On Fri, Mar 25, 2011 at 02:28:03AM +0100, Jan Kara wrote: > Hi, > > while working on changes to balance_dirty_pages() I was investigating why > NFS writeback is *so* bumpy when I do not call writeback_inodes_wb() from > balance_dirty_pages(). Take a single dd writing to NFS. What I can > see is that we quickly accumulate dirty pages upto limit - ~700 MB on that > machine. So flusher thread starts working and in an instant all these ~700 > MB transition from Dirty state to Writeback state. Then, as server acks > writes, Writeback pages slowly change to Unstable pages (at 100 MB/s rate > let's say) and then at one moment (commit to server happens) all pages > transition from Unstable to Clean state - the cycle begins from the start. > > The reason for this behavior seems to be a flaw in the logic in > over_bground_thresh() which checks: > global_page_state(NR_FILE_DIRTY) + > global_page_state(NR_UNSTABLE_NFS) > background_thresh > So at the moment all pages are turned Writeback, flusher thread goes to > sleep and doesn't do any background writeback, until we have accumulated > enough Stable pages to get over background_thresh. But NFS needs to have > ->write_inode() called so that it can sent commit requests to the server. > So effectively we end up sending commit only when background_thresh Unstable > pages have accumulated which creates the bumpyness. Previously this wasn't > a problem because balance_dirty_pages() ended up calling ->write_inode() > often enough for NFS to send commit requests reasonably often. > > Now I wouldn't write so long email about this if I knew how to cleanly fix > the check ;-). One way to "fix" the check would be to add there Writeback > pages: > NR_FILE_DIRTY + NR_WRITEBACK + NR_UNSTABLE_NFS > background_thresh > > This would work in the sense that it would keep flusher thread working but > a) for normal filesystems it would be working even if there's potentially > nothing to do (or it is not necessary to do anything) > b) NFS is picky when it sends commit requests (inode has to have more > Stable pages than Writeback pages if I'm reading the code in > nfs_commit_unstable_pages() right) so flusher thread may be working but > nothing really happens until enough stable pages accumulate. > > A check which kind of works but looks a bit hacky and is not perfect when > there are multiple files is: > NR_FILE_DIRTY + NR_UNSTABLE_NFS > background_thresh || > NR_UNSTABLE_NFS > NR_WRITEBACK (to match what NFS does) > > Any better idea for a fix? Have NFS account for it's writeback pages to also be accounted as NR_UNSTABLE_NFS pages? i.e. rather than incrementing NR_UNSTABLE_NFS at the writeback->unstable transition, account it at the dirty->writeback transition.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
