Dynamic Delay: new feature - Dynamic Delay (#276)

Calculate the delayed write rate based on the current state of the CF
based on the value of delayed_write_rate that the user initially set.
The calculation reduces speed linearly with regards to how much
the CF has exceeded the slowdown conditions (L0 files and pending bytes).
The slowdown for writing to the last memtable (when theres more than 3)
is 10 fold

db/column_family.cc

@@ -869,6 +869,39 @@ int GetL0ThresholdSpeedupCompaction(int level0_file_num_compaction_trigger,
 }
 }  // anonymous namespace
 
+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
+
+double ColumnFamilyData::TEST_CalculateWriteDelayDivider(
+    uint64_t compaction_needed_bytes,
+    const MutableCFOptions& mutable_cf_options,
+    WriteStallCause& write_stall_cause) {
+  return CalculateWriteDelayDividerAndMaybeUpdateWriteStallCause(
+      compaction_needed_bytes, mutable_cf_options, write_stall_cause);
+}
+
+std::unique_ptr<WriteControllerToken> ColumnFamilyData::DynamicSetupDelay(
+    WriteController* write_controller, uint64_t compaction_needed_bytes,
+    const MutableCFOptions& mutable_cf_options,
+    WriteStallCause& write_stall_cause) {
+  uint64_t max_write_rate = write_controller->max_delayed_write_rate();
+
+  const double rate_divider =
+      CalculateWriteDelayDividerAndMaybeUpdateWriteStallCause(
+          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;
+  }
+
+  return write_controller->GetDelayToken(write_rate);
+}
+
 std::pair<WriteStallCondition, WriteStallCause>
 ColumnFamilyData::GetWriteStallConditionAndCause(
     int num_unflushed_memtables, int num_l0_files,
@@ -907,6 +940,86 @@ ColumnFamilyData::GetWriteStallConditionAndCause(
   return {WriteStallCondition::kNormal, WriteStallCause::kNone};
 }
 
+// Delay divider is by how much we divide the users delayed_write_rate.
+// E.g. divider 10 will result in 10 Mb/s from users 100 Mb/s.
+// The rate is reduced linearly according to the range from slowdown to stop.
+double
+ColumnFamilyData::CalculateWriteDelayDividerAndMaybeUpdateWriteStallCause(
+    uint64_t compaction_needed_bytes,
+    const MutableCFOptions& mutable_cf_options,
+    WriteStallCause& write_stall_cause) {
+  assert(current_ != nullptr);
+
+  const auto* vstorage = current_->storage_info();
+
+  // Memtables
+  // this can only be entered when we're at the last memtable and theres more
+  // than 3. delay by 10X when writing to the last memtable.
+  double memtable_divider = 1;
+  auto num_unflushed_memtables = imm()->NumNotFlushed();
+  if (mutable_cf_options.max_write_buffer_number > 3 &&
+      num_unflushed_memtables >=
+          mutable_cf_options.max_write_buffer_number - 1 &&
+      num_unflushed_memtables - 1 >=
+          ioptions_.min_write_buffer_number_to_merge) {
+    memtable_divider = kMemtablePenalty;
+  }
+
+  // Pending Compaction Bytes
+  double pending_divider = 1;
+  auto soft_limit = mutable_cf_options.soft_pending_compaction_bytes_limit;
+  if (soft_limit > 0 && compaction_needed_bytes >= soft_limit) {
+    auto hard_limit = mutable_cf_options.hard_pending_compaction_bytes_limit;
+    // soft_limit != hard_limit here. we're in a kDelayed state here and not
+    // stop.
+    assert(hard_limit > soft_limit);
+    uint64_t soft_hard_range = hard_limit - soft_limit;
+    // change rate every 1G change or 100Mb if soft_hard_range is too small.
+    auto step_size = soft_hard_range > Gb ? Gb : 100 * Mb;
+    uint64_t num_steps = soft_hard_range / step_size;
+    auto extra_bytes = compaction_needed_bytes - soft_limit;
+    auto step_num = static_cast<uint64_t>(extra_bytes / step_size);
+    assert(step_num < num_steps);
+    if (num_steps > 0) {
+      pending_divider = 1 / (1 - (static_cast<double>(step_num) / num_steps));
+    }
+  }
+
+  double biggest_divider = 1;
+  if (memtable_divider > pending_divider) {
+    biggest_divider = memtable_divider;
+    write_stall_cause = WriteStallCause::kMemtableLimit;
+  } else if (pending_divider > 1) {
+    biggest_divider = pending_divider;
+    write_stall_cause = WriteStallCause::kPendingCompactionBytes;
+  }
+
+  // dont delay based on L0 when the user disables auto compactions
+  if (mutable_cf_options.disable_auto_compactions) {
+    return biggest_divider;
+  }
+
+  // L0 files
+  double l0_divider = 1;
+  const auto extra_l0_ssts = vstorage->NumLevelFiles(0) -
+                             mutable_cf_options.level0_slowdown_writes_trigger;
+  if (extra_l0_ssts > 0) {
+    const auto num_L0_steps = mutable_cf_options.level0_stop_writes_trigger -
+                              mutable_cf_options.level0_slowdown_writes_trigger;
+    assert(num_L0_steps > 0);
+    // since extra_l0_ssts == num_L0_steps then we're in a stop condition.
+    assert(extra_l0_ssts < num_L0_steps);
+    l0_divider = 1 / (1 - (extra_l0_ssts / num_L0_steps));
+  }
+
+  if (l0_divider > biggest_divider) {
+    biggest_divider = l0_divider;
+    write_stall_cause = WriteStallCause::kL0FileCountLimit;
+  }
+
+  return biggest_divider;
+}
+
 WriteStallCondition ColumnFamilyData::RecalculateWriteStallConditions(
     const MutableCFOptions& mutable_cf_options) {
   auto write_stall_condition = WriteStallCondition::kNormal;
@@ -925,6 +1038,18 @@ WriteStallCondition ColumnFamilyData::RecalculateWriteStallConditions(
 
