Re: RFC - how to balance Dirty+Writeback in the face of slow writeback.

[Nikita Danilov <nikita@clusterfs.com>]

Andrew Morton writes:

[...]

 > 
 > The way this code all works is pretty crude and simple: a process comes
 > in to to some writeback and it enters a polling loop:
 > 
 > 	while (we need to do writeback) {
 > 		for (each superblock) {
 > 			if (the superblock's backing_dev isn't congested) {
 > 				stuff some more IO down it()
 > 			}
 > 		}
 > 		take_a_nap();
 > 	}
 > 
 > so the process remains captured in that polling loop until the
 > dirty-memory-exceed condition subsides.  The reason why we avoid

Hm... wbc->nr_to_write is checked all the way down
(balance_dirty_pages(), writeback_inodes(), sync_sb_inodes(),
mpage_writepages()), so "occasional writer" cannot be stuck for more
than 32 + 16 pages, it seems.

Nikita.