Remove the unnecessary use of shared_ptr

Summary:
shared_ptr is slower than unique_ptr (which literally comes with no performance cost compare with raw pointers).
In memtable and memtable rep, we use shared_ptr when we'd actually should use unique_ptr.

According to igor's previous work, we are likely to make quite some performance gain from this diff.

Test Plan: make check

Reviewers: dhruba, igor, sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15213

db/memtable.cc

@@ -85,11 +85,11 @@ class MemTableIterator: public Iterator {
   MemTableIterator(MemTableRep* table, const ReadOptions& options)
     : iter_() {
     if (options.prefix) {
-      iter_ = table->GetPrefixIterator(*options.prefix);
+      iter_.reset(table->GetPrefixIterator(*options.prefix));
     } else if (options.prefix_seek) {
-      iter_ = table->GetDynamicPrefixIterator();
+      iter_.reset(table->GetDynamicPrefixIterator());
     } else {
-      iter_ = table->GetIterator();
+      iter_.reset(table->GetIterator());
     }
   }
 
@@ -110,7 +110,7 @@ class MemTableIterator: public Iterator {
   virtual Status status() const { return Status::OK(); }
 
  private:
-  std::shared_ptr<MemTableRep::Iterator> iter_;
+  std::unique_ptr<MemTableRep::Iterator> iter_;
   std::string tmp_;       // For passing to EncodeKey
 
   // No copying allowed
@@ -161,7 +161,7 @@ void MemTable::Add(SequenceNumber s, ValueType type,
 bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
                    MergeContext& merge_context, const Options& options) {
   Slice memkey = key.memtable_key();
-  std::shared_ptr<MemTableRep::Iterator> iter(
+  std::unique_ptr<MemTableRep::Iterator> iter(
       table_->GetIterator(key.user_key()));
   iter->Seek(memkey.data());
 
@@ -267,7 +267,7 @@ bool MemTable::Update(SequenceNumber seq, ValueType type,
   LookupKey lkey(key, seq);
   Slice memkey = lkey.memtable_key();
 
-  std::shared_ptr<MemTableRep::Iterator> iter(
+  std::unique_ptr<MemTableRep::Iterator> iter(
       table_->GetIterator(lkey.user_key()));
   iter->Seek(memkey.data());
 
@@ -329,7 +329,7 @@ size_t MemTable::CountSuccessiveMergeEntries(const LookupKey& key) {
   // A total ordered iterator is costly for some memtablerep (prefix aware
   // reps). By passing in the user key, we allow efficient iterator creation.
   // The iterator only needs to be ordered within the same user key.
-  std::shared_ptr<MemTableRep::Iterator> iter(
+  std::unique_ptr<MemTableRep::Iterator> iter(
       table_->GetIterator(key.user_key()));
   iter->Seek(memkey.data());
 

db/memtable.h

@@ -143,7 +143,7 @@ class MemTable {
   KeyComparator comparator_;
   int refs_;
   ArenaImpl arena_impl_;
-  shared_ptr<MemTableRep> table_;
+  unique_ptr<MemTableRep> table_;
 
   // These are used to manage memtable flushes to storage
   bool flush_in_progress_; // started the flush

include/rocksdb/memtablerep.h

@@ -111,27 +111,23 @@ class MemTableRep {
   };
 
   // Return an iterator over the keys in this representation.
-  virtual std::shared_ptr<Iterator> GetIterator() = 0;
+  virtual Iterator* GetIterator() = 0;
 
   // Return an iterator over at least the keys with the specified user key. The
   // iterator may also allow access to other keys, but doesn't have to. Default:
   // GetIterator().
-  virtual std::shared_ptr<Iterator> GetIterator(const Slice& user_key) {
-    return GetIterator();
-  }
+  virtual Iterator* GetIterator(const Slice& user_key) { return GetIterator(); }
 
   // Return an iterator over at least the keys with the specified prefix. The
   // iterator may also allow access to other keys, but doesn't have to. Default:
   // GetIterator().
-  virtual std::shared_ptr<Iterator> GetPrefixIterator(const Slice& prefix) {
+  virtual Iterator* GetPrefixIterator(const Slice& prefix) {
     return GetIterator();
   }
 
