From a6fe91ca47be99ab91dafc76de966619592dc0cf Mon Sep 17 00:00:00 2001
From: Nikita Mikhaylov <mikhaylovnikitka@gmail.com>
Date: Thu, 2 Sep 2021 17:40:29 +0000
Subject: [PATCH] get rid of half of allocations

---
 src/Common/PlainMultiSet.h                    | 114 ++++++++++++++
 src/Common/RingBuffer.h                       | 122 +++++++++++++++
 src/Common/ThreadPool.h                       |   1 -
 src/Common/tests/gtest_plain_multiset.cpp     |  22 +++
 src/Common/tests/gtest_ringbuffer.cpp         | 142 ++++++++++++++++++
 src/Interpreters/Context.cpp                  |  33 ++--
 .../MergeTree/MergeMutateExecutor.cpp         |  35 +++--
 src/Storages/MergeTree/MergeMutateExecutor.h  |  74 +++++----
 .../MergeTree/tests/gtest_executor.cpp        |  24 +--
 9 files changed, 505 insertions(+), 62 deletions(-)
 create mode 100644 src/Common/PlainMultiSet.h
 create mode 100644 src/Common/RingBuffer.h
 create mode 100644 src/Common/tests/gtest_plain_multiset.cpp
 create mode 100644 src/Common/tests/gtest_ringbuffer.cpp

diff --git a/src/Common/PlainMultiSet.h b/src/Common/PlainMultiSet.h
new file mode 100644
index 00000000000..d5b646a53db
--- /dev/null
+++ b/src/Common/PlainMultiSet.h
@@ -0,0 +1,114 @@
+#pragma once
+
+#include <vector>
+
+namespace DB
+{
+
+
+/**
+ * Class with O(n) complexity for all methods
+ * Size has to be fixed.
+ * The main reason to use this is to get rid of any allocations.
+ * Used is some executors, where the number of elements is really small.
+ */
+template <class T>
+class PlainMultiSet
+{
+public:
+
+    explicit PlainMultiSet(size_t capacity_)
+    {
+        buffer.resize(capacity_);
+    }
+
+
+    bool tryPush(T element)
+    {
+        for (auto & item : buffer)
+        {
+            if (item.state == State::EMPTY)
+            {
+                item.state = State::FILLED;
+                item.value = std::move(element);
+                ++count;
+                return true;
+            }
+        }
+
+
+        return false;
+    }
+
+    bool has(T element)
+    {
+        for (auto & item : buffer)
+            if (item.state == State::FILLED && item.value == element)
+                return true;
+
+        return false;
+    }
+
+
+    template <class Predicate>
+    std::vector<T> getAll(Predicate && predicate)
+    {
+        std::vector<T> suitable;
+        for (auto & item : buffer)
+            if (item.state == State::FILLED && predicate(item.value))
+                suitable.emplace_back(item.value);
+
+        return suitable;
+    }
+
+
+    bool tryErase(const T & element)
+    {
+        for (auto & item : buffer)
+        {
+            if (item.state == State::FILLED && item.value == element)
+            {
+                item.state = State::EMPTY;
+                item.value = T{};
+                --count;
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    size_t size()
+    {
+        return count;
+    }
+
+    void reserve(size_t new_capacity)
+    {
+        if (buffer.size() >= new_capacity)
+            return;
+
+        std::vector<Item> new_buffer(std::move(buffer));
+        new_buffer.reserve(new_capacity);
+
+        std::swap(new_buffer, buffer);
+    }
+
+private:
+    enum class State
+    {
+        EMPTY,
+        FILLED
+    };
+
+    struct Item
