Change the default value of background_schedule_pool_size to 2

Author: wudidapaopao

src/Core/ServerSettings.h

@@ -126,7 +126,7 @@ namespace DB
     M(UInt64, background_fetches_pool_size, 1, "The maximum number of threads that will be used for fetching data parts from another replica for *MergeTree-engine tables in a background.", 0) \
     M(UInt64, background_common_pool_size, 1, "The maximum number of threads that will be used for performing a variety of operations (mostly garbage collection) for *MergeTree-engine tables in a background.", 0) \
     M(UInt64, background_buffer_flush_schedule_pool_size, 2, "The maximum number of threads that will be used for performing flush operations for Buffer-engine tables in a background.", 0) \
-    M(UInt64, background_schedule_pool_size, 1, "The maximum number of threads that will be used for constantly executing some lightweight periodic operations.", 0) \
+    M(UInt64, background_schedule_pool_size, 2, "The maximum number of threads that will be used for constantly executing some lightweight periodic operations.", 0) \
     M(UInt64, background_message_broker_schedule_pool_size, 0, "The maximum number of threads that will be used for executing background operations for message streaming.", 0) \
     M(UInt64, background_distributed_schedule_pool_size, 0, "The maximum number of threads that will be used for executing distributed sends.", 0) \
     M(UInt64, tables_loader_foreground_pool_size, 0, "The maximum number of threads that will be used for foreground (that is being waited for by a query) loading of tables. Also used for synchronous loading of tables before the server start. Zero means use all CPUs.", 0) \