Simplify column family concurrency

Summary:
This patch changes concurrency guarantees around ColumnFamilySet::column_families_ and ColumnFamilySet::column_families_data_.

Before:
* When mutating: lock DB mutex and spin lock
* When reading: lock DB mutex OR spin lock

After:
* When mutating: lock DB mutex and be in write thread
* When reading: lock DB mutex or be in write thread

That way, we eliminate the spin lock that protects these hash maps and  simplify concurrency. That means we don't need to lock the spin lock during writing, since writing is mutually exclusive with column family create/drop (the only operations that mutate those hash maps).

With these new restrictions, I also needed to move column family create to the write thread (column family drop was already in the write thread).

Even though we don't need to lock the spin lock during write, impact on performance should be minimal -- the spin lock is almost never busy, so locking it is almost free.

This addresses task t5116919.

Test Plan:
make check

Stress test with lots and lots of column family drop and create:

   time ./db_stress --threads=30 --ops_per_thread=5000000 --max_key=5000 --column_families=200 --clear_column_family_one_in=100000 --verify_before_write=0  --reopen=15 --max_background_compactions=10 --max_background_flushes=10 --db=/fast-rocksdb-tmp/db_stress/

Reviewers: yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30651

db/column_family.cc

@@ -306,9 +306,10 @@ ColumnFamilyData::~ColumnFamilyData() {
   prev->next_ = next;
   next->prev_ = prev;
 
-  // it's nullptr for dummy CFD
-  if (column_family_set_ != nullptr) {
-    // remove from column_family_set
+  if (!dropped_ && column_family_set_ != nullptr) {
+    // If it's dropped, it's already removed from column family set
+    // If column_family_set_ == nullptr, this is dummy CFD and not in
+    // ColumnFamilySet
     column_family_set_->RemoveColumnFamily(this);
   }
 
@@ -353,6 +354,16 @@ ColumnFamilyData::~ColumnFamilyData() {
   }
 }
 
+void ColumnFamilyData::SetDropped() {
+  // can't drop default CF
+  assert(id_ != 0);
+  dropped_ = true;
+  write_controller_token_.reset();
+
+  // remove from column_family_set
+  column_family_set_->RemoveColumnFamily(this);
+}
+
 void ColumnFamilyData::RecalculateWriteStallConditions(
       const MutableCFOptions& mutable_cf_options) {
   if (current_ != nullptr) {
@@ -635,8 +646,7 @@ ColumnFamilySet::ColumnFamilySet(const std::string& dbname,
       env_options_(env_options),
       table_cache_(table_cache),
       write_buffer_(write_buffer),
-      write_controller_(write_controller),
-      spin_lock_(ATOMIC_FLAG_INIT) {
+      write_controller_(write_controller) {
   // initialize linked list
   dummy_cfd_->prev_ = dummy_cfd_;
   dummy_cfd_->next_ = dummy_cfd_;
@@ -693,7 +703,7 @@ size_t ColumnFamilySet::NumberOfColumnFamilies() const {
   return column_families_.size();
 }
 
-// under a DB mutex
+// under a DB mutex AND write thread
 ColumnFamilyData* ColumnFamilySet::CreateColumnFamily(
     const std::string& name, uint32_t id, Version* dummy_versions,
     const ColumnFamilyOptions& options) {
@@ -702,10 +712,8 @@ ColumnFamilyData* ColumnFamilySet::CreateColumnFamily(
       new ColumnFamilyData(id, name, dummy_versions, table_cache_,
                            write_buffer_, options, db_options_,
                            env_options_, this);
-  Lock();
   column_families_.insert({name, id});
   column_family_data_.insert({id, new_cfd});
-  Unlock();
   max_column_family_ = std::max(max_column_family_, id);
   // add to linked list
   new_cfd->next_ = dummy_cfd_;
@@ -719,14 +727,6 @@ ColumnFamilyData* ColumnFamilySet::CreateColumnFamily(
   return new_cfd;
 }
 
-void ColumnFamilySet::Lock() {
-  // spin lock
-  while (spin_lock_.test_and_set(std::memory_order_acquire)) {
-  }
-}
-
-void ColumnFamilySet::Unlock() { spin_lock_.clear(std::memory_order_release); }
-
 // REQUIRES: DB mutex held
 void ColumnFamilySet::FreeDeadColumnFamilies() {
   autovector<ColumnFamilyData*> to_delete;
@@ -741,30 +741,21 @@ void ColumnFamilySet::FreeDeadColumnFamilies() {
   }
 }
 
-// under a DB mutex
+// under a DB mutex AND from a write thread
 void ColumnFamilySet::RemoveColumnFamily(ColumnFamilyData* cfd) {
   auto cfd_iter = column_family_data_.find(cfd->GetID());
   assert(cfd_iter != column_family_data_.end());
-  Lock();
   column_family_data_.erase(cfd_iter);
   column_families_.erase(cfd->GetName());
-  Unlock();
 }
 