     bool was_stopped = write_controller->IsStopped();
     bool needed_delay = write_controller->NeedsDelay();
+    bool dynamic_delay = write_controller->is_dynamic_delay();
+
+    // GetWriteStallConditionAndCause returns the first condition met, so its
+    // possible that a later condition will require a harder rate limiting.
+    // calculate all conditions with DynamicSetupDelay and reavaluate the
+    // write_stall_cause. this is only relevant in the kDelayed case.
+    if (dynamic_delay &&
+        write_stall_condition == WriteStallCondition::kDelayed) {
+      write_controller_token_ =
+          DynamicSetupDelay(write_controller, compaction_needed_bytes,
+                            mutable_cf_options, write_stall_cause);
+    }
 
     if (write_stall_condition == WriteStallCondition::kStopped &&
         write_stall_cause == WriteStallCause::kMemtableLimit) {
@@ -960,10 +1085,12 @@ WriteStallCondition ColumnFamilyData::RecalculateWriteStallConditions(
           name_.c_str(), compaction_needed_bytes);
     } else if (write_stall_condition == WriteStallCondition::kDelayed &&
                write_stall_cause == WriteStallCause::kMemtableLimit) {
-      write_controller_token_ =
-          SetupDelay(write_controller, compaction_needed_bytes,
-                     prev_compaction_needed_bytes_, was_stopped,
-                     mutable_cf_options.disable_auto_compactions);
+      if (!dynamic_delay) {
+        write_controller_token_ =
+            SetupDelay(write_controller, compaction_needed_bytes,
+                       prev_compaction_needed_bytes_, was_stopped,
+                       mutable_cf_options.disable_auto_compactions);
+      }
       internal_stats_->AddCFStats(InternalStats::MEMTABLE_LIMIT_DELAYS, 1);
       ROCKS_LOG_WARN(
           ioptions_.logger,
@@ -975,13 +1102,15 @@ WriteStallCondition ColumnFamilyData::RecalculateWriteStallConditions(
           write_controller->delayed_write_rate());
     } else if (write_stall_condition == WriteStallCondition::kDelayed &&
                write_stall_cause == WriteStallCause::kL0FileCountLimit) {
-      // L0 is the last two files from stopping.
-      bool near_stop = vstorage->l0_delay_trigger_count() >=
-                       mutable_cf_options.level0_stop_writes_trigger - 2;
-      write_controller_token_ =
-          SetupDelay(write_controller, compaction_needed_bytes,
-                     prev_compaction_needed_bytes_, was_stopped || near_stop,
-                     mutable_cf_options.disable_auto_compactions);
+      if (!dynamic_delay) {
+        // L0 is the last two files from stopping.
+        bool near_stop = vstorage->l0_delay_trigger_count() >=
+                         mutable_cf_options.level0_stop_writes_trigger - 2;
+        write_controller_token_ =
+            SetupDelay(write_controller, compaction_needed_bytes,
+                       prev_compaction_needed_bytes_, was_stopped || near_stop,
