CompactedDBImpl::MultiGet() for better CuckooTable performance

Summary:
Add the MultiGet API to allow prefetching.
With file size of 1.5G, I configured it to have 0.9 hash ratio that can
fill With 115M keys and result in 2 hash functions, the lookup QPS is
~4.9M/s  vs. 3M/s for Get().
It is tricky to set the parameters right. Since files size is determined
by power-of-two factor, that means # of keys is fixed in each file. With
big file size (thus smaller # of files), we will have more chance to
waste lot of space in the last file - lower space utilization as a
result. Using smaller file size can improve the situation, but that
harms lookup speed.

Test Plan: db_bench

Reviewers: yhchiang, sdong, igor

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23673

db/db_bench.cc

@@ -167,6 +167,8 @@ DEFINE_int32(value_size, 100, "Size of each value");
 
 DEFINE_bool(use_uint64_comparator, false, "use Uint64 user comparator");
 
+DEFINE_int64(batch_size, 1, "Batch size");
+
 static bool ValidateKeySize(const char* flagname, int32_t value) {
   return true;
 }
@@ -1265,6 +1267,8 @@ class Benchmark {
       } else if (name == Slice("readrandomfast")) {
         method = &Benchmark::ReadRandomFast;
       } else if (name == Slice("multireadrandom")) {
+        entries_per_batch_ = FLAGS_batch_size;
+        fprintf(stderr, "entries_per_batch_ = %ld\n", entries_per_batch_);
         method = &Benchmark::MultiReadRandom;
       } else if (name == Slice("readmissing")) {
         ++key_size_;
@@ -2076,6 +2080,7 @@ class Benchmark {
   void ReadRandomFast(ThreadState* thread) {
     int64_t read = 0;
     int64_t found = 0;
+    int64_t nonexist = 0;
     ReadOptions options(FLAGS_verify_checksum, true);
     Slice key = AllocateKey();
     std::unique_ptr<const char[]> key_guard(key.data());
@@ -2096,13 +2101,17 @@ class Benchmark {
         if (db->Get(options, key, &value).ok()) {
           ++found;
         }
+        if (key_rand >= FLAGS_num) {
+          ++nonexist;
+        }
       }
       thread->stats.FinishedOps(db, 100);
     } while (!duration.Done(100));
 
     char msg[100];
-    snprintf(msg, sizeof(msg), "(%" PRIu64 " of %" PRIu64 " found)\n",
-             found, read);
+    snprintf(msg, sizeof(msg), "(%" PRIu64 " of %" PRIu64 " found, "
+             "issued %" PRIu64 " non-exist keys)\n",
+             found, read, nonexist);
 
     thread->stats.AddMessage(msg);
 

db/db_test.cc

@@ -1342,6 +1342,24 @@ TEST(DBTest, CompactedDB) {
   ASSERT_EQ(DummyString(kFileSize / 2, 'i'), Get("iii"));
   ASSERT_EQ(DummyString(kFileSize / 2, 'j'), Get("jjj"));
   ASSERT_EQ("NOT_FOUND", Get("kkk"));
+
+  // MultiGet
+  std::vector<std::string> values;
+  std::vector<Status> status_list = dbfull()->MultiGet(ReadOptions(),
+      std::vector<Slice>({Slice("aaa"), Slice("ccc"), Slice("eee"),
+                          Slice("ggg"), Slice("iii"), Slice("kkk")}),
+      &values);
+  ASSERT_EQ(status_list.size(), 6);
+  ASSERT_EQ(values.size(), 6);
+  ASSERT_OK(status_list[0]);
+  ASSERT_EQ(DummyString(kFileSize / 2, 'a'), values[0]);
+  ASSERT_TRUE(status_list[1].IsNotFound());
+  ASSERT_OK(status_list[2]);
+  ASSERT_EQ(DummyString(kFileSize / 2, 'e'), values[2]);
+  ASSERT_TRUE(status_list[3].IsNotFound());
+  ASSERT_OK(status_list[4]);
+  ASSERT_EQ(DummyString(kFileSize / 2, 'i'), values[4]);
+  ASSERT_TRUE(status_list[5].IsNotFound());
 }
 
