MemTableListVersion

Summary:
MemTableListVersion is to MemTableList what Version is to VersionSet. I took almost the same ideas to develop MemTableListVersion. The reason is to have copying std::list done in background, while flushing, rather than in foreground (MultiGet() and NewIterator()) under a mutex! Also, whenever we copied MemTableList, we copied also some MemTableList metadata (flush_requested_, commit_in_progress_, etc.), which was wasteful.

This diff avoids std::list copy under a mutex in both MultiGet() and NewIterator(). I created a small database with some number of immutable memtables, and creating 100.000 iterators in a single-thread (!) decreased from {188739, 215703, 198028} to {154352, 164035, 159817}. A lot of the savings come from code under a mutex, so we should see much higher savings with multiple threads. Creating new iterator is very important to LogDevice team.

I also think this diff will make SuperVersion obsolete for performance reasons. I will try it in the next diff. SuperVersion gave us huge savings on Get() code path, but I think that most of the savings came from copying MemTableList under a mutex. If we had MemTableListVersion, we would never need to copy the entire object (like we still do in NewIterator() and MultiGet())

Test Plan: `make check` works. I will also do `make valgrind_check` before commit

Reviewers: dhruba, haobo, kailiu, sdong, emayanke, tnovak

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15255

db/db_impl.cc

@@ -264,6 +264,7 @@ DBImpl::DBImpl(const Options& options, const std::string& dbname)
       bg_cv_(&mutex_),
       mem_rep_factory_(options_.memtable_factory.get()),
       mem_(new MemTable(internal_comparator_, options_)),
+      imm_(options_.min_write_buffer_number_to_merge),
       logfile_number_(0),
       super_version_(nullptr),
       super_version_number_(0),
@@ -360,7 +361,7 @@ DBImpl::~DBImpl() {
     delete mem_->Unref();
   }
 
-  imm_.UnrefAll(&to_delete);
+  imm_.current()->Unref(&to_delete);
   for (MemTable* m: to_delete) {
     delete m;
   }
@@ -508,7 +509,7 @@ bool DBImpl::SuperVersion::Unref() {
 
 void DBImpl::SuperVersion::Cleanup() {
   assert(refs.load(std::memory_order_relaxed) == 0);
-  imm.UnrefAll(&to_delete);
+  imm->Unref(&to_delete);
   MemTable* m = mem->Unref();
   if (m != nullptr) {
     to_delete.push_back(m);
@@ -516,13 +517,13 @@ void DBImpl::SuperVersion::Cleanup() {
   current->Unref();
 }
 
-void DBImpl::SuperVersion::Init(MemTable* new_mem, const MemTableList& new_imm,
+void DBImpl::SuperVersion::Init(MemTable* new_mem, MemTableListVersion* new_imm,
                                 Version* new_current) {
   mem = new_mem;
   imm = new_imm;
   current = new_current;
   mem->Ref();
-  imm.RefAll();
+  imm->Ref();
   current->Ref();
   refs.store(1, std::memory_order_relaxed);
 }
@@ -1221,7 +1222,7 @@ Status DBImpl::FlushMemTableToOutputFile(bool* madeProgress,
   mutex_.AssertHeld();
   assert(imm_.size() != 0);
 
