Storages: Update the length of Block queue and the number of read threads (#8702)

dbms/src/Interpreters/Settings.h

@@ -222,7 +222,7 @@ struct Settings
     M(SettingUInt64, dt_max_sharing_column_bytes_for_all, 2048 * Constant::MB, "Memory limitation for data sharing of all requests. 0 means disable data sharing")                                                                      \
     M(SettingUInt64, dt_max_sharing_column_count, 5, "ColumnPtr object limitation for data sharing of each DMFileReader::Stream. 0 means disable data sharing")                                                                         \
     M(SettingBool, dt_enable_bitmap_filter, true, "Use bitmap filter to read data or not")                                                                                                                                              \
-    M(SettingDouble, dt_read_thread_count_scale, 1.0, "Number of read thread = number of logical cpu cores * dt_read_thread_count_scale.  Only has meaning at server startup.")                                                         \
+    M(SettingDouble, dt_read_thread_count_scale, 2.0, "Number of read thread = number of logical cpu cores * dt_read_thread_count_scale.  Only has meaning at server startup.")                                                         \
     M(SettingDouble, dt_filecache_max_downloading_count_scale, 1.0, "Max downloading task count of FileCache = io thread count * dt_filecache_max_downloading_count_scale.")                                                            \
     M(SettingUInt64, dt_filecache_min_age_seconds, 1800, "Files of the same priority can only be evicted from files that were not accessed within `dt_filecache_min_age_seconds` seconds.")                                             \
     M(SettingUInt64, dt_small_file_size_threshold, 128 * 1024, "for dmfile, when the file size less than dt_small_file_size_threshold, it will be merged. If dt_small_file_size_threshold = 0, dmfile will just do as v2")              \

dbms/src/Storages/DeltaMerge/SegmentReadTaskPool.cpp

@@ -224,9 +224,10 @@ SegmentReadTaskPool::SegmentReadTaskPool(
     , unordered_input_stream_ref_count(0)
     , exception_happened(false)
     // 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))
+    // to encounter empty queues frequently, resulting in too much waiting and thread context switching.
+    // We limit the length of block queue to be 1.5 times of `num_streams_`, and in the extreme case,
+    // when `num_streams_` is 1, `block_slot_limit` is at least 2.
+    , block_slot_limit(std::ceil(num_streams_ * 1.5))
     // 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))