEFINE_int64(write_buffer_size, ROCKSDB_NAMESPACE::Options().write_buffer_size,
              "Number of bytes to buffer in memtable before compacting");
 
@@ -3627,6 +3630,7 @@ class Benchmark {
       options.write_buffer_manager.reset(
           new WriteBufferManager(FLAGS_db_write_buffer_size, cache_));
     }
+    options.arena_block_size = FLAGS_arena_block_size;
     options.write_buffer_size = FLAGS_write_buffer_size;
     options.max_write_buffer_number = FLAGS_max_write_buffer_number;
     options.min_write_buffer_number_to_merge =