db_bench: support seek to non-exist prefix (#5163)

Summary:
Add `--seek_missing_prefix` flag to db_bench to allow benchmarking seeking to non-existing prefix. Usage example:
```
./db_bench --db=/dev/shm/db_bench --use_existing_db=false --benchmarks=fillrandom --num=100000000 --prefix_size=9 --keys_per_prefix=10
./db_bench --db=/dev/shm/db_bench --use_existing_db=true --benchmarks=seekrandom --disable_auto_compactions=true --num=100000000 --prefix_size=9 --keys_per_prefix=10 --reads=1000 --prefix_same_as_start=true --seek_missing_prefix=true
```
Also adding `--total_order_seek` and `--prefix_same_as_start` flags.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5163

Differential Revision: D14935724

Pulled By: riversand963

fbshipit-source-id: 7c41023f007febe373eb1589861f215432a9e18a

tools/db_bench_tool.cc

@@ -1090,11 +1090,20 @@ static bool ValidatePrefixSize(const char* flagname, int32_t value) {
   }
   return true;
 }
+
 DEFINE_int32(prefix_size, 0, "control the prefix size for HashSkipList and "
              "plain table");
 DEFINE_int64(keys_per_prefix, 0, "control average number of keys generated "
              "per prefix, 0 means no special handling of the prefix, "
              "i.e. use the prefix comes with the generated random number.");
+DEFINE_bool(total_order_seek, false,
+            "Enable total order seek regardless of index format.");
+DEFINE_bool(prefix_same_as_start, false,
+            "Enforce iterator to return keys with prefix same as seek key.");
+DEFINE_bool(
+    seek_missing_prefix, false,
+    "Iterator seek to keys with non-exist prefixes. Require prefix_size > 8");
+
 DEFINE_int32(memtable_insert_with_hint_prefix_size, 0,
              "If non-zero, enable "
              "memtable insert with hint with the given prefix size.");
@@ -2529,6 +2538,17 @@ class Benchmark {
     }
   }
 
+  void GenerateKeyFromIntForSeek(uint64_t v, int64_t num_keys, Slice* key) {
+    GenerateKeyFromInt(v, num_keys, key);
+    if (FLAGS_seek_missing_prefix) {
+      assert(prefix_size_ > 8);
+      char* key_ptr = const_cast<char*>(key->data());
+      // This rely on GenerateKeyFromInt filling paddings with '0's.
+      // Putting a '1' will create a non-existing prefix.
+      key_ptr[8] = '1';
+    }
+  }
+
   std::string GetPathForMultiple(std::string base_name, size_t id) {
     if (!base_name.empty()) {
 #ifndef OS_WIN
@@ -4964,6 +4984,8 @@ void VerifyDBFromDB(std::string& truth_db_name) {
     int64_t found = 0;
     int64_t bytes = 0;
     ReadOptions options(FLAGS_verify_checksum, true);
+    options.total_order_seek = FLAGS_total_order_seek;
+    options.prefix_same_as_start = FLAGS_prefix_same_as_start;
     options.tailing = FLAGS_use_tailing_iterator;
 
     Iterator* single_iter = nullptr;
@@ -4988,7 +5010,8 @@ void VerifyDBFromDB(std::string& truth_db_name) {
     char value_buffer[256];
     while (!duration.Done(1)) {
       int64_t seek_pos = thread->rand.Next() % FLAGS_num;
-      GenerateKeyFromInt((uint64_t)seek_pos, FLAGS_num, &key);
+      GenerateKeyFromIntForSeek(static_cast<uint64_t>(seek_pos), FLAGS_num,
+                                &key);
       if (FLAGS_max_scan_distance != 0) {
         if (FLAGS_reverse_iterator) {
           GenerateKeyFromInt(
@@ -6256,6 +6279,11 @@ int db_bench_tool(int argc, char** argv) {
     FLAGS_stats_interval = 1000;
   }
 
+  if (FLAGS_seek_missing_prefix && FLAGS_prefix_size <= 8) {
+    fprintf(stderr, "prefix_size > 8 required by --seek_missing_prefix\n");
+    exit(1);
+  }
+
   rocksdb::Benchmark benchmark;
   benchmark.Run();