-  if (!imm_.IsFlushPending(options_.min_write_buffer_number_to_merge)) {
+  if (!imm_.IsFlushPending()) {
     Log(options_.info_log, "FlushMemTableToOutputFile already in progress");
     Status s = Status::IOError("FlushMemTableToOutputFile already in progress");
     return s;
@@ -1762,8 +1763,7 @@ void DBImpl::MaybeScheduleFlushOrCompaction() {
   } else if (shutting_down_.Acquire_Load()) {
     // DB is being deleted; no more background compactions
   } else {
-    bool is_flush_pending =
+    bool is_flush_pending = imm_.IsFlushPending();
-      imm_.IsFlushPending(options_.min_write_buffer_number_to_merge);
     if (is_flush_pending &&
         (bg_flush_scheduled_ < options_.max_background_flushes)) {
       // memtable flush needed
@@ -1798,8 +1798,7 @@ void DBImpl::BGWorkCompaction(void* db) {
 Status DBImpl::BackgroundFlush(bool* madeProgress,
                                DeletionState& deletion_state) {
   Status stat;
-  while (stat.ok() &&
+  while (stat.ok() && imm_.IsFlushPending()) {
-         imm_.IsFlushPending(options_.min_write_buffer_number_to_merge)) {
     Log(options_.info_log,
         "BackgroundCallFlush doing FlushMemTableToOutputFile, flush slots available %d",
         options_.max_background_flushes - bg_flush_scheduled_);
@@ -1919,7 +1918,7 @@ Status DBImpl::BackgroundCompaction(bool* madeProgress,
   mutex_.AssertHeld();
 
   // TODO: remove memtable flush from formal compaction
-  while (imm_.IsFlushPending(options_.min_write_buffer_number_to_merge)) {
+  while (imm_.IsFlushPending()) {
     Log(options_.info_log,
         "BackgroundCompaction doing FlushMemTableToOutputFile, compaction slots "
         "available %d",
@@ -2325,7 +2324,7 @@ Status DBImpl::DoCompactionWork(CompactionState* compact,
       const uint64_t imm_start = env_->NowMicros();
       LogFlush(options_.info_log);
       mutex_.Lock();
-      if (imm_.IsFlushPending(options_.min_write_buffer_number_to_merge)) {
+      if (imm_.IsFlushPending()) {
         FlushMemTableToOutputFile(nullptr, deletion_state);
         bg_cv_.SignalAll();  // Wakeup MakeRoomForWrite() if necessary
       }
@@ -2658,8 +2657,9 @@ Status DBImpl::DoCompactionWork(CompactionState* compact,
 namespace {
 struct IterState {
   port::Mutex* mu;
-  Version* version;
+  Version* version = nullptr;
-  std::vector<MemTable*> mem; // includes both mem_ and imm_
+  MemTable* mem = nullptr;
+  MemTableListVersion* imm = nullptr;
   DBImpl *db;
 };
 
@@ -2668,15 +2668,16 @@ static void CleanupIteratorState(void* arg1, void* arg2) {
   DBImpl::DeletionState deletion_state(state->db->GetOptions().
                                        max_write_buffer_number);
   state->mu->Lock();
-  for (unsigned int i = 0; i < state->mem.size(); i++) {
+  MemTable* m = state->mem->Unref();
-    MemTable* m = state->mem[i]->Unref();
+  if (m != nullptr) {
-    if (m != nullptr) {
+    deletion_state.memtables_to_free.push_back(m);
-      deletion_state.memtables_to_free.push_back(m);
-    }
   }
   if (state->version) {  // not set for memtable-only iterator
     state->version->Unref();
   }
+  if (state->imm) {  // not set for memtable-only iterator
+    state->imm->Unref(&deletion_state.memtables_to_free);
+  }
   // fast path FindObsoleteFiles
   state->db->FindObsoleteFiles(deletion_state, false, true);
   state->mu->Unlock();
@@ -2690,7 +2691,7 @@ Iterator* DBImpl::NewInternalIterator(const ReadOptions& options,
                                       SequenceNumber* latest_snapshot) {
   IterState* cleanup = new IterState;
   MemTable* mutable_mem;
-  std::vector<MemTable*> immutables;
+  MemTableListVersion* immutable_mems;
   Version* version;
 
   // Collect together all needed child iterators for mem
@@ -2699,27 +2700,22 @@ Iterator* DBImpl::NewInternalIterator(const ReadOptions& options,
   mem_->Ref();
   mutable_mem = mem_;
   // Collect together all needed child iterators for imm_
-  imm_.GetMemTables(&immutables);
+  immutable_mems = imm_.current();
-  for (unsigned int i = 0; i < immutables.size(); i++) {
+  immutable_mems->Ref();
-    immutables[i]->Ref();
-  }
   versions_->current()->Ref();
   version = versions_->current();
   mutex_.Unlock();
 
