On Wed, Oct 07, 2009 at 09:47:14PM +0800, Peter Staubach wrote: > Wu Fengguang wrote: > > Hi all, > > > > Here is a collection of writeback patches on > > > > - larger writeback chunk sizes > > - single per-bdi flush thread (killing the foreground throttling writeouts) > > - lumpy pageout > > - sync livelock prevention > > - writeback scheduling > > - random fixes > > > > Sorry for posting a too big series - there are many direct or implicit > > dependencies, and one patch lead to another before I can stop.. > > > > The lumpy pageout and nr_segments support is not complete and do not > > cover all filesystems for now. It may be better to first convert some of > > the ->writepages to the generic routines to avoid duplicate work. > > > > I managed to address many issues in past week, however there are still known > > problems. Hints from filesystem developers are highly appreciated. Thanks! > > > > The estimated writeback bandwidth is about 1/2 the real throughput > > for ext2/3/4 and btrfs; noticeable bigger than real throughput for NFS; and > > cannot be estimated at all for XFS. Very interesting.. > > > > NFS writeback is very bumpy. The page numbers and network throughput "freeze" > > together from time to time: > > > > Yes. It appears that the problem is that too many pages get dirtied > and the network/server get overwhelmed by the NFS client attempting > to write out all of the pages as quickly as it possibly can. In theory it should push pages as quickly as possible at first, to fill up the server side queue. > I think that it would be better if we could better match the > number of pages which can be dirty at any given point with the > bandwidth available through the network and the server file > system and storage. And then go into this steady state of matched network/disk bandwidth. > One approach that I have pondered is immediately queuing an > asynchronous request when enough pages are dirtied to be able > to completely fill an over the wire transfer. This sort of > seems like a per-file bdi, which doesn't seem quite like the > right approach to me. What would y'all think about that? Hmm, it sounds like unnecessary complexity. Because it is not going to help the busy-dirtier case anyway. And if we can do good on heavy IO, the pre-flushing policy becomes less interesting. > > > # vmmon -d 1 nr_writeback nr_dirty nr_unstable # (per 1-second samples) > > nr_writeback nr_dirty nr_unstable > > 11227 41463 38044 > > 11227 41463 38044 > > 11227 41463 38044 > > 11227 41463 38044 I guess in the above 4 seconds, either client or (more likely) server is blocked. A blocked server cannot send ACKs to knock down both nr_writeback/nr_unstable. And the stuck nr_writeback will freeze nr_dirty as well, because the dirtying process is throttled until it receives enough "PG_writeback cleared" event, however the bdi-flush thread is also blocked when trying to clear more PG_writeback, because the client side nr_writeback limit has been reached. In summary, server blocked => nr_writeback stuck => nr_writeback limit reached => bdi-flush blocked => no end_page_writeback() => dirtier blocked => nr_dirty stuck Thanks, Fengguang > > 11045 53987 6490 > > 11033 53120 8145 > > 11195 52143 10886 > > 11211 52144 10913 > > 11211 52144 10913 > > 11211 52144 10913 > > > > btrfs seems to maintain a private pool of writeback pages, which can go out of > > control: > > > > nr_writeback nr_dirty > > 261075 132 > > 252891 195 > > 244795 187 > > 236851 187 > > 228830 187 > > 221040 218 > > 212674 237 > > 204981 237 > > > > XFS has very interesting "bumpy writeback" behavior: it tends to wait > > collect enough pages and then write the whole world. > > > > nr_writeback nr_dirty > > 80781 0 > > 37117 37703 > > 37117 43933 > > 81044 6 > > 81050 0 > > 43943 10199 > > 43930 36355 > > 43930 36355 > > 80293 0 > > 80285 0 > > 80285 0 > > > > Thanks, > > Fengguang > > > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ -- 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
