On Thu, Sep 10, 2009 at 11:54:29PM +0800, Peter Zijlstra wrote: > On Thu, 2009-09-10 at 23:41 +0800, Wu Fengguang wrote: > > > > So btrfs_file_write() explicitly calls > > > > balance_dirty_pages_ratelimited_nr() to get throttled. > > > > > > Right, so what is wrong with than, and how does this patch fix that? > > > > > > [ the only thing you have to be careful with is that you don't > > > excessively grow the error bound on the dirty limit ] > > > > Then we could form a loop: > > > > btrfs_file_write(): dirty 1024 pages > > balance_dirty_pages(): write up to 12 pages (= ratelimit_pages * 1.5) > > > > in which the writeback rate cannot keep up with dirty rate, > > and the dirty pages go all the way beyond dirty_thresh. > > Ah, ok so this is to keep the error bound on the dirty limit bounded, > because we can break out of balance_dirty_pages() early, the /* We've > done our duty */ break. > > Which unbalances the duty vs the dirty ratio. Right! > I figure that with the task dirty limit stuff we could maybe try to get > rid of this break.. worth a try. Be careful. Without that break, the time a task get throttled in a single trip may go out of control. For example, task B get blocked for 1000 seconds because there is a task A keep dirtying pages, in the mean time task A's dirty thresh going down slowly, but still larger than B's. > > Sorry for writing such a vague changelog! > > np, as long as we get there :-) > > Change makes sense now, thanks! May I add you ack? Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