   // Return an iterator that has a special Seek semantics. The result of
   // a Seek might only include keys with the same prefix as the target key.
-  virtual std::shared_ptr<Iterator> GetDynamicPrefixIterator() {
-    return GetIterator();
-  }
+  virtual Iterator* GetDynamicPrefixIterator() { return GetIterator(); }
 
  protected:
   // When *key is an internal key concatenated with the value, returns the
@@ -144,8 +140,8 @@ class MemTableRep {
 class MemTableRepFactory {
  public:
   virtual ~MemTableRepFactory() { };
-  virtual std::shared_ptr<MemTableRep> CreateMemTableRep(
-    MemTableRep::KeyComparator&, Arena*) = 0;
+  virtual MemTableRep* CreateMemTableRep(MemTableRep::KeyComparator&,
+                                         Arena*) = 0;
   virtual const char* Name() const = 0;
 };
 
@@ -161,8 +157,8 @@ class VectorRepFactory : public MemTableRepFactory {
   const size_t count_;
 public:
   explicit VectorRepFactory(size_t count = 0) : count_(count) { }
-  virtual std::shared_ptr<MemTableRep> CreateMemTableRep(
-    MemTableRep::KeyComparator&, Arena*) override;
+  virtual MemTableRep* CreateMemTableRep(MemTableRep::KeyComparator&,
+                                         Arena*) override;
   virtual const char* Name() const override {
     return "VectorRepFactory";
   }
@@ -171,8 +167,8 @@ public:
 // This uses a skip list to store keys. It is the default.
 class SkipListFactory : public MemTableRepFactory {
 public:
-  virtual std::shared_ptr<MemTableRep> CreateMemTableRep(
-    MemTableRep::KeyComparator&, Arena*) override;
+ virtual MemTableRep* CreateMemTableRep(MemTableRep::KeyComparator&,
+                                        Arena*) override;
   virtual const char* Name() const override {
     return "SkipListFactory";
   }

util/hash_skiplist_rep.cc

@@ -31,17 +31,15 @@ class HashSkipListRep : public MemTableRep {
 
   virtual ~HashSkipListRep();
 
-  virtual std::shared_ptr<MemTableRep::Iterator> GetIterator() override;
+  virtual MemTableRep::Iterator* GetIterator() override;
 
-  virtual std::shared_ptr<MemTableRep::Iterator> GetIterator(
-      const Slice& slice) override;
+  virtual MemTableRep::Iterator* GetIterator(const Slice& slice) override;
 
-  virtual std::shared_ptr<MemTableRep::Iterator> GetPrefixIterator(
-      const Slice& prefix) override;
-
-  virtual std::shared_ptr<MemTableRep::Iterator> GetDynamicPrefixIterator()
+  virtual MemTableRep::Iterator* GetPrefixIterator(const Slice& prefix)
       override;
 
+  virtual MemTableRep::Iterator* GetDynamicPrefixIterator() override;
+
  private:
   friend class DynamicIterator;
   typedef SkipList<const char*, MemTableRep::KeyComparator&> Bucket;
@@ -208,18 +206,15 @@ class HashSkipListRep : public MemTableRep {
     virtual void SeekToLast() { }
    private:
   };
-
-  std::shared_ptr<EmptyIterator> empty_iterator_;
 };
 
 HashSkipListRep::HashSkipListRep(MemTableRep::KeyComparator& compare,
-    Arena* arena, const SliceTransform* transform, size_t bucket_size)
+                                 Arena* arena, const SliceTransform* transform,
+                                 size_t bucket_size)
     : bucket_size_(bucket_size),
       transform_(transform),
       compare_(compare),
-    arena_(arena),
-    empty_iterator_(std::make_shared<EmptyIterator>()) {
-
+      arena_(arena) {
   buckets_ = new port::AtomicPointer[bucket_size];
 
   for (size_t i = 0; i < bucket_size_; ++i) {
@@ -263,7 +258,7 @@ size_t HashSkipListRep::ApproximateMemoryUsage() {
   return sizeof(buckets_);
 }
 
-std::shared_ptr<MemTableRep::Iterator> HashSkipListRep::GetIterator() {
+MemTableRep::Iterator* HashSkipListRep::GetIterator() {
   auto list = new Bucket(compare_, arena_);
   for (size_t i = 0; i < bucket_size_; ++i) {
     auto bucket = GetBucket(i);
@@ -274,35 +269,30 @@ std::shared_ptr<MemTableRep::Iterator> HashSkipListRep::GetIterator() {
       }
     }
   }
-  return std::make_shared<Iterator>(list);
+  return new Iterator(list);
 }
 
-std::shared_ptr<MemTableRep::Iterator> HashSkipListRep::GetPrefixIterator(
-  const Slice& prefix) {
+MemTableRep::Iterator* HashSkipListRep::GetPrefixIterator(const Slice& prefix) {
   auto bucket = GetBucket(prefix);
   if (bucket == nullptr) {
-    return empty_iterator_;
+    return new EmptyIterator();
   }
-  return std::make_shared<Iterator>(bucket, false);
+  return new Iterator(bucket, false);
 }
 