+    {
+        T value;
+        State state{State::EMPTY};
+    };
+
+    size_t count{0};
+    std::vector<Item> buffer;
+};
+
+}
diff --git a/src/Common/RingBuffer.h b/src/Common/RingBuffer.h
new file mode 100644
index 00000000000..a619ae2dab6
--- /dev/null
+++ b/src/Common/RingBuffer.h
@@ -0,0 +1,122 @@
+#pragma once
+
+#include <vector>
+#include <iostream>
+
+namespace DB
+{
+
+/**
+ * A ring buffer of fixed size.
+ * With an ability to expand / narrow.
+ * When narrowing only first N elements remain.
+ */
+
+template <class T>
+class RingBuffer {
+public:
+    explicit RingBuffer(size_t capacity_) : capacity(capacity_)
+    {
+        buffer.assign(capacity, {});
+    }
+
+    size_t size() const
+    {
+        return count;
+    }
+
+    bool empty() const
+    {
+        return count == 0;
+    }
+
+    bool tryPush(T element)
+    {
+        if (count == capacity) {
+            return false;
+        }
+        buffer[(position + count) % capacity] = element;
+        ++count;
+        return true;
+    }
+
+    bool tryPop(T * element)
+    {
+        if (empty()) {
+            return false;
+        }
+        *element = std::move(buffer[position]);
+        --count;
+        position = (position + 1) % capacity;
+        return true;
+    }
+
+    template <typename Predicate>
+    void removeElements(Predicate && predicate)
+    {
+        /// Shift all elements to the beginning of the buffer
+        std::rotate(buffer.begin(), buffer.begin() + position, buffer.end());
+
+        /// Remove elements
+        auto end_removed = std::remove_if(buffer.begin(), buffer.begin() + count, predicate);
+
+        size_t new_count = std::distance(buffer.begin(), end_removed);
+
+        for (size_t i = new_count; i < count; ++i)
+            buffer[i] = T{};
+
+        count = new_count;
+        position = 0;
+    }
+
+
+    void resize(size_t new_capacity)
+    {
+        if (new_capacity > capacity)
+            expand(new_capacity);
+        else if (new_capacity < capacity)
+            narrow(new_capacity);
+    }
+
+private:
+
+    void expand(size_t new_capacity)
+    {
+        bool overflow = (position + count) > capacity;
+        buffer.resize(new_capacity);
+
+        if (overflow)
+        {
+            size_t count_before_end = capacity - position;
+            for (size_t i = 0; i < count_before_end; ++i)
+                buffer[new_capacity - i] = buffer[capacity - i];
+            position = new_capacity - count_before_end;
+        }
+
+        capacity = new_capacity;
+    }
+
+    void narrow(size_t new_capacity)
+    {
+        std::vector<T> new_buffer(new_capacity);
+
+        count = std::min(new_capacity, count);
+        for (size_t i = 0; i < count; ++i)
+            new_buffer[i] = buffer[(position + i) % capacity];
+
+        std::swap(buffer, new_buffer);
+
+        position = 0;
+        capacity = new_capacity;
+    }
+
+
+    std::vector<T> buffer;
+    size_t position{0};
+    size_t count{0};
+    size_t capacity{0};
+};
+
+
+
+}
diff --git a/src/Common/ThreadPool.h b/src/Common/ThreadPool.h
index 1da5d25eef0..1cac87abb70 100644
--- a/src/Common/ThreadPool.h
+++ b/src/Common/ThreadPool.h
@@ -107,7 +107,6 @@ private:
     std::list<Thread> threads;
     std::exception_ptr first_exception;
 
