tls: maintain a free slot index set in TLS InstanceImpl to allocate in O(1…

source/common/thread_local/thread_local_impl.cc

@@ -1,5 +1,6 @@
 #include "common/thread_local/thread_local_impl.h"
 
+#include <algorithm>
 #include <atomic>
 #include <cstdint>
 #include <list>
@@ -24,16 +25,16 @@ SlotPtr InstanceImpl::allocateSlot() {
   ASSERT(std::this_thread::get_id() == main_thread_id_);
   ASSERT(!shutdown_);
 
-  for (uint64_t i = 0; i < slots_.size(); i++) {
-    if (slots_[i] == nullptr) {
-      std::unique_ptr<SlotImpl> slot(new SlotImpl(*this, i));
-      slots_[i] = slot.get();
-      return slot;
-    }
+  if (free_slot_indexes_.empty()) {
+    std::unique_ptr<SlotImpl> slot(new SlotImpl(*this, slots_.size()));
+    slots_.push_back(slot.get());
+    return slot;
   }
-
-  std::unique_ptr<SlotImpl> slot(new SlotImpl(*this, slots_.size()));
-  slots_.push_back(slot.get());
+  uint32_t idx = free_slot_indexes_.front();
+  free_slot_indexes_.pop_front();
+  ASSERT(idx < slots_.size());
+  std::unique_ptr<SlotImpl> slot(new SlotImpl(*this, idx));
+  slots_[idx] = slot.get();
   return slot;
 }
 
@@ -73,6 +74,10 @@ void InstanceImpl::removeSlot(SlotImpl& slot) {
 
   const uint64_t index = slot.index_;
   slots_[index] = nullptr;
+  ASSERT(std::find(free_slot_indexes_.begin(), free_slot_indexes_.end(), index) ==
+             free_slot_indexes_.end(),
+         fmt::format("slot index {} already in free slot set!", index));
+  free_slot_indexes_.push_back(index);
   runOnAllThreads([index]() -> void {
     // This runs on each thread and clears the slot, making it available for a new allocations.
     // This is safe even if a new allocation comes in, because everything happens with post() and

source/common/thread_local/thread_local_impl.h

@@ -9,6 +9,8 @@
 
 #include "common/common/logger.h"
 
+#include "absl/container/flat_hash_set.h"
+
 namespace Envoy {
 namespace ThreadLocal {
 
@@ -57,6 +59,9 @@ class InstanceImpl : Logger::Loggable<Logger::Id::main>, public Instance {
 
   static thread_local ThreadLocalData thread_local_data_;
   std::vector<SlotImpl*> slots_;
+  // A list of index of freed slots.
+  std::list<uint32_t> free_slot_indexes_;
+
   std::list<std::reference_wrapper<Event::Dispatcher>> registered_threads_;
   std::thread::id main_thread_id_;
   Event::Dispatcher* main_thread_dispatcher_{};