MergingIterator uses autovector instead of vector

Summary:
Use autovector in MergingIterator so that if there are 4 or less child iterators in it, iterator wrappers are inline, which is more likely to be cache friendly.

Based on one test run with a shadow traffic of one product, it reduces CPU of MergingIterator::Seek() by half.

Test Plan: make all check

Reviewers: haobo, yhchiang, igor, dhruba

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18531

table/iterator_wrapper.h

@@ -20,6 +20,13 @@ class IteratorWrapper {
   explicit IteratorWrapper(Iterator* iter): iter_(nullptr) {
     Set(iter);
   }
+  IteratorWrapper(const IteratorWrapper&) {
+    // Iterator wrapper exclusively owns iter_ so it cannot be copied.
+    // Didn't delete the function because vector<IteratorWrapper> requires
+    // this function to compile.
+    assert(false);
+  }
+  void operator=(const IteratorWrapper&) = delete;
   ~IteratorWrapper() { delete iter_; }
   Iterator* iter() const { return iter_; }
 
@@ -35,7 +42,6 @@ class IteratorWrapper {
     }
   }
 
-
   // Iterator interface methods
   bool Valid() const        { return valid_; }
   Slice key() const         { assert(Valid()); return key_; }

table/merger.cc

@@ -19,6 +19,7 @@
 #include "table/iterator_wrapper.h"
 #include "util/stop_watch.h"
 #include "util/perf_context_imp.h"
+#include "util/autovector.h"
 
 namespace rocksdb {
 namespace {
@@ -43,16 +44,18 @@ MinIterHeap NewMinIterHeap(const Comparator* comparator) {
   return MinIterHeap(MinIteratorComparator(comparator));
 }
 
+const size_t kNumIterReserve = 4;
+
 class MergingIterator : public Iterator {
  public:
   MergingIterator(const Comparator* comparator, Iterator** children, int n)
       : comparator_(comparator),
-        children_(n),
         current_(nullptr),
         use_heap_(true),
         direction_(kForward),
         maxHeap_(NewMaxIterHeap(comparator_)),
         minHeap_(NewMinIterHeap(comparator_)) {
+    children_.resize(n);
     for (int i = 0; i < n; i++) {
       children_[i].Set(children[i]);
     }
@@ -240,7 +243,7 @@ class MergingIterator : public Iterator {
   void ClearHeaps();
 
   const Comparator* comparator_;
-  std::vector<IteratorWrapper> children_;
+  autovector<IteratorWrapper, kNumIterReserve> children_;
   IteratorWrapper* current_;
   // If the value is true, both of iterators in the heap and current_
   // contain valid rows. If it is false, only current_ can possibly contain