+// under a DB mutex OR from a write thread
 bool ColumnFamilyMemTablesImpl::Seek(uint32_t column_family_id) {
   if (column_family_id == 0) {
     // optimization for common case
     current_ = column_family_set_->GetDefault();
   } else {
-    // maybe outside of db mutex, should lock
-    column_family_set_->Lock();
     current_ = column_family_set_->GetColumnFamily(column_family_id);
-    column_family_set_->Unlock();
-    // TODO(icanadi) Maybe remove column family from the hash table when it's
-    // dropped?
-    if (current_ != nullptr && current_->IsDropped()) {
-      current_ = nullptr;
-    }
   }
   handle_.SetCFD(current_);
   return current_ != nullptr;

db/column_family.h

@@ -123,8 +123,7 @@ extern ColumnFamilyOptions SanitizeOptions(const InternalKeyComparator* icmp,
 
 class ColumnFamilySet;
 
-// This class keeps all the data that a column family needs. It's mosly dumb and
-// used just to provide access to metadata.
+// This class keeps all the data that a column family needs.
 // Most methods require DB mutex held, unless otherwise noted
 class ColumnFamilyData {
  public:
@@ -145,7 +144,10 @@ class ColumnFamilyData {
     return --refs_ == 0;
   }
 
-  // This can only be called from single-threaded VersionSet::LogAndApply()
+  // SetDropped() can only be called under following conditions:
+  // 1) Holding a DB mutex,
+  // 2) from single-threaded write thread, AND
+  // 3) from single-threaded VersionSet::LogAndApply()
   // After dropping column family no other operation on that column family
   // will be executed. All the files and memory will be, however, kept around
   // until client drops the column family handle. That way, client can still
@@ -153,17 +155,12 @@ class ColumnFamilyData {
   // Column family can be dropped and still alive. In that state:
   // *) Column family is not included in the iteration.
   // *) Compaction and flush is not executed on the dropped column family.
-  // *) Client can continue writing and reading from column family. However, all
-  // writes stay in the current memtable.
+  // *) Client can continue reading from column family. Writes will fail unless
+  // WriteOptions::ignore_missing_column_families is true
   // When the dropped column family is unreferenced, then we:
   // *) delete all memory associated with that column family
   // *) delete all the files associated with that column family
-  void SetDropped() {
-    // can't drop default CF
-    assert(id_ != 0);
-    dropped_ = true;
-    write_controller_token_.reset();
-  }
+  void SetDropped();
   bool IsDropped() const { return dropped_; }
 
   // thread-safe
@@ -348,18 +345,21 @@ class ColumnFamilyData {
 };
 
 // ColumnFamilySet has interesting thread-safety requirements
-// * CreateColumnFamily() or RemoveColumnFamily() -- need to protect by DB
-// mutex. Inside, column_family_data_ and column_families_ will be protected
-// by Lock() and Unlock(). CreateColumnFamily() should ONLY be called from
-// VersionSet::LogAndApply() in the normal runtime. It is also called
-// during Recovery and in DumpManifest(). RemoveColumnFamily() is called
-// from ColumnFamilyData destructor
+// * CreateColumnFamily() or RemoveColumnFamily() -- need to be protected by DB
+// mutex AND executed in the write thread.
+// CreateColumnFamily() should ONLY be called from VersionSet::LogAndApply() AND
+// single-threaded write thread. It is also called during Recovery and in
+// DumpManifest().
+// RemoveColumnFamily() is only called from SetDropped(). DB mutex needs to be
+// held and it needs to be executed from the write thread. SetDropped() also
+// guarantees that it will be called only from single-threaded LogAndApply(),
+// but this condition is not that important.
 // * Iteration -- hold DB mutex, but you can release it in the body of
 // iteration. If you release DB mutex in body, reference the column
 // family before the mutex and unreference after you unlock, since the column
 // family might get dropped when the DB mutex is released
 // * GetDefault() -- thread safe
-// * GetColumnFamily() -- either inside of DB mutex or call Lock() <-> Unlock()
+// * GetColumnFamily() -- either inside of DB mutex or from a write thread
 // * GetNextColumnFamilyID(), GetMaxColumnFamily(), UpdateMaxColumnFamily(),
 // NumberOfColumnFamilies -- inside of DB mutex
 class ColumnFamilySet {
@@ -410,9 +410,6 @@ class ColumnFamilySet {
   iterator begin() { return iterator(dummy_cfd_->next_); }
   iterator end() { return iterator(dummy_cfd_); }
 
