db_bench background work thread pool size arguments

Summary:
The background thread pools' sizes weren't easily configurable by `max_background_compactions` and `max_background_flushes` in multi-instance setups. Introduced separate arguments for their sizes.
Closes https://github.com/facebook/rocksdb/pull/2680

Differential Revision: D5550675

Pulled By: ajkr

fbshipit-source-id: bab5f0a7bc5db63bb084d0c10facbe437096367d

tools/db_bench_tool.cc

@@ -322,6 +322,14 @@ DEFINE_int32(num_bottom_pri_threads, 0,
              "The number of threads in the bottom-priority thread pool (used "
              "by universal compaction only).");
 
+DEFINE_int32(num_high_pri_threads, 0,
+             "The maximum number of concurrent background compactions"
+             " that can occur in parallel.");
+
+DEFINE_int32(num_low_pri_threads, 0,
+             "The maximum number of concurrent background compactions"
+             " that can occur in parallel.");
+
 DEFINE_int32(max_background_compactions,
              rocksdb::Options().max_background_compactions,
              "The maximum number of concurrent background compactions"
@@ -5241,13 +5249,14 @@ int db_bench_tool(int argc, char** argv) {
 
   FLAGS_rep_factory = StringToRepFactory(FLAGS_memtablerep.c_str());
 
-  // The number of background threads should be at least as much the
-  // max number of concurrent compactions.
-  FLAGS_env->SetBackgroundThreads(FLAGS_max_background_compactions);
-  FLAGS_env->SetBackgroundThreads(FLAGS_max_background_flushes,
+  // Note options sanitization may increase thread pool sizes according to
+  // max_background_flushes/max_background_compactions/max_background_jobs
+  FLAGS_env->SetBackgroundThreads(FLAGS_num_high_pri_threads,
                                   rocksdb::Env::Priority::HIGH);
   FLAGS_env->SetBackgroundThreads(FLAGS_num_bottom_pri_threads,
                                   rocksdb::Env::Priority::BOTTOM);
+  FLAGS_env->SetBackgroundThreads(FLAGS_num_low_pri_threads,
+                                  rocksdb::Env::Priority::LOW);
 
   // Choose a location for the test database if none given with --db=<path>
   if (FLAGS_db.empty()) {