In background loop run purge to completion before doing other background operations: it does not make sense to flush buffer pool pages if they are soon modified again by purge
trx0purge.c:
  Increase purge_sys->n_pages_handled for every undo log we purge, even if that log would be only a hundred bytes: that way we get the purge batches of 20 pages to set a fresh purge view (limit) more often, and we can reduce the number of old row versions purge has to look at when it decides if it can remove some delete-marked index record