-  std::vector<Iterator*> list;
+  std::vector<Iterator*> iterator_list;
-  list.push_back(mutable_mem->NewIterator(options));
+  iterator_list.push_back(mutable_mem->NewIterator(options));
-  cleanup->mem.push_back(mutable_mem);
+  cleanup->mem = mutable_mem;
-
+  cleanup->imm = immutable_mems;
   // Collect all needed child iterators for immutable memtables
-  for (MemTable* m : immutables) {
+  immutable_mems->AddIterators(options, &iterator_list);
-    list.push_back(m->NewIterator(options));
-    cleanup->mem.push_back(m);
-  }
   // Collect iterators for files in L0 - Ln
-  version->AddIterators(options, storage_options_, &list);
+  version->AddIterators(options, storage_options_, &iterator_list);
-  Iterator* internal_iter =
+  Iterator* internal_iter = NewMergingIterator(
-      NewMergingIterator(&internal_comparator_, &list[0], list.size());
+      &internal_comparator_, &iterator_list[0], iterator_list.size());
   cleanup->version = version;
   cleanup->mu = &mutex_;
   cleanup->db = this;
@@ -2738,19 +2734,15 @@ std::pair<Iterator*, Iterator*> DBImpl::GetTailingIteratorPair(
     uint64_t* superversion_number) {
 
   MemTable* mutable_mem;
-  std::vector<MemTable*> immutables;
+  MemTableListVersion* immutable_mems;
   Version* version;
 
-  immutables.reserve(options_.max_write_buffer_number);
-
   // get all child iterators and bump their refcounts under lock
   mutex_.Lock();
   mutable_mem = mem_;
   mutable_mem->Ref();
-  imm_.GetMemTables(&immutables);
+  immutable_mems = imm_.current();
-  for (size_t i = 0; i < immutables.size(); ++i) {
+  immutable_mems->Ref();
-    immutables[i]->Ref();
-  }
   version = versions_->current();
   version->Ref();
   if (superversion_number != nullptr) {
@@ -2760,7 +2752,7 @@ std::pair<Iterator*, Iterator*> DBImpl::GetTailingIteratorPair(
 
   Iterator* mutable_iter = mutable_mem->NewIterator(options);
   IterState* mutable_cleanup = new IterState();
-  mutable_cleanup->mem.push_back(mutable_mem);
+  mutable_cleanup->mem = mutable_mem;
   mutable_cleanup->db = this;
   mutable_cleanup->mu = &mutex_;
   mutable_iter->RegisterCleanup(CleanupIteratorState, mutable_cleanup, nullptr);
@@ -2772,10 +2764,8 @@ std::pair<Iterator*, Iterator*> DBImpl::GetTailingIteratorPair(
   Iterator* immutable_iter;
   IterState* immutable_cleanup = new IterState();
   std::vector<Iterator*> list;
-  for (MemTable* m : immutables) {
+  immutable_mems->AddIterators(options, &list);
-    list.push_back(m->NewIterator(options));
+  immutable_cleanup->imm = immutable_mems;
-    immutable_cleanup->mem.push_back(m);
-  }
   version->AddIterators(options, storage_options_, &list);
   immutable_cleanup->version = version;
   immutable_cleanup->db = this;
@@ -2832,7 +2822,7 @@ void DBImpl::InstallSuperVersion(DeletionState& deletion_state) {
 DBImpl::SuperVersion* DBImpl::InstallSuperVersion(
     SuperVersion* new_superversion) {
   mutex_.AssertHeld();
-  new_superversion->Init(mem_, imm_, versions_->current());
+  new_superversion->Init(mem_, imm_.current(), versions_->current());
   SuperVersion* old_superversion = super_version_;
   super_version_ = new_superversion;
   ++super_version_number_;
@@ -2875,7 +2865,7 @@ Status DBImpl::GetImpl(const ReadOptions& options,
   if (get_version->mem->Get(lkey, value, &s, merge_context, options_)) {
     // Done
     RecordTick(options_.statistics.get(), MEMTABLE_HIT);
-  } else if (get_version->imm.Get(lkey, value, &s, merge_context, options_)) {
+  } else if (get_version->imm->Get(lkey, value, &s, merge_context, options_)) {
     // Done
     RecordTick(options_.statistics.get(), MEMTABLE_HIT);
   } else {
@@ -2930,10 +2920,10 @@ std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
   }
 