-  void Lock();
-  void Unlock();
-
   // REQUIRES: DB mutex held
   // Don't call while iterating over ColumnFamilySet
   void FreeDeadColumnFamilies();
@@ -424,9 +421,12 @@ class ColumnFamilySet {
   void RemoveColumnFamily(ColumnFamilyData* cfd);
 
   // column_families_ and column_family_data_ need to be protected:
-  // * when mutating: 1. DB mutex locked first, 2. spinlock locked second
-  // * when reading, either: 1. lock DB mutex, or 2. lock spinlock
-  //  (if both, respect the ordering to avoid deadlock!)
+  // * when mutating both conditions have to be satisfied:
+  // 1. DB mutex locked
+  // 2. thread currently in single-threaded write thread
+  // * when reading, at least one condition needs to be satisfied:
+  // 1. DB mutex locked
+  // 2. accessed from a single-threaded write thread
   std::unordered_map<std::string, uint32_t> column_families_;
   std::unordered_map<uint32_t, ColumnFamilyData*> column_family_data_;
 
@@ -444,7 +444,6 @@ class ColumnFamilySet {
   Cache* table_cache_;
   WriteBuffer* write_buffer_;
   WriteController* write_controller_;
-  std::atomic_flag spin_lock_;
 };
 
 // We use ColumnFamilyMemTablesImpl to provide WriteBatch a way to access
@@ -459,17 +458,22 @@ class ColumnFamilyMemTablesImpl : public ColumnFamilyMemTables {
 
   // sets current_ to ColumnFamilyData with column_family_id
   // returns false if column family doesn't exist
+  // REQUIRES: under a DB mutex OR from a write thread
   bool Seek(uint32_t column_family_id) override;
 
   // Returns log number of the selected column family
+  // REQUIRES: under a DB mutex OR from a write thread
   uint64_t GetLogNumber() const override;
 
   // REQUIRES: Seek() called first
+  // REQUIRES: under a DB mutex OR from a write thread
   virtual MemTable* GetMemTable() const override;
 
   // Returns column family handle for the selected column family
+  // REQUIRES: under a DB mutex OR from a write thread
   virtual ColumnFamilyHandle* GetColumnFamilyHandle() override;
 
+  // REQUIRES: under a DB mutex OR from a write thread
   virtual void CheckMemtableFull() override;
 
  private:

db/db_impl.cc

@@ -2579,9 +2579,17 @@ Status DBImpl::CreateColumnFamily(const ColumnFamilyOptions& cf_options,
     // LogAndApply will both write the creation in MANIFEST and create
     // ColumnFamilyData object
     Options opt(db_options_, cf_options);
-    s = versions_->LogAndApply(nullptr,
-        MutableCFOptions(opt, ImmutableCFOptions(opt)),
-        &edit, &mutex_, db_directory_.get(), false, &cf_options);
+    {  // write thread
+      WriteThread::Writer w(&mutex_);
+      s = write_thread_.EnterWriteThread(&w, 0);
+      assert(s.ok() && !w.done);  // No timeout and nobody should do our job
+      // LogAndApply will both write the creation in MANIFEST and create
+      // ColumnFamilyData object
+      s = versions_->LogAndApply(
+          nullptr, MutableCFOptions(opt, ImmutableCFOptions(opt)), &edit,
+          &mutex_, db_directory_.get(), false, &cf_options);
+      write_thread_.ExitWriteThread(&w, &w, s);
+    }
     if (s.ok()) {
       single_column_family_mode_ = false;
       auto* cfd =

db/write_batch.cc

@@ -280,6 +280,8 @@ void WriteBatch::PutLogData(const Slice& blob) {
 }
 
 namespace {
+// This class can *only* be used from a single-threaded write thread, because it
+// calls ColumnFamilyMemTablesImpl::Seek()
 class MemTableInserter : public WriteBatch::Handler {
  public:
   SequenceNumber sequence_;
@@ -305,6 +307,8 @@ class MemTableInserter : public WriteBatch::Handler {
   }
 
   bool SeekToColumnFamily(uint32_t column_family_id, Status* s) {
+    // We are only allowed to call this from a single-threaded write thread
+    // (or while holding DB mutex)
     bool found = cf_mems_->Seek(column_family_id);
     if (!found) {
       if (ignore_missing_column_families_) {
@@ -485,6 +489,11 @@ class MemTableInserter : public WriteBatch::Handler {
 };
 }  // namespace
 
+// This function can only be called in these conditions:
+// 1) During Recovery()
+// 2) during Write(), in a single-threaded write thread
+// The reason is that it calles ColumnFamilyMemTablesImpl::Seek(), which needs
+// to be called from a single-threaded write thread (or while holding DB mutex)
 Status WriteBatchInternal::InsertInto(const WriteBatch* b,
                                       ColumnFamilyMemTables* memtables,
                                       bool ignore_missing_column_families,