+                       mutable_cf_options.disable_auto_compactions);
+      }
       internal_stats_->AddCFStats(InternalStats::L0_FILE_COUNT_LIMIT_DELAYS, 1);
       if (compaction_picker_->IsLevel0CompactionInProgress()) {
         internal_stats_->AddCFStats(
@@ -998,19 +1127,21 @@ WriteStallCondition ColumnFamilyData::RecalculateWriteStallConditions(
       // If the distance to hard limit is less than 1/4 of the gap between soft
       // and
       // hard bytes limit, we think it is near stop and speed up the slowdown.
-      bool near_stop =
-          mutable_cf_options.hard_pending_compaction_bytes_limit > 0 &&
-          (compaction_needed_bytes -
-           mutable_cf_options.soft_pending_compaction_bytes_limit) >
-              3 *
-                  (mutable_cf_options.hard_pending_compaction_bytes_limit -
-                   mutable_cf_options.soft_pending_compaction_bytes_limit) /
-                  4;
+      if (!dynamic_delay) {
+        bool near_stop =
+            mutable_cf_options.hard_pending_compaction_bytes_limit > 0 &&
+            (compaction_needed_bytes -
+             mutable_cf_options.soft_pending_compaction_bytes_limit) >
+                3 *
+                    (mutable_cf_options.hard_pending_compaction_bytes_limit -
+                     mutable_cf_options.soft_pending_compaction_bytes_limit) /
+                    4;
 
-      write_controller_token_ =
-          SetupDelay(write_controller, compaction_needed_bytes,
-                     prev_compaction_needed_bytes_, was_stopped || near_stop,
-                     mutable_cf_options.disable_auto_compactions);
+        write_controller_token_ =
+            SetupDelay(write_controller, compaction_needed_bytes,
+                       prev_compaction_needed_bytes_, was_stopped || near_stop,
+                       mutable_cf_options.disable_auto_compactions);
+      }
       internal_stats_->AddCFStats(
           InternalStats::PENDING_COMPACTION_BYTES_LIMIT_DELAYS, 1);
       ROCKS_LOG_WARN(

db/column_family.h

@@ -474,6 +474,24 @@ class ColumnFamilyData {
   WriteStallCondition RecalculateWriteStallConditions(
       const MutableCFOptions& mutable_cf_options);
 
+  // REQUIREMENT: db mutex must be held
+  double TEST_CalculateWriteDelayDivider(
+      uint64_t compaction_needed_bytes,
+      const MutableCFOptions& mutable_cf_options,
+      WriteStallCause& write_stall_cause);
+
+ private:
+  std::unique_ptr<WriteControllerToken> DynamicSetupDelay(
+      WriteController* write_controller, uint64_t compaction_needed_bytes,
+      const MutableCFOptions& mutable_cf_options,
+      WriteStallCause& write_stall_cause);
+
+  double CalculateWriteDelayDividerAndMaybeUpdateWriteStallCause(
+      uint64_t compaction_needed_bytes,
+      const MutableCFOptions& mutable_cf_options,
+      WriteStallCause& write_stall_cause);
+
+ public:
   void set_initialized() { initialized_.store(true); }
 
   bool initialized() const { return initialized_.load(); }