   MemTable* mem = mem_;
-  MemTableList imm = imm_;
+  MemTableListVersion* imm = imm_.current();
   Version* current = versions_->current();
   mem->Ref();
-  imm.RefAll();
+  imm->Ref();
   current->Ref();
 
   // Unlock while reading from files and memtables
@@ -2965,7 +2955,7 @@ std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
     LookupKey lkey(keys[i], snapshot);
     if (mem->Get(lkey, value, &s, merge_context, options_)) {
       // Done
-    } else if (imm.Get(lkey, value, &s, merge_context, options_)) {
+    } else if (imm->Get(lkey, value, &s, merge_context, options_)) {
       // Done
     } else {
       current->Get(options, lkey, value, &s, &merge_context, &stats, options_);
@@ -2984,7 +2974,7 @@ std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
     MaybeScheduleFlushOrCompaction();
   }
   MemTable* m = mem->Unref();
-  imm.UnrefAll(&to_delete);
+  imm->Unref(&to_delete);
   current->Unref();
   mutex_.Unlock();
 

db/db_impl.h

@@ -140,10 +140,10 @@ class DBImpl : public DB {
   // holds references to memtable, all immutable memtables and version
   struct SuperVersion {
     MemTable* mem;
-    MemTableList imm;
+    MemTableListVersion* imm;
     Version* current;
     std::atomic<uint32_t> refs;
-    // We need to_delete because during Cleanup(), imm.UnrefAll() returns
+    // We need to_delete because during Cleanup(), imm->Unref() returns
     // all memtables that we need to free through this vector. We then
     // delete all those memtables outside of mutex, during destruction
     std::vector<MemTable*> to_delete;
@@ -161,7 +161,7 @@ class DBImpl : public DB {
     // that needs to be deleted in to_delete vector. Unrefing those
     // objects needs to be done in the mutex
     void Cleanup();
-    void Init(MemTable* new_mem, const MemTableList& new_imm,
+    void Init(MemTable* new_mem, MemTableListVersion* new_imm,
               Version* new_current);
   };
 

db/memtablelist.cc

@@ -16,41 +16,85 @@ namespace rocksdb {
 
 class InternalKeyComparator;
 class Mutex;
-class MemTableListIterator;
 class VersionSet;
 
-using std::list;
+MemTableListVersion::MemTableListVersion(MemTableListVersion* old) {
-
+  if (old != nullptr) {
-// Increase reference count on all underling memtables
+    memlist_ = old->memlist_;
-void MemTableList::RefAll() {
+    size_ = old->size_;
-  for (auto &memtable : memlist_) {
+    for (auto& m : memlist_) {
-    memtable->Ref();
+      m->Ref();
+    }
   }
 }
 
