Add -report_open_timing to db_bench (#8464)

Summary:
Hello and thanks for RocksDB,

This PR adds support for ```-report_open_timing true``` to ```db_bench```.
It can be useful when tuning RocksDB on filesystem/env with high latencies for file level operations (create/delete/rename...) seen during ```((Optimistic)Transaction)DB::Open```.

Some examples:

```
> db_bench -benchmarks updaterandom -num 1 -db /dev/shm/db_bench
> db_bench -benchmarks updaterandom -num 0 -db /dev/shm/db_bench -use_existing_db true -report_open_timing true -readonly true 2>&1 | grep OpenDb
OpenDb:     3.90133 milliseconds
> db_bench -benchmarks updaterandom -num 0 -db /dev/shm/db_bench -use_existing_db true -report_open_timing true -use_secondary_db true 2>&1 | grep OpenDb
OpenDb:     3.33414 milliseconds
> db_bench -benchmarks updaterandom -num 0 -db /dev/shm/db_bench -use_existing_db true -report_open_timing true 2>&1 | grep -A1 OpenDb
OpenDb:     6.05423 milliseconds

> db_bench -benchmarks updaterandom -num 1
> db_bench -benchmarks updaterandom -num 0 -use_existing_db true -report_open_timing true -readonly true 2>&1 | grep OpenDb
OpenDb:     4.06859 milliseconds
> db_bench -benchmarks updaterandom -num 0 -use_existing_db true -report_open_timing true -use_secondary_db true 2>&1 | grep OpenDb
OpenDb:     2.85794 milliseconds
> db_bench -benchmarks updaterandom -num 0 -use_existing_db true -report_open_timing true 2>&1 | grep OpenDb
OpenDb:     6.46376 milliseconds

> db_bench -benchmarks updaterandom -num 1 -db /clustered_fs/db_bench
> db_bench -benchmarks updaterandom -num 0 -db /clustered_fs/db_bench -use_existing_db true -report_open_timing true -readonly true 2>&1 | grep OpenDb
OpenDb:     3.79805 milliseconds
> db_bench -benchmarks updaterandom -num 0 -db /clustered_fs/db_bench -use_existing_db true -report_open_timing true -use_secondary_db true 2>&1 | grep OpenDb
OpenDb:     3.00174 milliseconds
> db_bench -benchmarks updaterandom -num 0 -db /clustered_fs/db_bench -use_existing_db true -report_open_timing true 2>&1 | grep OpenDb
OpenDb:     24.8732 milliseconds
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8464

Reviewed By: hx235

Differential Revision: D29398096

Pulled By: zhichao-cao

fbshipit-source-id: 8f05dc3284f084612a3f30234e39e1c37548f50c

tools/db_bench_tool.cc

@@ -1410,6 +1410,7 @@ DEFINE_int32(skip_list_lookahead, 0, "Used with skip_list memtablerep; try "
              "position");
 DEFINE_bool(report_file_operations, false, "if report number of file "
             "operations");
+DEFINE_bool(report_open_timing, false, "if report open timing");
 DEFINE_int32(readahead_size, 0, "Iterator readahead size");
 
 DEFINE_bool(read_with_latest_user_timestamp, true,
@@ -4401,6 +4402,7 @@ class Benchmark {
 
   void OpenDb(Options options, const std::string& db_name,
       DBWithColumnFamilies* db) {
+    uint64_t open_start = FLAGS_report_open_timing ? FLAGS_env->NowNanos() : 0;
     Status s;
     // Open with column families if necessary.
     if (FLAGS_num_column_families > 1) {
@@ -4541,6 +4543,11 @@ class Benchmark {
     } else {
       s = DB::Open(options, db_name, &db->db);
     }
+    if (FLAGS_report_open_timing) {
+      std::cout << "OpenDb:     "
+                << (FLAGS_env->NowNanos() - open_start) / 1000000.0
+                << " milliseconds\n";
+    }
     if (!s.ok()) {
       fprintf(stderr, "open error: %s\n", s.ToString().c_str());
       exit(1);