- Add benchmark for hashtable reconstruction.

- Optimize by not initializing hash_node._node char array.
- Also skip reconstruction, if it is in initial state.

3 files changed, 26 insertions, 1 deletions

diff --git a/vespalib/src/tests/stllike/hashtable_test.cpp b/vespalib/src/tests/stllike/hashtable_test.cpp
index cbd8b28d9a8..5f6f1601c84 100644
--- a/vespalib/src/tests/stllike/hashtable_test.cpp
+++ b/vespalib/src/tests/stllike/hashtable_test.cpp
@@ -134,6 +134,27 @@ TEST("require that hashtable<vector<int>> can be copied") {
     EXPECT_EQUAL(6, (*table.find(2))[2]);
 }
 
+/**
+ * Test to profile destruction and recreation of hash map.
+ * It revealed some unexpected behaviour. Results with 10k iterations on 2018 macbook pro 2.6 Ghz i7
+ * 1 - previous - 14.7s hash_node() : _node(), _next(invalid) {}
+ * 2 - test     -  6.6s hash_node() : _next(invalid) { memset(_node, 0, sizeof(node)); }
+ * 3 - current  -  2.3s hash_node() : _next(invalid) {}
+ */
+TEST("benchmark clear") {
+    vespalib::hash_map<uint32_t, uint32_t> m(1000000);
+    constexpr size_t NUM_ITER = 10; // Set to 1k-10k to get measurable numbers 10k ~= 2.3s
+    for (size_t i(0); i < NUM_ITER; i++) {
+        m[46] = 17;
+        EXPECT_FALSE(m.empty());
+        EXPECT_EQUAL(1u, m.size());
+        EXPECT_EQUAL(1048576u, m.capacity());
+        m.clear();
+        EXPECT_TRUE(m.empty());
+        EXPECT_EQUAL(1048576u, m.capacity());
+    }
+}
+
 }  // namespace
 
 TEST_MAIN() { TEST_RUN_ALL(); }
diff --git a/vespalib/src/vespa/vespalib/stllike/hashtable.h b/vespalib/src/vespa/vespalib/stllike/hashtable.h
index b94672aaa06..ede18f89dc2 100644
--- a/vespalib/src/vespa/vespalib/stllike/hashtable.h
+++ b/vespalib/src/vespa/vespalib/stllike/hashtable.h
@@ -102,7 +102,9 @@ class hash_node {
 public:
     using next_t=hashtable_base::next_t;
     enum {npos=-1u, invalid=-2u};
-    hash_node() : _node(), _next(invalid) {}
+    hash_node()
+        : _next(invalid)
+    {}
     hash_node(const V & node, next_t next=npos)
         : _next(next)
     {
diff --git a/vespalib/src/vespa/vespalib/stllike/hashtable.hpp b/vespalib/src/vespa/vespalib/stllike/hashtable.hpp
index d80113a8f55..494dc223f5b 100644
--- a/vespalib/src/vespa/vespalib/stllike/hashtable.hpp
+++ b/vespalib/src/vespa/vespalib/stllike/hashtable.hpp
@@ -146,6 +146,8 @@ hashtable<Key, Value, Hash, Equal, KeyExtract, Modulator>::erase(const Key & key
 template< typename Key, typename Value, typename Hash, typename Equal, typename KeyExtract, typename Modulator >
 void
 hashtable<Key, Value, Hash, Equal, KeyExtract, Modulator>::clear() {
+    if (_count == 0) return; // Already empty and properly initialized
+
     _nodes.clear();
     _count = 0;
     _nodes.resize(getTableSize());