-// Drop reference count on all underling memtables. If the
+void MemTableListVersion::Ref() { ++refs_; }
-// refcount of an underlying memtable drops to zero, then
+
-// return it in to_delete vector.
+void MemTableListVersion::Unref(std::vector<MemTable*>* to_delete) {
-void MemTableList::UnrefAll(std::vector<MemTable*>* to_delete) {
+  --refs_;
-  for (auto &memtable : memlist_) {
+  if (refs_ == 0) {
-    MemTable* m = memtable->Unref();
+    // if to_delete is equal to nullptr it means we're confident
-    if (m != nullptr) {
+    // that refs_ will not be zero
-      to_delete->push_back(m);
+    assert(to_delete != nullptr);
+    for (const auto& m : memlist_) {
+      MemTable* x = m->Unref();
+      if (x != nullptr) {
+        to_delete->push_back(x);
+      }
     }
+    delete this;
   }
 }
 
+int MemTableListVersion::size() const { return size_; }
+
 // Returns the total number of memtables in the list
-int MemTableList::size() {
+int MemTableList::size() const {
-  assert(num_flush_not_started_ <= size_);
+  assert(num_flush_not_started_ <= current_->size_);
-  return size_;
+  return current_->size_;
+}
+
+// Search all the memtables starting from the most recent one.
+// Return the most recent value found, if any.
+// Operands stores the list of merge operations to apply, so far.
+bool MemTableListVersion::Get(const LookupKey& key, std::string* value,
+                              Status* s, MergeContext& merge_context,
+                              const Options& options) {
+  for (auto& memtable : memlist_) {
+    if (memtable->Get(key, value, s, merge_context, options)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+void MemTableListVersion::AddIterators(const ReadOptions& options,
+                                       std::vector<Iterator*>* iterator_list) {
+  for (auto& m : memlist_) {
+    iterator_list->push_back(m->NewIterator(options));
+  }
+}
+
+void MemTableListVersion::Add(MemTable* m) {
+  assert(refs_ == 1);  // only when refs_ == 1 is MemTableListVersion mutable
+  m->Ref();
+  memlist_.push_front(m);
+  ++size_;
+}
+
+void MemTableListVersion::Remove(MemTable* m) {
+  assert(refs_ == 1);  // only when refs_ == 1 is MemTableListVersion mutable
+  MemTable* x __attribute__((unused)) = m->Unref();
+  assert(x == nullptr);  // it still needs to be alive!
+  memlist_.remove(m);
+  --size_;
 }
 
 // Returns true if there is at least one memtable on which flush has
 // not yet started.
-bool MemTableList::IsFlushPending(int min_write_buffer_number_to_merge) {
+bool MemTableList::IsFlushPending() {
   if ((flush_requested_ && num_flush_not_started_ >= 1) ||
-      (num_flush_not_started_ >= min_write_buffer_number_to_merge)) {
+      (num_flush_not_started_ >= min_write_buffer_number_to_merge_)) {
     assert(imm_flush_needed.NoBarrier_Load() != nullptr);
     return true;
   }
@@ -59,7 +103,8 @@ bool MemTableList::IsFlushPending(int min_write_buffer_number_to_merge) {
 
 // Returns the memtables that need to be flushed.
 void MemTableList::PickMemtablesToFlush(std::vector<MemTable*>* ret) {
-  for (auto it = memlist_.rbegin(); it != memlist_.rend(); it++) {
+  const auto& memlist = current_->memlist_;
+  for (auto it = memlist.rbegin(); it != memlist.rend(); ++it) {
     MemTable* m = *it;
     if (!m->flush_in_progress_) {
       assert(!m->flush_completed_);
@@ -122,8 +167,8 @@ Status MemTableList::InstallMemtableFlushResults(
   // scan all memtables from the earliest, and commit those
   // (in that order) that have finished flushing. Memetables
   // are always committed in the order that they were created.
-  while (!memlist_.empty() && s.ok()) {
+  while (!current_->memlist_.empty() && s.ok()) {
-    MemTable* m = memlist_.back(); // get the last element
+    MemTable* m = current_->memlist_.back();  // get the last element
     if (!m->flush_completed_) {
       break;
     }
@@ -135,6 +180,10 @@ Status MemTableList::InstallMemtableFlushResults(
     // this can release and reacquire the mutex.
     s = vset->LogAndApply(&m->edit_, mu);
 