-
     template <typename ReturnType>
     ReturnType scheduleImpl(Job job, int priority, std::optional<uint64_t> wait_microseconds);
 
diff --git a/src/Common/tests/gtest_plain_multiset.cpp b/src/Common/tests/gtest_plain_multiset.cpp
new file mode 100644
index 00000000000..2018053d6a7
--- /dev/null
+++ b/src/Common/tests/gtest_plain_multiset.cpp
@@ -0,0 +1,22 @@
+#include <gtest/gtest.h>
+
+#include <random>
+
+#include <Common/PlainMultiSet.h>
+
+using namespace DB;
+
+
+TEST(PlainMultiSet, Simple)
+{
+    PlainMultiSet<int> set(10);
+
+    ASSERT_TRUE(set.tryPush(1));
+    ASSERT_TRUE(set.tryPush(1));
+    ASSERT_TRUE(set.tryPush(2));
+    ASSERT_TRUE(set.tryPush(3));
+
+    ASSERT_TRUE(set.has(1));
+    ASSERT_TRUE(set.has(2));
+    ASSERT_TRUE(set.has(3));
+}
diff --git a/src/Common/tests/gtest_ringbuffer.cpp b/src/Common/tests/gtest_ringbuffer.cpp
new file mode 100644
index 00000000000..e56739b21cc
--- /dev/null
+++ b/src/Common/tests/gtest_ringbuffer.cpp
@@ -0,0 +1,142 @@
+#include <gtest/gtest.h>
+
+#include <random>
+
+#include <Common/RingBuffer.h>
+
+using namespace DB;
+
+TEST(RingBuffer, Empty)
+{
+    RingBuffer<int> buffer(1);
+
+    ASSERT_TRUE(buffer.size() == 0u); // NOLINT
+    ASSERT_TRUE(buffer.empty());
+}
+
+TEST(RingBuffer, PushAndPop)
+{
+    RingBuffer<int> buffer(2);
+
+    int i;
+    ASSERT_TRUE(true == buffer.tryPush(0));
+    ASSERT_TRUE(true == buffer.tryPush(1));
+    ASSERT_TRUE(false == buffer.tryPush(2));
+
+    ASSERT_TRUE(2u == buffer.size());
+    ASSERT_TRUE(false == buffer.empty());
+
+    ASSERT_TRUE(true == buffer.tryPop(&i));
+    ASSERT_TRUE(0 == i);
+    ASSERT_TRUE(true == buffer.tryPop(&i));
+    ASSERT_TRUE(1 == i);
+
+    ASSERT_TRUE(false == buffer.tryPop(&i));
+    ASSERT_TRUE(buffer.empty());
+    ASSERT_TRUE(true == buffer.empty());
+}
+
+TEST(RingBuffer, Random)
+{
+    std::random_device device;
+    std::mt19937 generator(device());
+
+    std::uniform_int_distribution<> distribution(0, 1);
+
+    RingBuffer<int> buffer(10);
+
+    int next_element = 0;
+    int next_received_element = 0;
+    for (int i = 0; i < 100000; ++i) {
+        if (distribution(generator) == 0)
+        {
+            if (buffer.tryPush(next_element))
+                next_element++;
+        }
+        else
+        {
+            int element;
+            if (buffer.tryPop(&element))
+            {
+                ASSERT_TRUE(next_received_element == element);
+                next_received_element++;
+            }
+        }
+    }
+}
+
+
+TEST(RingBuffer, Resize)
+{
+    RingBuffer<int> buffer(10);
+
+    for (size_t i = 0; i < 10; ++i)
+        ASSERT_TRUE(buffer.tryPush(i));
+
+    buffer.resize(0);
+
+    ASSERT_TRUE(buffer.empty());
+    ASSERT_EQ(buffer.size(), 0u);
+
+    ASSERT_FALSE(buffer.tryPush(42));
+
+    int value;
+    ASSERT_FALSE(buffer.tryPop(&value));
+
+    buffer.resize(1);
+
+    ASSERT_TRUE(buffer.tryPush(42));
+    ASSERT_TRUE(buffer.tryPop(&value));
+    ASSERT_EQ(value, 42);
+
+    buffer.resize(42);
+
+    for (size_t i = 0; i < 42; ++i)
+        ASSERT_TRUE(buffer.tryPush(i));
+
+    buffer.resize(56);
+
+    for (size_t i = 0; i < 42; ++i)
+    {
+        ASSERT_TRUE(buffer.tryPop(&value));
+        ASSERT_EQ(value, i);
+    }
+
+    for (size_t i = 0; i < 56; ++i)
+        ASSERT_TRUE(buffer.tryPush(i));
+
+    buffer.resize(13);
+
+    for (size_t i = 0; i < 13; ++i)
+    {
+        ASSERT_TRUE(buffer.tryPop(&value));
+        ASSERT_EQ(value, i);
+    }
+}
+
+
+TEST(RingBuffer, removeElements)
+{
+    RingBuffer<int> buffer(10);
+
+    for (size_t i = 0; i < 10; ++i)
+        ASSERT_TRUE(buffer.tryPush(i));
+
+    int value;
+    ASSERT_TRUE(buffer.tryPop(&value));
+    ASSERT_TRUE(buffer.tryPop(&value));
+    ASSERT_TRUE(buffer.tryPop(&value));
+
+    buffer.removeElements([](int current) { return current % 2 == 0; });
+
+    ASSERT_EQ(buffer.size(), 4);
+
+    ASSERT_TRUE(buffer.tryPop(&value));
+    ASSERT_EQ(value, 3);
+    ASSERT_TRUE(buffer.tryPop(&value));
+    ASSERT_EQ(value, 5);
+    ASSERT_TRUE(buffer.tryPop(&value));
+    ASSERT_EQ(value, 7);
+    ASSERT_TRUE(buffer.tryPop(&value));
+    ASSERT_EQ(value, 9);
+}
diff --git a/src/Interpreters/Context.cpp b/src/Interpreters/Context.cpp
index c1e92f1be0e..2c7bf9e6eca 100644
--- a/src/Interpreters/Context.cpp
+++ b/src/Interpreters/Context.cpp
@@ -2729,21 +2729,30 @@ PartUUIDsPtr Context::getIgnoredPartUUIDs() const
 
