Global delay bug fix: use credits only under mutex (#438)

While at it, rename and reorder some members

db/column_family.cc

@@ -872,7 +872,6 @@ int GetL0ThresholdSpeedupCompaction(int level0_file_num_compaction_trigger,
 namespace {
 const uint64_t Gb = 1ull << 30;
 const uint64_t Mb = 1ull << 20;
-const uint64_t kMinWriteRate = 16 * 1024u;  // Minimum write rate 16KB/s.
 const int kMemtablePenalty = 10;
 }  // namespace
 
@@ -899,8 +898,8 @@ std::unique_ptr<WriteControllerToken> ColumnFamilyData::DynamicSetupDelay(
           compaction_needed_bytes, mutable_cf_options, write_stall_cause);
   assert(rate_divider >= 1);
   auto write_rate = static_cast<uint64_t>(max_write_rate / rate_divider);
-  if (write_rate < kMinWriteRate) {
-    write_rate = kMinWriteRate;
+  if (write_rate < WriteController::kMinWriteRate) {
+    write_rate = WriteController::kMinWriteRate;
   }
 
   // GetDelayToken returns a DelayWriteToken and also sets the

db/write_controller.cc

@@ -24,7 +24,7 @@ std::unique_ptr<WriteControllerToken> WriteController::GetStopToken() {
 std::unique_ptr<WriteControllerToken> WriteController::GetDelayToken(
     uint64_t write_rate) {
   {
-    std::lock_guard<std::mutex> lock(mu_);
+    std::lock_guard<std::mutex> lock(metrics_mu_);
     if (0 == total_delayed_++) {
       // Starting delay, so reset counters.
       next_refill_time_ = 0;
@@ -41,12 +41,12 @@ std::unique_ptr<WriteControllerToken> WriteController::GetDelayToken(
 uint64_t WriteController::TEST_GetMapMinRate() { return GetMapMinRate(); }
 
 void WriteController::AddToDbRateMap(CfIdToRateMap* cf_map) {
-  std::lock_guard<std::mutex> lock(mu_for_map_);
+  std::lock_guard<std::mutex> lock(map_mu_);
   db_id_to_write_rate_map_.insert(cf_map);
 }
 
 void WriteController::RemoveFromDbRateMap(CfIdToRateMap* cf_map) {
-  std::lock_guard<std::mutex> lock(mu_for_map_);
+  std::lock_guard<std::mutex> lock(map_mu_);
   db_id_to_write_rate_map_.erase(cf_map);
 }
 
@@ -78,7 +78,7 @@ bool WriteController::IsMinRate(uint32_t id, CfIdToRateMap* cf_map) {
 
 void WriteController::DeleteSelfFromMapAndMaybeUpdateDelayRate(
     uint32_t id, CfIdToRateMap* cf_map) {
-  std::lock_guard<std::mutex> lock(mu_for_map_);
+  std::lock_guard<std::mutex> lock(map_mu_);
   bool was_min = IsMinRate(id, cf_map);
   cf_map->erase(id);
   if (was_min) {
@@ -94,7 +94,7 @@ void WriteController::DeleteSelfFromMapAndMaybeUpdateDelayRate(
 uint64_t WriteController::InsertToMapAndGetMinRate(uint32_t id,
                                                    CfIdToRateMap* cf_map,
                                                    uint64_t cf_write_rate) {
-  std::lock_guard<std::mutex> lock(mu_for_map_);
+  std::lock_guard<std::mutex> lock(map_mu_);
   bool was_min = IsMinRate(id, cf_map);
   cf_map->insert_or_assign(id, cf_write_rate);
   if (cf_write_rate <= delayed_write_rate()) {
@@ -107,7 +107,7 @@ uint64_t WriteController::InsertToMapAndGetMinRate(uint32_t id,
 }
 
 void WriteController::WaitOnCV(const ErrorHandler& error_handler) {
-  std::unique_lock<std::mutex> lock(stop_mutex_);
+  std::unique_lock<std::mutex> lock(stop_mu_);
   while (error_handler.GetBGError().ok() && IsStopped()) {
     TEST_SYNC_POINT("WriteController::WaitOnCV");
     stop_cv_.wait(lock);
@@ -135,10 +135,11 @@ uint64_t WriteController::GetDelay(SystemClock* clock, uint64_t num_bytes) {
     return 0;
   }
 
