Storage: Avoid wasting CPU with inefficient scheduling in SegmentRead…

…TaskScheduler (#6496)

* Fix useless scheduling.

* ci

* format

* ci

* Refine

* add comment

* fix

Co-authored-by: JaySon <tshent@qq.com>
Co-authored-by: Ti Chi Robot <ti-community-prow-bot@tidb.io>

dbms/src/Storages/DeltaMerge/DeltaMergeStore.cpp

@@ -918,7 +918,8 @@ BlockInputStreams DeltaMergeStore::readRaw(const Context & db_context,
         std::move(tasks),
         after_segment_read,
         req_info,
-        enable_read_thread);
+        enable_read_thread,
+        final_num_stream);
 
     BlockInputStreams res;
     for (size_t i = 0; i < final_num_stream; ++i)
@@ -1003,7 +1004,8 @@ BlockInputStreams DeltaMergeStore::read(const Context & db_context,
         std::move(tasks),
         after_segment_read,
         log_tracing_id,
-        enable_read_thread);
+        enable_read_thread,
+        final_num_stream);
 
     BlockInputStreams res;
     for (size_t i = 0; i < final_num_stream; ++i)

dbms/src/Storages/DeltaMerge/ReadThread/MergedTask.cpp

@@ -123,4 +123,17 @@ void MergedTaskPool::push(const MergedTaskPtr & t)
     std::lock_guard lock(mtx);
     merged_task_pool.push_back(t);
 }
+
+bool MergedTaskPool::has(UInt64 pool_id)
+{
+    std::lock_guard lock(mtx);
+    for (const auto & t : merged_task_pool)
+    {
+        if (t->containPool(pool_id))
+        {
+            return true;
+        }
+    }
+    return false;
+}
 } // namespace DB::DM

dbms/src/Storages/DeltaMerge/ReadThread/MergedTask.h

@@ -161,6 +161,7 @@ class MergedTaskPool
 
     MergedTaskPtr pop(uint64_t pool_id);
     void push(const MergedTaskPtr & t);
+    bool has(UInt64 pool_id);
 
 private:
     std::mutex mtx;

dbms/src/Storages/DeltaMerge/ReadThread/SegmentReadTaskScheduler.cpp

@@ -118,15 +118,21 @@ SegmentReadTaskPools SegmentReadTaskScheduler::getPoolsUnlock(const std::vector<
     return pools;
 }
 
+bool SegmentReadTaskScheduler::needScheduleToRead(const SegmentReadTaskPoolPtr & pool)
+{
+    return pool->getFreeBlockSlots() > 0 && // Block queue is not full and
+        (merged_task_pool.has(pool->poolId()) || // can schedule a segment from MergedTaskPool or
+         pool->getFreeActiveSegments() > 0); // schedule a new segment.
+}
+
 SegmentReadTaskPoolPtr SegmentReadTaskScheduler::scheduleSegmentReadTaskPoolUnlock()
 {
     int64_t pool_count = read_pools.size(); // All read task pool need to be scheduled, including invalid read task pool.
     for (int64_t i = 0; i < pool_count; i++)
     {
         auto pool = read_pools.next();
-        // If pool->getFreeBlockSlots() > 0, schedule it for read blocks.
         // If !pool->valid(), schedule it for clean MergedTaskPool.
-        if (pool != nullptr && (pool->getFreeBlockSlots() > 0 || !pool->valid()))
+        if (pool != nullptr && (needScheduleToRead(pool) || !pool->valid()))
         {
             return pool;
         }

dbms/src/Storages/DeltaMerge/ReadThread/SegmentReadTaskScheduler.h

@@ -62,7 +62,7 @@ class SegmentReadTaskScheduler
     bool isStop() const;
     bool schedule();
     void schedLoop();
-
+    bool needScheduleToRead(const SegmentReadTaskPoolPtr & pool);
     SegmentReadTaskPools getPoolsUnlock(const std::vector<uint64_t> & pool_ids);
     // <seg_id, pool_ids>
     std::optional<std::pair<uint64_t, std::vector<uint64_t>>> scheduleSegmentUnlock(const SegmentReadTaskPoolPtr & pool);

dbms/src/Storages/DeltaMerge/SegmentReadTaskPool.cpp

@@ -205,7 +205,7 @@ std::unordered_map<uint64_t, std::vector<uint64_t>>::const_iterator SegmentReadT
 {
     auto target = segments.end();
     std::lock_guard lock(mutex);
-    if (getFreeActiveSegmentCountUnlock() <= 0)
+    if (getFreeActiveSegmentsUnlock() <= 0)
     {
         return target;
     }
@@ -269,33 +269,29 @@ void SegmentReadTaskPool::pushBlock(Block && block)
     q.push(std::move(block), nullptr);
 }
 
-int64_t SegmentReadTaskPool::increaseUnorderedInputStreamRefCount()
+Int64 SegmentReadTaskPool::increaseUnorderedInputStreamRefCount()
 {
     return unordered_input_stream_ref_count.fetch_add(1, std::memory_order_relaxed);
 }
-int64_t SegmentReadTaskPool::decreaseUnorderedInputStreamRefCount()
+Int64 SegmentReadTaskPool::decreaseUnorderedInputStreamRefCount()
 {
     return unordered_input_stream_ref_count.fetch_sub(1, std::memory_order_relaxed);
 }
 
