Messages in this thread | | | Date | Sat, 1 Aug 2009 12:02:24 +0800 | From | Wu Fengguang <> | Subject | Re: Bug in kernel 2.6.31, Slow wb_kupdate writeout |
| |
On Thu, Jul 30, 2009 at 03:34:12PM -0700, Martin Bligh wrote: > > The test case above on a 4G machine is only generating 1G of dirty data. > > I ran the same test case on the 16G, resulting in only background > > writeout. The relevant bit here being that the background writeout > > finished quickly, writing at disk speed. > > > > I re-ran the same test, but using 300 100MB files instead. While the > > dd's are running, we are going at ~80MB/sec (this is disk speed, it's an > > x25-m). When the dd's are done, it continues doing 80MB/sec for 10 > > seconds or so. Then the remainder (about 2G) is written in bursts at > > disk speeds, but with some time in between. > > OK, I think the test case is sensitive to how many files you have - if > we punt them to the back of the list, and yet we still have 299 other > ones, it may well be able to keep the disk spinning despite the bug > I outlined.Try using 30 1GB files? > > Though it doesn't seem to happen with just one dd streamer, and > I don't see why the bug doesn't trigger in that case either.
I guess the bug is not related to number dd streamers, but whether there is a stream of newly dirtied inodes (atime dirtiness would be enough). Because wb_kupdate() itself won't give up on congestion, but redirty_tail() would refresh the inode dirty time if there are newly dirtied inodes in front. And we cannot claim it to be a bug of the list based redirty_tail(), since we call it with the belief that the inode is somehow blocked. In this manner redirty_tail() can refresh the inode dirty time (and therefore delay its writeback for up to 30s) at will.
> I believe the bugfix is correct independent of any bdi changes?
Agreed.
Thanks, Fengguang
| |