 void Context::initializeBackgroundExecutors()
 {
-    shared->merge_mutate_executor = MergeTreeBackgroundExecutor::create(MergeTreeBackgroundExecutor::Type::MERGE_MUTATE);
-    shared->moves_executor = MergeTreeBackgroundExecutor::create(MergeTreeBackgroundExecutor::Type::MOVE);
-    shared->fetch_executor = MergeTreeBackgroundExecutor::create(MergeTreeBackgroundExecutor::Type::FETCH);
+    shared->merge_mutate_executor = MergeTreeBackgroundExecutor::create
+    (
+        MergeTreeBackgroundExecutor::Type::MERGE_MUTATE,
+        [this] () { return getSettingsRef().background_pool_size; },
+        [this] () { return getSettingsRef().background_pool_size; },
+        CurrentMetrics::BackgroundPoolTask
+    );
 
-    shared->merge_mutate_executor->setThreadsCount([this] () { return getSettingsRef().background_pool_size; });
-    shared->merge_mutate_executor->setTasksCount([this] () { return getSettingsRef().background_pool_size; });
-    shared->merge_mutate_executor->setMetric(CurrentMetrics::BackgroundPoolTask);
+    shared->moves_executor = MergeTreeBackgroundExecutor::create
+    (
+        MergeTreeBackgroundExecutor::Type::MOVE,
+        [this] () { return getSettingsRef().background_move_pool_size; },
+        [this] () { return getSettingsRef().background_move_pool_size; },
+        CurrentMetrics::BackgroundMovePoolTask
+    );
 
-    shared->moves_executor->setThreadsCount([this] () { return getSettingsRef().background_move_pool_size; });
-    shared->moves_executor->setTasksCount([this] () { return getSettingsRef().background_move_pool_size; });
-    shared->moves_executor->setMetric(CurrentMetrics::BackgroundMovePoolTask);
 
-    shared->fetch_executor->setThreadsCount([this] () { return getSettingsRef().background_fetches_pool_size; });
-    shared->fetch_executor->setTasksCount([this] () { return getSettingsRef().background_fetches_pool_size; });
-    shared->fetch_executor->setMetric(CurrentMetrics::BackgroundFetchesPoolTask);
+    shared->fetch_executor = MergeTreeBackgroundExecutor::create
+    (
+        MergeTreeBackgroundExecutor::Type::FETCH,
+        [this] () { return getSettingsRef().background_fetches_pool_size; },
+        [this] () { return getSettingsRef().background_fetches_pool_size; },
+        CurrentMetrics::BackgroundFetchesPoolTask
+    );
 }
 
 
diff --git a/src/Storages/MergeTree/MergeMutateExecutor.cpp b/src/Storages/MergeTree/MergeMutateExecutor.cpp
index c7b025a74f3..5aecf85a7a4 100644
--- a/src/Storages/MergeTree/MergeMutateExecutor.cpp
+++ b/src/Storages/MergeTree/MergeMutateExecutor.cpp
@@ -32,12 +32,10 @@ void MergeTreeBackgroundExecutor::removeTasksCorrespondingToStorage(StorageID id
         /// Mark this StorageID as deleting
         currently_deleting.emplace(id);
 
-        std::erase_if(pending, [&] (auto item) -> bool { return item->task->getStorageID() == id; });
+        pending.removeElements([&] (auto item) -> bool { return item->task->getStorageID() == id; });
 
         /// Find pending to wait
-        for (const auto & item : active)
-            if (item->task->getStorageID() == id)
-                tasks_to_wait.emplace_back(item);
+        tasks_to_wait = active.getAll([&] (auto item) -> bool { return item->task->getStorageID() == id; });
     }
 