+    // we will be changing the version in the next code path,
+    // so we better create a new one, since versions are immutable
+    InstallNewVersion();
+
     // All the later memtables that have the same filenum
     // are part of the same batch. They can be committed now.
     uint64_t mem_id = 1;  // how many memtables has been flushed.
@@ -144,7 +193,7 @@ Status MemTableList::InstallMemtableFlushResults(
             "Level-0 commit table #%lu: memtable #%lu done",
             (unsigned long)m->file_number_,
             (unsigned long)mem_id);
-        memlist_.remove(m);
+        current_->Remove(m);
         assert(m->file_number_ > 0);
 
         // pending_outputs can be cleared only after the newly created file
@@ -155,7 +204,6 @@ Status MemTableList::InstallMemtableFlushResults(
         if (m->Unref() != nullptr) {
           to_delete->push_back(m);
         }
-        size_--;
       } else {
         //commit failed. setup state so that we can flush again.
         Log(info_log,
@@ -172,7 +220,7 @@ Status MemTableList::InstallMemtableFlushResults(
         s = Status::IOError("Unable to commit flushed memtable");
       }
       ++mem_id;
-    } while (!memlist_.empty() && (m = memlist_.back()) &&
+    } while (!current_->memlist_.empty() && (m = current_->memlist_.back()) &&
              m->file_number_ == file_number);
   }
   commit_in_progress_ = false;
@@ -181,9 +229,9 @@ Status MemTableList::InstallMemtableFlushResults(
 
 // New memtables are inserted at the front of the list.
 void MemTableList::Add(MemTable* m) {
-  assert(size_ >= num_flush_not_started_);
+  assert(current_->size_ >= num_flush_not_started_);
-  size_++;
+  InstallNewVersion();
-  memlist_.push_front(m);
+  current_->Add(m);
   m->MarkImmutable();
   num_flush_not_started_++;
   if (num_flush_not_started_ == 1) {
@@ -194,28 +242,20 @@ void MemTableList::Add(MemTable* m) {
 // Returns an estimate of the number of bytes of data in use.
 size_t MemTableList::ApproximateMemoryUsage() {
   size_t size = 0;
-  for (auto &memtable : memlist_) {
+  for (auto& memtable : current_->memlist_) {
     size += memtable->ApproximateMemoryUsage();
   }
   return size;
 }
 
-// Search all the memtables starting from the most recent one.
+void MemTableList::InstallNewVersion() {
-// Return the most recent value found, if any.
+  if (current_->refs_ == 1) {
-// Operands stores the list of merge operations to apply, so far.
+    // we're the only one using the version, just keep using it
-bool MemTableList::Get(const LookupKey& key, std::string* value, Status* s,
+  } else {
-                       MergeContext& merge_context, const Options& options) {
+    // somebody else holds the current version, we need to create new one
-  for (auto &memtable : memlist_) {
+    MemTableListVersion* version = current_;
-    if (memtable->Get(key, value, s, merge_context, options)) {
+    current_ = new MemTableListVersion(current_);
-      return true;
+    version->Unref();
-    }
-  }
-  return false;
-}
-
-void MemTableList::GetMemTables(std::vector<MemTable*>* output) {
-  for (auto &memtable : memlist_) {
-    output->push_back(memtable);
   }
 }
 

db/memtablelist.h

@@ -7,8 +7,10 @@
 #pragma once
 #include <string>
 #include <list>
-#include <deque>
+#include <vector>
 #include "rocksdb/db.h"
+#include "rocksdb/options.h"
+#include "rocksdb/iterator.h"
 #include "db/dbformat.h"
 #include "db/skiplist.h"
 #include "memtable.h"
@@ -17,44 +19,71 @@ namespace rocksdb {
 
 class InternalKeyComparator;
 class Mutex;
-class MemTableListIterator;
 
