MDEV-24949: Enabling idle flushing (possible regression from MDEV-23855)

- Currently page cleaner thread will stop flushing if
  dirty_pct < innodb_max_dirty_pages_pct_lwm.

- If the server is not performing any activity then said resources/time
  could be used to flush the pending dirty pages and keep buffer pool
  clean for the next burst of the cycle. This flushing is called idle flushing.

- flushing logic underwent a complete revamp in 10.5.7/8
  and as part of the revamp idle flushing logic got removed.

- New proposed logic of idle flushing is based on updated logic of the
  page cleaner that will enable idle flushing if
  - buf page cleaner is idle
  - there are dirty pages (< innodb_max_dirty_pages_pct_lwm)
  - server is not performing any activity

  Logic will kickstart the idle flushing bounded by innodb_io_capacity.

(Thanks to Marko Makela for reviewing the patch and idea
 right from the its inception).

1 files changed, 9 insertions, 0 deletions

diff --git a/storage/innobase/include/buf0buf.h b/storage/innobase/include/buf0buf.h
index 12fb139f1bf..deff8054362 100644
--- a/storage/innobase/include/buf0buf.h
+++ b/storage/innobase/include/buf0buf.h
@@ -1946,6 +1946,8 @@ public:
 private:
   /** whether the page cleaner needs wakeup from indefinite sleep */
   bool page_cleaner_is_idle;
+  /** track server activity count for signaling idle flushing */
+  ulint last_activity_count;
 public:
   /** signalled to wake up the page_cleaner; protected by flush_list_mutex */
   pthread_cond_t do_flush_list;
@@ -1966,6 +1968,13 @@ public:
     page_cleaner_is_idle= deep_sleep;
   }
 
+  /** Update server last activity count */
+  void update_last_activity_count(ulint activity_count)
+  {
+    mysql_mutex_assert_owner(&flush_list_mutex);
+    last_activity_count= activity_count;
+  }
+
   // n_flush_LRU + n_flush_list is approximately COUNT(io_fix()==BUF_IO_WRITE)
   // in flush_list