On Dec 12, 2007 12:55 PM, Peter Zijlstra <[EMAIL PROTECTED]> wrote: > > On Mon, 2007-12-10 at 18:02 -0800, Michael Rubin wrote: > > From: Michael Rubin <[EMAIL PROTECTED]> > The part I miss here is the rationale on _how_ you solve the problem. > > The patch itself is simple enough, but I've been staring at this code > for a while now, and I'm just not getting it.
Apologies for the lack of rationale. I have been staring at this code for awhile also and it makes my head hurt. I have a patch coming (hopefully today) that proposes using one data structure with a more consistent priority scheme for 2.6.25. To me it's simpler, but I am biased. The problem we encounter when we append to a large file at a fast rate while also writing to smaller files is that the wb_kupdate thread does not keep up with disk traffic. In this workload often the inodes end up at fs/fs-writeback.c:287 after do_writepages, since do_writepages did not write all the pages. This can be due to congestion but I think there are other causes also since I have observed so. The first issue is that the inode is put on the s_more_io queue. This ensures that more_io is set at the end of sync_sb_inodes. The result from that is the wb_kupdate routine will perform a sleep at mm/page-writeback.c:642. This slows us down enough that the wb_kupdate cannot keep up with traffic. The other issue is that the inode that has been placed on the s_more_io queue cannot be processed by sync_sb_inodes until the entire s_io list is empty. With lots of small files that are not being dirtied as quickly as the one large inode on the s_more_io queue the inode with the most pages being dirtied is not given attention and wb_kupdate cannot keep up again. mrubin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