 
@@ -66,13 +64,24 @@ void MergeTreeBackgroundExecutor::schedulerThreadFunction()
         if (shutdown_suspend)
             break;
 
-        auto item = std::move(pending.front());
-        pending.pop_front();
+        ItemPtr item;
+        if (!pending.tryPop(&item))
+            continue;
 
-        active.emplace(item);
+        active.tryPush(item);
+
+
+        try
+        {
+            /// This is needed to increase / decrease the number of threads at runtime
+            if (update_timer.compareAndRestartDeferred(1.))
+                updateConfiguration();
+        }
+        catch (...)
+        {
+            tryLogCurrentException(__PRETTY_FUNCTION__);
+        }
 
-        /// This is needed to increase / decrease the number of threads at runtime
-        updatePoolConfiguration();
 
         bool res = pool.trySchedule([this, item] ()
         {
@@ -80,7 +89,7 @@ void MergeTreeBackgroundExecutor::schedulerThreadFunction()
 
             auto check_if_deleting = [&] () -> bool
             {
-                active.erase(item);
+                active.tryErase(item);
 
                 for (auto & id : currently_deleting)
                 {
@@ -108,7 +117,7 @@ void MergeTreeBackgroundExecutor::schedulerThreadFunction()
                     if (check_if_deleting())
                         return;
 
-                    pending.emplace_back(item);
+                    pending.tryPush(item);
                     has_tasks.notify_one();
                     return;
                 }
@@ -130,8 +139,8 @@ void MergeTreeBackgroundExecutor::schedulerThreadFunction()
 
         if (!res)
         {
-            active.erase(item);
-            pending.emplace_back(item);
+            active.tryErase(item);
+            pending.tryPush(item);
         }
 
     }
diff --git a/src/Storages/MergeTree/MergeMutateExecutor.h b/src/Storages/MergeTree/MergeMutateExecutor.h
index db1e89dd1bd..accb9bf2e0f 100644
--- a/src/Storages/MergeTree/MergeMutateExecutor.h
+++ b/src/Storages/MergeTree/MergeMutateExecutor.h
@@ -5,10 +5,13 @@
 #include <atomic>
 #include <mutex>
 #include <condition_variable>
+#include <unordered_set>
 
 #include <common/shared_ptr_helper.h>
 #include <Common/ThreadPool.h>
-#include <Common/ArenaAllocator.h>
+#include <Common/Stopwatch.h>
+#include <Common/RingBuffer.h>
+#include <Common/PlainMultiSet.h>
 #include <Storages/MergeTree/ExecutableTask.h>
 #include <Storages/MergeTree/MergeTreeData.h>
 
@@ -61,9 +64,19 @@ public:
         MOVE
     };
 
-    explicit MergeTreeBackgroundExecutor(Type type_) : type(type_)
+    MergeTreeBackgroundExecutor(
+        Type type_,
+        CountGetter && threads_count_getter_,
+        CountGetter && max_task_count_getter_,
+        CurrentMetrics::Metric metric_)
+        : type(type_)
+        , threads_count_getter(threads_count_getter_)
+        , max_task_count_getter(max_task_count_getter_)
+        , metric(metric_)
     {
         name = toString(type);
+
+        updateConfiguration();
         scheduler = ThreadFromGlobalPool([this]() { schedulerThreadFunction(); });
     }
 
@@ -72,21 +85,6 @@ public:
         wait();
     }
 
-    void setThreadsCount(CountGetter && getter)
-    {
-        threads_count_getter = getter;
-    }
-
-    void setTasksCount(CountGetter && getter)
-    {
-        max_task_count_getter = getter;
-    }
-
-    void setMetric(CurrentMetrics::Metric metric_)
-    {
-        metric = metric_;
-    }
-
     bool trySchedule(ExecutableTaskPtr task)
     {
         std::lock_guard lock(mutex);
@@ -95,10 +93,13 @@ public:
             return false;
 
         auto & value = CurrentMetrics::values[metric];
-        if (value.load() >= static_cast<int64_t>(max_task_count_getter()))
+        if (value.load() >= static_cast<int64_t>(max_tasks_count))
             return false;
 
-        pending.emplace_back(std::make_shared<Item>(std::move(task), metric));
+        if (!pending.tryPush(std::make_shared<Item>(std::move(task), metric)))
+            return false;
+
+
         has_tasks.notify_one();
         return true;
     }
@@ -133,12 +134,27 @@ public:
 
 private:
 