 // Make sure that when options.block_cache is set, after a new table is

utilities/compacted_db/compacted_db_impl.cc

@@ -23,8 +23,7 @@ CompactedDBImpl::CompactedDBImpl(
 CompactedDBImpl::~CompactedDBImpl() {
 }
 
-Status CompactedDBImpl::Get(const ReadOptions& options,
-     ColumnFamilyHandle*, const Slice& key, std::string* value) {
+size_t CompactedDBImpl::FindFile(const Slice& key) {
   size_t left = 0;
   size_t right = files_.num_files - 1;
   while (left < right) {
@@ -40,7 +39,12 @@ Status CompactedDBImpl::Get(const ReadOptions& options,
       right = mid;
     }
   }
-  const FdWithKeyRange& f = files_.files[right];
+  return right;
+}
+
+Status CompactedDBImpl::Get(const ReadOptions& options,
+     ColumnFamilyHandle*, const Slice& key, std::string* value) {
+  const FdWithKeyRange& f = files_.files[FindFile(key)];
 
   bool value_found;
   MergeContext merge_context;
@@ -64,6 +68,50 @@ Status CompactedDBImpl::Get(const ReadOptions& options,
   return Status::NotFound();
 }
 
+std::vector<Status> CompactedDBImpl::MultiGet(const ReadOptions& options,
+    const std::vector<ColumnFamilyHandle*>&,
+    const std::vector<Slice>& keys, std::vector<std::string>* values) {
+  autovector<TableReader*, 16> reader_list;
+  for (const auto& key : keys) {
+    const FdWithKeyRange& f = files_.files[FindFile(key)];
+    if (user_comparator_->Compare(key, ExtractUserKey(f.smallest_key)) < 0) {
+      reader_list.push_back(nullptr);
+    } else {
+      LookupKey lkey(key, kMaxSequenceNumber);
+      f.fd.table_reader->Prepare(lkey.internal_key());
+      reader_list.push_back(f.fd.table_reader);
+    }
+  }
+  std::vector<Status> statuses(keys.size(), Status::NotFound());
+  values->resize(keys.size());
+  bool value_found;
+  MergeContext merge_context;
+  Version::Saver saver;
+  saver.ucmp = user_comparator_;
+  saver.value_found = &value_found;
+  saver.merge_operator = nullptr;
+  saver.merge_context = &merge_context;
+  saver.logger = info_log_;
+  saver.statistics = statistics_;
+  int idx = 0;
+  for (auto* r : reader_list) {
+    if (r != nullptr) {
+      saver.state = Version::kNotFound;
+      saver.user_key = keys[idx];
+      saver.value = &(*values)[idx];
+      LookupKey lkey(keys[idx], kMaxSequenceNumber);
+      r->Get(options, lkey.internal_key(),
+             reinterpret_cast<void*>(&saver), SaveValue,
+             MarkKeyMayExist);
+      if (saver.state == Version::kFound) {
+        statuses[idx] = Status::OK();
+      }
+    }
+    ++idx;
+  }
+  return statuses;
+}
+
 Status CompactedDBImpl::Init(const Options& options) {
   mutex_.Lock();
   ColumnFamilyDescriptor cf(kDefaultColumnFamilyName,

utilities/compacted_db/compacted_db_impl.h

@@ -24,6 +24,12 @@ class CompactedDBImpl : public DBImpl {
   virtual Status Get(const ReadOptions& options,
                      ColumnFamilyHandle* column_family, const Slice& key,
                      std::string* value) override;
+  using DB::MultiGet;
+  virtual std::vector<Status> MultiGet(
+      const ReadOptions& options,
+      const std::vector<ColumnFamilyHandle*>&,
+      const std::vector<Slice>& keys, std::vector<std::string>* values)
+    override;
 
   using DBImpl::Put;
   virtual Status Put(const WriteOptions& options,
@@ -74,6 +80,7 @@ class CompactedDBImpl : public DBImpl {
 
  private:
   friend class DB;
+  inline size_t FindFile(const Slice& key);
   Status Init(const Options& options);
 
   ColumnFamilyData* cfd_;