Small improvements to DBGet microbenchmark (#11498)

Summary:
Follow a couple best practices:

- Allowed Google benchmark to decide number of iterations. Previously we hardcoded a value, which circumvented benchmark's heuristic for iterating until the result is stable.
- Made each iteration do similar work. Previously, an iteration could do different work depending if the key was found in the first, second, third, or no L0 file.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11498

Test Plan: none as I am unable to prove it is better

Reviewed By: hx235

Differential Revision: D46339050

Pulled By: ajkr

fbshipit-source-id: fcfc6da4111c5b3ae86d79d908afc5f61f96675b

microbench/db_basic_bench.cc

@@ -571,36 +571,34 @@ static void DBGet(benchmark::State& state) {
   options.table_factory.reset(NewBlockBasedTableFactory(table_options));
 
   auto rnd = Random(301 + state.thread_index());
-  KeyGenerator kg(&rnd, key_num);
 
   if (state.thread_index() == 0) {
+    KeyGenerator kg_seq(key_num /* max_key */);
     SetupDB(state, options, &db, "DBGet");
 
-    // load db
+    // Load all valid keys into DB. That way, iterations in `!negative_query`
+    // runs can always find the key even though it is generated from a random
+    // number.
     auto wo = WriteOptions();
     wo.disableWAL = true;
     for (uint64_t i = 0; i < key_num; i++) {
-      Status s = db->Put(wo, kg.Next(),
+      Status s = db->Put(wo, kg_seq.Next(),
                          rnd.RandomString(static_cast<int>(per_key_size)));
       if (!s.ok()) {
         state.SkipWithError(s.ToString().c_str());
       }
     }
 
-    FlushOptions fo;
-    Status s = db->Flush(fo);
+    // Compact whole DB into one level, so each iteration will consider the same
+    // number of files (one).
+    Status s = db->CompactRange(CompactRangeOptions(), nullptr /* begin */,
+                                nullptr /* end */);
     if (!s.ok()) {
       state.SkipWithError(s.ToString().c_str());
     }
-
-    auto db_full = static_cast_with_check<DBImpl>(db.get());
-    s = db_full->WaitForCompact(WaitForCompactOptions());
-    if (!s.ok()) {
-      state.SkipWithError(s.ToString().c_str());
-      return;
-    }
   }
 
+  KeyGenerator kg_rnd(&rnd, key_num /* max_key */);
   auto ro = ReadOptions();
   if (mmap) {
     ro.verify_checksums = false;
@@ -609,7 +607,7 @@ static void DBGet(benchmark::State& state) {
   if (negative_query) {
     for (auto _ : state) {
       std::string val;
-      Status s = db->Get(ro, kg.NextNonExist(), &val);
+      Status s = db->Get(ro, kg_rnd.NextNonExist(), &val);
       if (s.IsNotFound()) {
         not_found++;
       }
@@ -617,7 +615,7 @@ static void DBGet(benchmark::State& state) {
   } else {
     for (auto _ : state) {
       std::string val;
-      Status s = db->Get(ro, kg.Next(), &val);
+      Status s = db->Get(ro, kg_rnd.Next(), &val);
       if (s.IsNotFound()) {
         not_found++;
       }
@@ -636,7 +634,7 @@ static void DBGet(benchmark::State& state) {
       state.counters["get_p99"] = histogram_data.percentile99 * std::milli::den;
     }
 
-    TeardownDB(state, db, options, kg);
+    TeardownDB(state, db, options, kg_rnd);
   }
 }
 
@@ -662,9 +660,8 @@ static void DBGetArguments(benchmark::internal::Benchmark* b) {
                "negative_query", "enable_filter", "mmap"});
 }
 
-static constexpr uint64_t kDBGetNum = 1l << 20;
-BENCHMARK(DBGet)->Threads(1)->Iterations(kDBGetNum)->Apply(DBGetArguments);
-BENCHMARK(DBGet)->Threads(8)->Iterations(kDBGetNum / 8)->Apply(DBGetArguments);
+BENCHMARK(DBGet)->Threads(1)->Apply(DBGetArguments);
+BENCHMARK(DBGet)->Threads(8)->Apply(DBGetArguments);
 
 static void SimpleGetWithPerfContext(benchmark::State& state) {
   // setup DB