-//
+// keeps a list of immutable memtables in a vector. the list is immutable
+// if refcount is bigger than one. It is used as a state for Get() and
+// Iterator code paths
+class MemTableListVersion {
+ public:
+  explicit MemTableListVersion(MemTableListVersion* old = nullptr);
+
+  void Ref();
+  void Unref(std::vector<MemTable*>* to_delete = nullptr);
+
+  int size() const;
+
+  // Search all the memtables starting from the most recent one.
+  // Return the most recent value found, if any.
+  bool Get(const LookupKey& key, std::string* value, Status* s,
+           MergeContext& merge_context, const Options& options);
+
+  void AddIterators(const ReadOptions& options,
+                    std::vector<Iterator*>* iterator_list);
+
+  // REQUIRE: m is mutable memtable
+  void Add(MemTable* m);
+  // REQUIRE: m is mutable memtable
+  void Remove(MemTable* m);
+
+ private:
+  friend class MemTableList;
+  std::list<MemTable*> memlist_;
+  int size_ = 0;
+  int refs_ = 1;
+};
+
 // This class stores references to all the immutable memtables.
 // The memtables are flushed to L0 as soon as possible and in
 // any order. If there are more than one immutable memtable, their
 // flushes can occur concurrently.  However, they are 'committed'
 // to the manifest in FIFO order to maintain correctness and
 // recoverability from a crash.
-//
 class MemTableList {
  public:
   // A list of memtables.
-  MemTableList() : size_(0), num_flush_not_started_(0),
+  explicit MemTableList(int min_write_buffer_number_to_merge)
-    commit_in_progress_(false),
+      : min_write_buffer_number_to_merge_(min_write_buffer_number_to_merge),
-    flush_requested_(false) {
+        current_(new MemTableListVersion()),
+        num_flush_not_started_(0),
+        commit_in_progress_(false),
+        flush_requested_(false) {
     imm_flush_needed.Release_Store(nullptr);
+    current_->Ref();
   }
-  ~MemTableList() {};
+  ~MemTableList() {}
+
+  MemTableListVersion* current() { return current_; }
 
   // so that backgrund threads can detect non-nullptr pointer to
   // determine whether this is anything more to start flushing.
   port::AtomicPointer imm_flush_needed;
 
-  // Increase reference count on all underling memtables
-  void RefAll();
-
-  // Drop reference count on all underling memtables. If the refcount
-  // on an underlying memtable drops to zero, then return it in
-  // to_delete vector.
-  void UnrefAll(std::vector<MemTable*>* to_delete);
-
   // Returns the total number of memtables in the list
-  int size();
+  int size() const;
 
   // Returns true if there is at least one memtable on which flush has
   // not yet started.
-  bool IsFlushPending(int min_write_buffer_number_to_merge);
+  bool IsFlushPending();
 
   // Returns the earliest memtables that needs to be flushed. The returned
   // memtables are guaranteed to be in the ascending order of created time.
@@ -75,14 +104,6 @@ class MemTableList {
   // Returns an estimate of the number of bytes of data in use.
   size_t ApproximateMemoryUsage();
 
-  // Search all the memtables starting from the most recent one.
-  // Return the most recent value found, if any.
-  bool Get(const LookupKey& key, std::string* value, Status* s,
-           MergeContext& merge_context, const Options& options);
-
-  // Returns the list of underlying memtables.
-  void GetMemTables(std::vector<MemTable*>* list);
-
   // Request a flush of all existing memtables to storage
   void FlushRequested() { flush_requested_ = true; }
 
@@ -91,8 +112,12 @@ class MemTableList {
   // void operator=(const MemTableList&);
 
  private:
-  std::list<MemTable*> memlist_;
+  // DB mutex held
-  int size_;
+  void InstallNewVersion();
+
+  int min_write_buffer_number_to_merge_;
+
+  MemTableListVersion* current_;
 
   // the number of elements that still need flushing
   int num_flush_not_started_;