db_bench add an option of --base_background_compactions

Summary: As title.

Test Plan: Run the benchmark with and without the parameter.

Reviewers: yiwu, andrewkr, lightmark, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61491

tools/db_bench_tool.cc

@@ -303,6 +303,11 @@ DEFINE_int32(max_background_compactions,
              "The maximum number of concurrent background compactions"
              " that can occur in parallel.");
 
+DEFINE_int32(base_background_compactions,
+             rocksdb::Options().base_background_compactions,
+             "The base number of concurrent background compactions"
+             " to occur in parallel.");
+
 DEFINE_uint64(subcompactions, 1,
               "Maximum number of subcompactions to divide L0-L1 compactions "
               "into.");
@@ -2524,6 +2529,7 @@ class Benchmark {
       FLAGS_min_write_buffer_number_to_merge;
     options.max_write_buffer_number_to_maintain =
         FLAGS_max_write_buffer_number_to_maintain;
+    options.base_background_compactions = FLAGS_base_background_compactions;
     options.max_background_compactions = FLAGS_max_background_compactions;
     options.max_subcompactions = static_cast<uint32_t>(FLAGS_subcompactions);
     options.max_background_flushes = FLAGS_max_background_flushes;