-std::shared_ptr<MemTableRep::Iterator> HashSkipListRep::GetIterator(
-    const Slice& slice) {
+MemTableRep::Iterator* HashSkipListRep::GetIterator(const Slice& slice) {
   return GetPrefixIterator(transform_->Transform(slice));
 }
 
-std::shared_ptr<MemTableRep::Iterator>
-    HashSkipListRep::GetDynamicPrefixIterator() {
-  return std::make_shared<DynamicIterator>(*this);
+MemTableRep::Iterator* HashSkipListRep::GetDynamicPrefixIterator() {
+  return new DynamicIterator(*this);
 }
 
 } // anon namespace
 
-std::shared_ptr<MemTableRep>
-HashSkipListRepFactory::CreateMemTableRep(MemTableRep::KeyComparator &compare,
-                                          Arena *arena) {
-  return std::make_shared<HashSkipListRep>(compare, arena, transform_,
-      bucket_count_);
+MemTableRep* HashSkipListRepFactory::CreateMemTableRep(
+    MemTableRep::KeyComparator& compare, Arena* arena) {
+  return new HashSkipListRep(compare, arena, transform_, bucket_count_);
 }
 
 MemTableRepFactory* NewHashSkipListRepFactory(

util/hash_skiplist_rep.h

@@ -21,8 +21,8 @@ class HashSkipListRepFactory : public MemTableRepFactory {
 
   virtual ~HashSkipListRepFactory() { delete transform_; }
 
-  virtual std::shared_ptr<MemTableRep> CreateMemTableRep(
-      MemTableRep::KeyComparator& compare, Arena* arena) override;
+  virtual MemTableRep* CreateMemTableRep(MemTableRep::KeyComparator& compare,
+                                         Arena* arena) override;
 
   virtual const char* Name() const override {
     return "HashSkipListRepFactory";

util/skiplistrep.cc

@@ -90,15 +90,15 @@ public:
   // Unhide default implementations of GetIterator
   using MemTableRep::GetIterator;
 
-  virtual std::shared_ptr<MemTableRep::Iterator> GetIterator() override {
-    return std::make_shared<SkipListRep::Iterator>(&skip_list_);
+  virtual MemTableRep::Iterator* GetIterator() override {
+    return new SkipListRep::Iterator(&skip_list_);
   }
 };
 }
 
-std::shared_ptr<MemTableRep> SkipListFactory::CreateMemTableRep (
+MemTableRep* SkipListFactory::CreateMemTableRep(
     MemTableRep::KeyComparator& compare, Arena* arena) {
-    return std::shared_ptr<MemTableRep>(new SkipListRep(compare, arena));
+  return new SkipListRep(compare, arena);
 }
 
 } // namespace rocksdb

util/vectorrep.cc

@@ -88,7 +88,7 @@ class VectorRep : public MemTableRep {
   using MemTableRep::GetIterator;
 
   // Return an iterator over the keys in this representation.
-  virtual std::shared_ptr<MemTableRep::Iterator> GetIterator() override;
+  virtual MemTableRep::Iterator* GetIterator() override;
 
  private:
   friend class Iterator;
@@ -228,22 +228,22 @@ void VectorRep::Iterator::SeekToLast() {
   }
 }
 
-std::shared_ptr<MemTableRep::Iterator> VectorRep::GetIterator() {
+MemTableRep::Iterator* VectorRep::GetIterator() {
   ReadLock l(&rwlock_);
   // Do not sort here. The sorting would be done the first time
   // a Seek is performed on the iterator.
   if (immutable_) {
-    return std::make_shared<Iterator>(this, bucket_, compare_);
+    return new Iterator(this, bucket_, compare_);
   } else {
     std::shared_ptr<Bucket> tmp;
     tmp.reset(new Bucket(*bucket_)); // make a copy
-    return std::make_shared<Iterator>(nullptr, tmp, compare_);
+    return new Iterator(nullptr, tmp, compare_);
   }
 }
 } // anon namespace
 
-std::shared_ptr<MemTableRep> VectorRepFactory::CreateMemTableRep(
+MemTableRep* VectorRepFactory::CreateMemTableRep(
     MemTableRep::KeyComparator& compare, Arena* arena) {
-  return std::make_shared<VectorRep>(compare, arena, count_);
+  return new VectorRep(compare, arena, count_);
 }
 } // namespace rocksdb