Avoid switching empty memtable in certain cases (#4792)

Summary:
in certain cases, we do not perform memtable switching if the active
memtable of the column family is empty. Two exceptions:
1. In manual flush, if cached_recoverable_state_empty_ is false, then we need
   to switch memtable due to requirement of transaction.
2. In switch WAL, we need to switch memtable anyway because we have to seal the
   memtable if the WAL on which it depends will be closed.

This change can potentially delay the occurence of write stalls because number
of memtables increase more slowly.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4792

Differential Revision: D13499501

Pulled By: riversand963

fbshipit-source-id: 91c9b17ae753578578039f3851667d93610005e1

db/db_impl_compaction_flush.cc

@@ -1464,11 +1464,16 @@ Status DBImpl::FlushMemTable(ColumnFamilyData* cfd,
       write_thread_.EnterUnbatched(&w, &mutex_);
     }
 
-    if (cfd->imm()->NumNotFlushed() != 0 || !cfd->mem()->IsEmpty() ||
-        !cached_recoverable_state_empty_.load()) {
+    if (!cfd->mem()->IsEmpty() || !cached_recoverable_state_empty_.load()) {
       s = SwitchMemtable(cfd, &context);
-      flush_memtable_id = cfd->imm()->GetLatestMemTableID();
-      flush_req.emplace_back(cfd, flush_memtable_id);
+    }
+
+    if (s.ok()) {
+      if (cfd->imm()->NumNotFlushed() != 0 || !cfd->mem()->IsEmpty() ||
+          !cached_recoverable_state_empty_.load()) {
+        flush_memtable_id = cfd->imm()->GetLatestMemTableID();
+        flush_req.emplace_back(cfd, flush_memtable_id);
+      }
     }
 
     if (s.ok() && !flush_req.empty()) {
@@ -1542,6 +1547,9 @@ Status DBImpl::AtomicFlushMemTables(
       }
     }
     for (auto cfd : cfds) {
+      if (cfd->mem()->IsEmpty() && cached_recoverable_state_empty_.load()) {
+        continue;
+      }
       cfd->Ref();
       s = SwitchMemtable(cfd, &context);
       cfd->Unref();