PlainTableIterator not to store copied key in std::string

Summary:
Move PlainTableIterator's copied key from std::string local buffer to avoid paying the extra costs in std::string related to sharing. Reuse the same buffer class in DbIter. Move the class to dbformat.h.

This patch improves iterator performance significantly. Running this benchmark:

./table_reader_bench --num_keys2=17 --iterator --plain_table --time_unit=nanosecond

The average latency is improved to about 750 nanoseconds from 1100 nanoseconds.

Test Plan:
Add a unit test.
make all check

Reviewers: haobo, ljin

Reviewed By: haobo

CC: igor, yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D17547

db/db_iter.cc

@@ -39,71 +39,6 @@ static void DumpInternalIter(Iterator* iter) {
 
 namespace {
 
-class IterLookupKey {
- public:
-  IterLookupKey() : key_(space_), buf_size_(sizeof(space_)), key_size_(0) {}
-
-  ~IterLookupKey() { Clear(); }
-
-  Slice GetKey() const {
-    if (key_ != nullptr) {
-      return Slice(key_, key_size_);
-    } else {
-      return Slice();
-    }
-  }
-
-  bool Valid() const { return key_ != nullptr; }
-
-  void Clear() {
-    if (key_ != nullptr && key_ != space_) {
-      delete[] key_;
-    }
-    key_ = space_;
-    buf_size_ = sizeof(buf_size_);
-  }
-
-  // Enlarge the buffer size if needed based on key_size.
-  // By default, static allocated buffer is used. Once there is a key
-  // larger than the static allocated buffer, another buffer is dynamically
-  // allocated, until a larger key buffer is requested. In that case, we
-  // reallocate buffer and delete the old one.
-  void EnlargeBufferIfNeeded(size_t key_size) {
-    // If size is smaller than buffer size, continue using current buffer,
-    // or the static allocated one, as default
-    if (key_size > buf_size_) {
-      // Need to enlarge the buffer.
-      Clear();
-      key_ = new char[key_size];
-      buf_size_ = key_size;
-    }
-    key_size_ = key_size;
-  }
-
-  void SetUserKey(const Slice& user_key) {
-    size_t size = user_key.size();
-    EnlargeBufferIfNeeded(size);
-    memcpy(key_, user_key.data(), size);
-  }
-
-  void SetInternalKey(const Slice& user_key, SequenceNumber s) {
-    size_t usize = user_key.size();
-    EnlargeBufferIfNeeded(usize + sizeof(uint64_t));
-    memcpy(key_, user_key.data(), usize);
-    EncodeFixed64(key_ + usize, PackSequenceAndType(s, kValueTypeForSeek));
-  }
-
- private:
-  char* key_;
-  size_t buf_size_;
-  size_t key_size_;
-  char space_[32];  // Avoid allocation for short keys
-
-  // No copying allowed
-  IterLookupKey(const IterLookupKey&) = delete;
-  void operator=(const LookupKey&) = delete;
-};
-
 // Memtables and sstables that make the DB representation contain
 // (userkey,seq,type) => uservalue entries.  DBIter
 // combines multiple entries for the same userkey found in the DB
@@ -191,7 +126,7 @@ class DBIter: public Iterator {
   SequenceNumber const sequence_;
 
   Status status_;
-  IterLookupKey saved_key_;   // == current key when direction_==kReverse
+  IterKey saved_key_;   // == current key when direction_==kReverse
   std::string saved_value_;   // == current raw value when direction_==kReverse
   std::string skip_key_;
   Direction direction_;

db/dbformat.h

@@ -238,4 +238,74 @@ inline LookupKey::~LookupKey() {
   if (start_ != space_) delete[] start_;
 }
 