-int64_t SegmentReadTaskPool::getFreeBlockSlots() const
+Int64 SegmentReadTaskPool::getFreeBlockSlots() const
 {
-    auto block_slots = unordered_input_stream_ref_count.load(std::memory_order_relaxed);
-    if (block_slots < 3)
-    {
-        block_slots = 3;
-    }
-    return block_slots - blk_stat.pendingCount();
+    return block_slot_limit - blk_stat.pendingCount();
 }
 
-int64_t SegmentReadTaskPool::getFreeActiveSegmentCountUnlock()
+Int64 SegmentReadTaskPool::getFreeActiveSegments() const
 {
-    auto active_segment_limit = unordered_input_stream_ref_count.load(std::memory_order_relaxed);
-    if (active_segment_limit < 2)
-    {
-        active_segment_limit = 2;
-    }
-    return active_segment_limit - static_cast<int64_t>(active_segment_ids.size());
+    std::lock_guard lock(mutex);
+    return getFreeActiveSegmentsUnlock();
+}
+
+Int64 SegmentReadTaskPool::getFreeActiveSegmentsUnlock() const
+{
+    return active_segment_limit - static_cast<Int64>(active_segment_ids.size());
 }
 
 bool SegmentReadTaskPool::exceptionHappened() const

dbms/src/Storages/DeltaMerge/SegmentReadTaskPool.h

@@ -168,7 +168,8 @@ class SegmentReadTaskPool : private boost::noncopyable
         SegmentReadTasks && tasks_,
         AfterSegmentRead after_segment_read_,
         const String & tracing_id,
-        bool enable_read_thread_)
+        bool enable_read_thread_,
+        Int64 num_streams_)
         : pool_id(nextPoolId())
         , table_id(table_id_)
         , dm_context(dm_context_)
@@ -183,6 +184,13 @@ class SegmentReadTaskPool : private boost::noncopyable
         , unordered_input_stream_ref_count(0)
         , exception_happened(false)
         , mem_tracker(current_memory_tracker == nullptr ? nullptr : current_memory_tracker->shared_from_this())
+        // If the queue is too short, only 1 in the extreme case, it may cause the computation thread
+        // to encounter empty queues frequently, resulting in too much waiting and thread context
+        // switching, so we limit the lower limit to 3, which provides two blocks of buffer space.
+        , block_slot_limit(std::max(num_streams_, 3))
+        // Limiting the minimum number of reading segments to 2 is to avoid, as much as possible,
+        // situations where the computation may be faster and the storage layer may not be able to keep up.
+        , active_segment_limit(std::max(num_streams_, 2))
     {}
 
     ~SegmentReadTaskPool()
@@ -223,9 +231,10 @@ class SegmentReadTaskPool : private boost::noncopyable
         const std::unordered_map<uint64_t, std::vector<uint64_t>> & segments,
         uint64_t expected_merge_count);
 
-    int64_t increaseUnorderedInputStreamRefCount();
-    int64_t decreaseUnorderedInputStreamRefCount();
-    int64_t getFreeBlockSlots() const;
+    Int64 increaseUnorderedInputStreamRefCount();
+    Int64 decreaseUnorderedInputStreamRefCount();
+    Int64 getFreeBlockSlots() const;
+    Int64 getFreeActiveSegments() const;
     bool valid() const;
     void setException(const DB::Exception & e);
 
@@ -240,7 +249,7 @@ class SegmentReadTaskPool : private boost::noncopyable
     }
 
 private:
-    int64_t getFreeActiveSegmentCountUnlock();
+    Int64 getFreeActiveSegmentsUnlock() const;
     bool exceptionHappened() const;
     void finishSegment(const SegmentPtr & seg);
     void pushBlock(Block && block);
@@ -255,13 +264,13 @@ class SegmentReadTaskPool : private boost::noncopyable
     const ReadMode read_mode;
     SegmentReadTasksWrapper tasks_wrapper;
     AfterSegmentRead after_segment_read;
-    std::mutex mutex;
+    mutable std::mutex mutex;
     std::unordered_set<uint64_t> active_segment_ids;
     WorkQueue<Block> q;
     BlockStat blk_stat;
     LoggerPtr log;
 
-    std::atomic<int64_t> unordered_input_stream_ref_count;
+    std::atomic<Int64> unordered_input_stream_ref_count;
 
     std::atomic<bool> exception_happened;
     DB::Exception exception;
@@ -275,6 +284,9 @@ class SegmentReadTaskPool : private boost::noncopyable
     // std::once_flag and std::call_once to prevent duplicated add.
     std::once_flag add_to_scheduler;
 
+    const Int64 block_slot_limit;
+    const Int64 active_segment_limit;
+
     inline static std::atomic<uint64_t> pool_id_gen{1};
     inline static BlockStat global_blk_stat;
     static uint64_t nextPoolId()