-  auto credits = credit_in_bytes_.load();
-  while (credits >= num_bytes) {
-    if (credit_in_bytes_.compare_exchange_weak(credits, credits - num_bytes))
-      return 0;
+  std::lock_guard<std::mutex> lock(metrics_mu_);
+
+  if (credit_in_bytes_ >= num_bytes) {
+    credit_in_bytes_ -= num_bytes;
+    return 0;
   }
   // The frequency to get time inside DB mutex is less than one per refill
   // interval.
@@ -148,8 +149,6 @@ uint64_t WriteController::GetDelay(SystemClock* clock, uint64_t num_bytes) {
   // Refill every 1 ms
   const uint64_t kMicrosPerRefill = 1000;
 
-  std::lock_guard<std::mutex> lock(mu_);
-
   if (next_refill_time_ == 0) {
     // Start with an initial allotment of bytes for one interval
     next_refill_time_ = time_now;
@@ -188,7 +187,7 @@ uint64_t WriteController::NowMicrosMonotonic(SystemClock* clock) {
 void WriteController::NotifyCV() {
   assert(total_stopped_ >= 1);
   {
-    std::lock_guard<std::mutex> lock(stop_mutex_);
+    std::lock_guard<std::mutex> lock(stop_mu_);
     --total_stopped_;
   }
   stop_cv_.notify_all();

db/write_controller.h

@@ -29,18 +29,21 @@ class WriteController {
   explicit WriteController(bool dynamic_delay,
                            uint64_t _delayed_write_rate = 1024u * 1024u * 16u,
                            int64_t low_pri_rate_bytes_per_sec = 1024 * 1024)
-      : total_stopped_(0),
+      : dynamic_delay_(dynamic_delay),
+        total_stopped_(0),
         total_delayed_(0),
         total_compaction_pressure_(0),
         credit_in_bytes_(0),
         next_refill_time_(0),
-        dynamic_delay_(dynamic_delay),
         low_pri_rate_limiter_(
             NewGenericRateLimiter(low_pri_rate_bytes_per_sec)) {
     set_max_delayed_write_rate(_delayed_write_rate);
   }
   ~WriteController() = default;
 
+  static constexpr uint64_t kMinWriteRate =
+      16 * 1024u;  // Minimum write rate 16KB/s.
+
   // When an actor (column family) requests a stop token, all writes will be
   // stopped until the stop token is released (deleted)
   std::unique_ptr<WriteControllerToken> GetStopToken();
@@ -92,8 +95,6 @@ class WriteController {
 
   bool is_dynamic_delay() const { return dynamic_delay_; }
 
-  std::mutex& GetMapMutex() { return mu_for_map_; }
-
   using CfIdToRateMap = std::unordered_map<uint32_t, uint64_t>;
 
   void AddToDbRateMap(CfIdToRateMap* cf_map);
@@ -115,8 +116,20 @@ class WriteController {
   bool IsMinRate(uint32_t id, CfIdToRateMap* cf_map);
 
   // returns the min rate from db_id_to_write_rate_map_
+  // REQUIRES: write_controller map_mu_ mutex held.
   uint64_t GetMapMinRate();
 
+  // Whether Speedb's dynamic delay is used
+  bool dynamic_delay_;
+
+  std::mutex map_mu_;
+  std::unordered_set<CfIdToRateMap*> db_id_to_write_rate_map_;
+
+  // The mutex used by stop_cv_
+  std::mutex stop_mu_;
+  std::condition_variable stop_cv_;
+  // end of methods and members used when dynamic_delay_ == true.
+
   uint64_t NowMicrosMonotonic(SystemClock* clock);
 
   friend class WriteControllerToken;
@@ -129,7 +142,7 @@ class WriteController {
   std::atomic<int> total_compaction_pressure_;
 
   // mutex to protect below 4 members
-  std::mutex mu_;
+  std::mutex metrics_mu_;
   // Number of bytes allowed to write without delay
   std::atomic<uint64_t> credit_in_bytes_;
   // Next time that we can add more credit of bytes
@@ -139,16 +152,6 @@ class WriteController {
   // Current write rate (bytes / second)
   std::atomic<uint64_t> delayed_write_rate_;
 
-  // Whether Speedb's dynamic delay is used
-  bool dynamic_delay_;
-
-  std::mutex mu_for_map_;
-  std::unordered_set<CfIdToRateMap*> db_id_to_write_rate_map_;
-
-  std::condition_variable stop_cv_;
-  // The mutex used by stop_cv_
-  std::mutex stop_mutex_;
-
   std::unique_ptr<RateLimiter> low_pri_rate_limiter_;
 };