add an iterator refresh option for SeekRandom

Summary: One more option to allow iterator refreshing when using normal iterator

Test Plan: ran db_bench

Reviewers: haobo, sdong, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18849

db/db_bench.cc

@@ -465,6 +465,8 @@ static auto FLAGS_compaction_fadvice_e =
 
 DEFINE_bool(use_tailing_iterator, false,
             "Use tailing iterator to access a series of keys instead of get");
+DEFINE_int64(iter_refresh_interval_us, -1,
+             "How often to refresh iterators. Disable refresh when -1");
 
 DEFINE_bool(use_adaptive_mutex, rocksdb::Options().use_adaptive_mutex,
             "Use adaptive mutex");
@@ -2009,12 +2011,31 @@ class Benchmark {
         multi_iters.push_back(db->NewIterator(options));
       }
     }
+    uint64_t last_refresh = FLAGS_env->NowMicros();
 
     Slice key = AllocateKey();
     std::unique_ptr<const char[]> key_guard(key.data());
 
     Duration duration(FLAGS_duration, reads_);
     while (!duration.Done(1)) {
+      if (!FLAGS_use_tailing_iterator && FLAGS_iter_refresh_interval_us >= 0) {
+        uint64_t now = FLAGS_env->NowMicros();
+        if (now - last_refresh > (uint64_t)FLAGS_iter_refresh_interval_us) {
+          if (db_ != nullptr) {
+            delete single_iter;
+            single_iter = db_->NewIterator(options);
+          } else {
+            for (auto iter : multi_iters) {
+              delete iter;
+            }
+            multi_iters.clear();
+            for (DB* db : multi_dbs_) {
+              multi_iters.push_back(db->NewIterator(options));
+            }
+          }
+        }
+        last_refresh = now;
+      }
       // Pick a Iterator to use
       Iterator* iter_to_use = single_iter;
       if (single_iter == nullptr) {