-    void updatePoolConfiguration()
+    void updateConfiguration()
     {
-        const auto max_threads = threads_count_getter();
-        pool.setMaxFreeThreads(0);
-        pool.setMaxThreads(max_threads);
-        pool.setQueueSize(max_threads);
+        auto new_threads_count = threads_count_getter();
+        auto new_max_tasks_count = max_task_count_getter();
+
+        try
+        {
+            pending.resize(new_max_tasks_count);
+            active.reserve(new_max_tasks_count);
+
+            pool.setMaxFreeThreads(0);
+            pool.setMaxThreads(new_threads_count);
+            pool.setQueueSize(new_max_tasks_count);
+        }
+        catch (...)
+        {
+            tryLogCurrentException(__PRETTY_FUNCTION__);
+        }
+
+        threads_count = new_threads_count;
+        max_tasks_count = new_max_tasks_count;
     }
 
     void schedulerThreadFunction();
@@ -151,6 +167,11 @@ private:
     CountGetter max_task_count_getter;
     CurrentMetrics::Metric metric;
 
+    size_t threads_count{0};
+    size_t max_tasks_count{0};
+
+    AtomicStopwatch update_timer;
+
     struct Item
     {
         explicit Item(ExecutableTaskPtr && task_, CurrentMetrics::Metric metric_)
@@ -169,8 +190,9 @@ private:
 
     using ItemPtr = std::shared_ptr<Item>;
 
-    std::deque<ItemPtr> pending;
-    std::set<ItemPtr> active;
+    /// Initially it will be empty
+    RingBuffer<ItemPtr> pending{0};
+    PlainMultiSet<ItemPtr> active{0};
     std::set<StorageID> currently_deleting;
 
     std::mutex remove_mutex;
diff --git a/src/Storages/MergeTree/tests/gtest_executor.cpp b/src/Storages/MergeTree/tests/gtest_executor.cpp
index 0a330a0af76..664b09aa82f 100644
--- a/src/Storages/MergeTree/tests/gtest_executor.cpp
+++ b/src/Storages/MergeTree/tests/gtest_executor.cpp
@@ -54,14 +54,16 @@ private:
 
 TEST(Executor, RemoveTasks)
 {
-    auto executor = DB::MergeTreeBackgroundExecutor::create(DB::MergeTreeBackgroundExecutor::Type::MERGE_MUTATE);
-
     const size_t tasks_kinds = 25;
     const size_t batch = 100;
 
-    executor->setThreadsCount([]() { return tasks_kinds; });
-    executor->setTasksCount([] () { return tasks_kinds * batch; });
-    executor->setMetric(CurrentMetrics::BackgroundPoolTask);
+    auto executor = DB::MergeTreeBackgroundExecutor::create
+    (
+        DB::MergeTreeBackgroundExecutor::Type::MERGE_MUTATE,
+        [] () { return tasks_kinds; },
+        [] () { return tasks_kinds * batch; },
+        CurrentMetrics::BackgroundPoolTask
+    );
 
     for (size_t i = 0; i < batch; ++i)
         for (size_t j = 0; j < tasks_kinds; ++j)
@@ -93,16 +95,18 @@ TEST(Executor, RemoveTasks)
 
 TEST(Executor, RemoveTasksStress)
 {
-    auto executor = DB::MergeTreeBackgroundExecutor::create(DB::MergeTreeBackgroundExecutor::Type::MERGE_MUTATE);
-
     const size_t tasks_kinds = 25;
     const size_t batch = 100;
     const size_t schedulers_count = 5;
     const size_t removers_count = 5;
 
-    executor->setThreadsCount([]() { return tasks_kinds; });
-    executor->setTasksCount([] () { return tasks_kinds * batch * (schedulers_count + removers_count); });
-    executor->setMetric(CurrentMetrics::BackgroundPoolTask);
+    auto executor = DB::MergeTreeBackgroundExecutor::create
+    (
+        DB::MergeTreeBackgroundExecutor::Type::MERGE_MUTATE,
+        [] () { return tasks_kinds; },
+        [] () { return tasks_kinds * batch * (schedulers_count + removers_count); },
+        CurrentMetrics::BackgroundPoolTask
+    );
 
     std::barrier barrier(schedulers_count + removers_count);