diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3bd50074..43b627b0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -562,8 +562,8 @@ set(CXX_FLAGS -Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-sec
 
 if(USE_SANITIZE)
   message(STATUS "Sanitize: ${USE_SANITIZE}")
-  set(CXX_FLAGS "${CXX_FLAGS};-fsanitize=address;-fno-omit-frame-pointer;-fno-common")
-  set(CMAKE_EXE_LINKER_FLAGS "-fsanitize=address")
+  set(CXX_FLAGS "${CXX_FLAGS};-fsanitize=thread")
+  set(CMAKE_EXE_LINKER_FLAGS "-fsanitize=thread")
 endif()
 
 target_compile_options(OpenSTA
diff --git a/include/sta/Search.hh b/include/sta/Search.hh
index 5828f7ba..86f43c1e 100644
--- a/include/sta/Search.hh
+++ b/include/sta/Search.hh
@@ -583,12 +583,14 @@ protected:
   // Entries in tags_ may be missing where previous filter tags were deleted.
   TagIndex tag_capacity_;
   Tag **tags_;
+  Tag **tags_prev_;
   TagIndex tag_next_;
   // Holes in tags_ left by deleting filter tags.
   std::vector<TagIndex> tag_free_indices_;
   std::mutex tag_lock_;
   TagGroupSet *tag_group_set_;
   TagGroup **tag_groups_;
+  TagGroup **tag_groups_prev_;
   TagGroupIndex tag_group_next_;
   // Holes in tag_groups_ left by deleting filter tag groups.
   std::vector<TagIndex> tag_group_free_indices_;
diff --git a/parasitics/ConcreteParasitics.cc b/parasitics/ConcreteParasitics.cc
index d636ea05..5a78c154 100644
--- a/parasitics/ConcreteParasitics.cc
+++ b/parasitics/ConcreteParasitics.cc
@@ -946,9 +946,9 @@ ConcreteParasitics::findPiElmore(const Pin *drvr_pin,
 				 const RiseFall *rf,
 				 const ParasiticAnalysisPt *ap) const
 {
+  LockGuard lock(lock_);
   if (!drvr_parasitic_map_.empty()) {
     int ap_rf_index = parasiticAnalysisPtIndex(ap, rf);
-    LockGuard lock(lock_);
     ConcreteParasitic **parasitics = drvr_parasitic_map_.findKey(drvr_pin);
     if (parasitics) {
       ConcreteParasitic *parasitic = parasitics[ap_rf_index];
diff --git a/search/PathGroup.cc b/search/PathGroup.cc
index 0b61358e..c9a3cb21 100644
--- a/search/PathGroup.cc
+++ b/search/PathGroup.cc
@@ -95,17 +95,22 @@ PathGroup::~PathGroup()
 bool
 PathGroup::savable(PathEnd *path_end)
 {
+  float threshold;
+  {
+    LockGuard lock(lock_);
+    threshold = threshold_;
+  }
   bool savable = false;
   if (compare_slack_) {
     // Crpr increases the slack, so check the slack
     // without crpr first because it is expensive to find.
     Slack slack = path_end->slackNoCrpr(sta_);
     if (!delayIsInitValue(slack, min_max_)
- 	&& delayLessEqual(slack, threshold_, sta_)
+ 	&& delayLessEqual(slack, threshold, sta_)
  	&& delayLessEqual(slack, slack_max_, sta_)) {
       // Now check with crpr.