+class IterKey {
+ public:
+  IterKey() : key_(space_), buf_size_(sizeof(space_)), key_size_(0) {}
+
+  ~IterKey() { Clear(); }
+
+  Slice GetKey() const {
+    if (key_ != nullptr) {
+      return Slice(key_, key_size_);
+    } else {
+      return Slice();
+    }
+  }
+
+  bool Valid() const { return key_ != nullptr; }
+
+  void Clear() {
+    if (key_ != nullptr && key_ != space_) {
+      delete[] key_;
+    }
+    key_ = space_;
+    buf_size_ = sizeof(buf_size_);
+  }
+
+  // Enlarge the buffer size if needed based on key_size.
+  // By default, static allocated buffer is used. Once there is a key
+  // larger than the static allocated buffer, another buffer is dynamically
+  // allocated, until a larger key buffer is requested. In that case, we
+  // reallocate buffer and delete the old one.
+  void EnlargeBufferIfNeeded(size_t key_size) {
+    // If size is smaller than buffer size, continue using current buffer,
+    // or the static allocated one, as default
+    if (key_size > buf_size_) {
+      // Need to enlarge the buffer.
+      Clear();
+      key_ = new char[key_size];
+      buf_size_ = key_size;
+    }
+    key_size_ = key_size;
+  }
+
+  void SetUserKey(const Slice& user_key) {
+    size_t size = user_key.size();
+    EnlargeBufferIfNeeded(size);
+    memcpy(key_, user_key.data(), size);
+  }
+
+  void SetInternalKey(const Slice& user_key, SequenceNumber s,
+                      ValueType value_type = kValueTypeForSeek) {
+    size_t usize = user_key.size();
+    EnlargeBufferIfNeeded(usize + sizeof(uint64_t));
+    memcpy(key_, user_key.data(), usize);
+    EncodeFixed64(key_ + usize, PackSequenceAndType(s, value_type));
+  }
+
+  void SetInternalKey(const ParsedInternalKey& parsed_key) {
+    SetInternalKey(parsed_key.user_key, parsed_key.sequence, parsed_key.type);
+  }
+
+ private:
+  char* key_;
+  size_t buf_size_;
+  size_t key_size_;
+  char space_[32];  // Avoid allocation for short keys
+
+  // No copying allowed
+  IterKey(const IterKey&) = delete;
+  void operator=(const IterKey&) = delete;
+};
+
 }  // namespace rocksdb

db/plain_table_db_test.cc

@@ -429,6 +429,48 @@ TEST(PlainTableDBTest, Iterator) {
   }
 }
 
+std::string MakeLongKey(size_t length, char c) {
+  return std::string(length, c);
+}
+
+TEST(PlainTableDBTest, IteratorLargeKeys) {
+  Options options = CurrentOptions();
+  options.table_factory.reset(NewTotalOrderPlainTableFactory(0, 0, 16));
+  options.create_if_missing = true;
+  options.prefix_extractor.reset();
+  DestroyAndReopen(&options);
+
+  std::string key_list[] = {
+      MakeLongKey(30, '0'),
+      MakeLongKey(16, '1'),
+      MakeLongKey(32, '2'),
+      MakeLongKey(60, '3'),
+      MakeLongKey(90, '4'),
+      MakeLongKey(50, '5'),
+      MakeLongKey(26, '6')
+  };
+
+  for (size_t i = 0; i < 7; i++) {
+    ASSERT_OK(Put(key_list[i], std::to_string(i)));
+  }
+
+  dbfull()->TEST_FlushMemTable();
+
+  Iterator* iter = dbfull()->NewIterator(ro_);
+  iter->Seek(key_list[0]);
+
+  for (size_t i = 0; i < 7; i++) {
+    ASSERT_TRUE(iter->Valid());
+    ASSERT_EQ(key_list[i], iter->key().ToString());
+    ASSERT_EQ(std::to_string(i), iter->value().ToString());
+    iter->Next();
+  }
+
+  ASSERT_TRUE(!iter->Valid());
+
+  delete iter;
+}
+
 // A test comparator which compare two strings in this way:
 // (1) first compare prefix of 8 bytes in alphabet order,
 // (2) if two strings share the same prefix, sort the other part of the string

table/plain_table_reader.cc

@@ -81,10 +81,9 @@ class PlainTableIterator : public Iterator {
   bool use_prefix_seek_;
   uint32_t offset_;
   uint32_t next_offset_;
-  Slice key_;
+  IterKey key_;
   Slice value_;
   Status status_;
-  std::string tmp_str_;
   // No copying allowed
   PlainTableIterator(const PlainTableIterator&) = delete;
   void operator=(const Iterator&) = delete;
@@ -720,9 +719,7 @@ void PlainTableIterator::Next() {
     status_ = table_->Next(&next_offset_, &parsed_key, &value_);
     if (status_.ok()) {
       // Make a copy in this case. TODO optimize.
-      tmp_str_.clear();
+      key_.SetInternalKey(parsed_key);
-      AppendInternalKey(&tmp_str_, parsed_key);
-      key_ = Slice(tmp_str_);
     } else {
       offset_ = next_offset_ = table_->data_end_offset_;
     }
@@ -735,7 +732,7 @@ void PlainTableIterator::Prev() {
 
 Slice PlainTableIterator::key() const {
   assert(Valid());
-  return key_;
+  return key_.GetKey();
 }
 
 Slice PlainTableIterator::value() const {