With my M2 instance indexers in "
Update on Schedule" mode...
var/log/cron.log receives this warning near constantly:
Could not acquire lock for cron job:
indexer_update_all_views  
The cron process in the process list terminates... plus I have a constant "X in the backlog" message for a few indexes.
I cannot figure out what this problem is about... I've tried truncating all *_cl tables and rebuilding the index... but still the site eventually goes back to items permanently stuck the backlog, a cron job not terminating, and this log message always appearing.
The only solution I have right now to restore the site's performance is to leave the indexer in Update on Save mode.