       slack = path_end->slack(sta_);
-      savable = delayLessEqual(slack, threshold_, sta_)
+      savable = delayLessEqual(slack, threshold, sta_)
  	&& delayLessEqual(slack, slack_max_, sta_)
  	&& delayGreaterEqual(slack, slack_min_, sta_);
     }
@@ -113,7 +118,7 @@ PathGroup::savable(PathEnd *path_end)
   else {
     const Arrival &arrival = path_end->dataArrivalTime(sta_);
     savable = !delayIsInitValue(arrival, min_max_)
-      && delayGreaterEqual(arrival, threshold_, min_max_, sta_);
+      && delayGreaterEqual(arrival, threshold, min_max_, sta_);
   }
   return savable;
 }
diff --git a/search/Search.cc b/search/Search.cc
index 1a358e54..3f1b7bb9 100644
--- a/search/Search.cc
+++ b/search/Search.cc
@@ -237,8 +237,10 @@ Search::init(StaState *sta)
   clk_info_set_ = new ClkInfoSet(ClkInfoLess(sta));
   tag_next_ = 0;
   tags_ = new Tag*[tag_capacity_];
+  tags_prev_ = nullptr;
   tag_group_capacity_ = 127;
   tag_groups_ = new TagGroup*[tag_group_capacity_];
+  tag_groups_prev_ = nullptr;
   tag_group_next_ = 0;
   tag_group_set_ = new TagGroupSet(tag_group_capacity_);
   pending_latch_outputs_ = new VertexSet(graph_);
@@ -270,7 +272,9 @@ Search::~Search()
   delete tag_set_;
   delete clk_info_set_;
   delete [] tags_;
+  delete [] tags_prev_;
   delete [] tag_groups_;
+  delete [] tag_groups_prev_;
   delete tag_group_set_;
   delete search_adj_;
   delete eval_pred_;
@@ -2647,15 +2651,15 @@ Search::findTagGroup(TagGroupBldr *tag_bldr)
     // std::vector doesn't seem to follow this protocol so multi-thread
     // search fails occasionally if a vector is used for tag_groups_.
     if (tag_group_next_ == tag_group_capacity_) {
-      TagGroupIndex new_capacity = nextMersenne(tag_group_capacity_);
-      TagGroup **new_tag_groups = new TagGroup*[new_capacity];
-      memcpy(new_tag_groups, tag_groups_,
+      TagGroupIndex tag_capacity = tag_group_capacity_ * 2;
+      TagGroup **tag_groups = new TagGroup*[tag_capacity];
+      memcpy(tag_groups, tag_groups_,
              tag_group_capacity_ * sizeof(TagGroup*));
-      TagGroup **old_tag_groups = tag_groups_;
-      tag_groups_ = new_tag_groups;
-      tag_group_capacity_ = new_capacity;
-      delete [] old_tag_groups;
-      tag_group_set_->reserve(new_capacity);
+      delete [] tag_groups_prev_;
+      tag_groups_prev_ = tag_groups_;
+      tag_groups_ = tag_groups;
+      tag_group_capacity_ = tag_capacity;
+      tag_group_set_->reserve(tag_capacity);
     }
     if (tag_group_next_ > tag_group_index_max)
       report_->critical(1510, "max tag group index exceeded");
@@ -2888,14 +2892,14 @@ Search::findTag(const RiseFall *rf,
     // std::vector doesn't seem to follow this protocol so multi-thread
     // search fails occasionally if a vector is used for tags_.
     if (tag_next_ == tag_capacity_) {
-      TagIndex new_capacity = nextMersenne(tag_capacity_);
-      Tag **new_tags = new Tag*[new_capacity];
-      memcpy(new_tags, tags_, tag_capacity_ * sizeof(Tag*));
-      Tag **old_tags = tags_;
-      tags_ = new_tags;
-      delete [] old_tags;
-      tag_capacity_ = new_capacity;
-      tag_set_->reserve(new_capacity);
+      TagIndex tag_capacity = tag_capacity_ * 2;
+      Tag **tags = new Tag*[tag_capacity];
+      memcpy(tags, tags_, tag_capacity_ * sizeof(Tag*));
+      delete [] tags_prev_;
+      tags_prev_ = tags_;
+      tags_ = tags;
+      tag_capacity_ = tag_capacity;
+      tag_set_->reserve(tag_capacity);
     }
     if (tag_next_ == tag_index_max)
       report_->critical(1511, "max tag index exceeded");