Reserve reasonably sized buffer upfront to keep memory footprint stable

2 files changed, 22 insertions, 9 deletions

diff --git a/searchlib/src/vespa/searchlib/attribute/changevector.h b/searchlib/src/vespa/searchlib/attribute/changevector.h
index af4a4fd6618..c3b4d0ce3b0 100644
--- a/searchlib/src/vespa/searchlib/attribute/changevector.h
+++ b/searchlib/src/vespa/searchlib/attribute/changevector.h
@@ -4,6 +4,7 @@
 
 #include <vespa/vespalib/stllike/hash_map.h>
 #include <vespa/searchcommon/common/undefinedvalues.h>
+#include <vector>
 
 namespace vespalib { class MemoryUsage; }
 
@@ -141,8 +142,8 @@ protected:
 template <typename T>
 class ChangeVectorT : public ChangeVectorBase {
 private:
-    typedef vespalib::hash_map<uint32_t, uint32_t> Map;
-    typedef vespalib::Array<T> Vector;
+    using Map = vespalib::hash_map<uint32_t, uint32_t>;
+    using Vector = std::vector<T>;
 public:
     ChangeVectorT();
     ~ChangeVectorT();
@@ -175,8 +176,8 @@ public:
     vespalib::MemoryUsage getMemoryUsage() const;
 private:
     void linkIn(uint32_t doc, size_t index, size_t last);
-    Vector _v;
-    Map    _docs;
+    Vector   _v;
+    Map      _docs;
     uint32_t _tail;
 };
 
diff --git a/searchlib/src/vespa/searchlib/attribute/changevector.hpp b/searchlib/src/vespa/searchlib/attribute/changevector.hpp
index da18da4e9d5..dcb31ebae73 100644
--- a/searchlib/src/vespa/searchlib/attribute/changevector.hpp
+++ b/searchlib/src/vespa/searchlib/attribute/changevector.hpp
@@ -3,16 +3,28 @@
 #pragma once
 
 #include "changevector.h"
-#include <vespa/vespalib/util/array.hpp>
 #include <vespa/vespalib/util/memoryusage.h>
 
 namespace search {
 
+namespace {
+
+// This number is selected to be large enough to hold bursts between commits
+constexpr size_t NUM_ELEMS_TO_RESERVE = 200;
+
+}
+
 template <typename T>
-ChangeVectorT<T>::ChangeVectorT() : _tail(0) { }
-    
+ChangeVectorT<T>::ChangeVectorT()
+    : _v(),
+      _docs(NUM_ELEMS_TO_RESERVE*2),
+      _tail(0)
+{
+    _v.reserve(vespalib::roundUp2inN(NUM_ELEMS_TO_RESERVE, sizeof(T)));
+}
+
 template <typename T>
-ChangeVectorT<T>::~ChangeVectorT() { }
+ChangeVectorT<T>::~ChangeVectorT() = default;
 
 template <typename T>
 void
@@ -38,7 +50,7 @@ ChangeVectorT<T>::push_back(uint32_t doc, Accessor & ac)
     if (ac.size() <= 0) { return; }
 
     size_t index(size());
-    _v.reserve(vespalib::roundUp2inN(index + ac.size()));
+    _v.reserve(vespalib::roundUp2inN(index + ac.size(), sizeof(T)));
     for (size_t i(0), m(ac.size()); i < m; i++, ac.next()) {
         _v.push_back(T(ChangeBase::APPEND, doc, typename T::DataType(ac.value()), ac.weight()));
         _v.back().setNext(index + i + 1);