On Tue, Dec 14, 2010 at 05:15:51AM +0800, Trond Myklebust wrote: > On Mon, 2010-12-13 at 22:47 +0800, Wu Fengguang wrote: > > plain text document attachment (writeback-nfs-in-commit.patch) > > When doing 10+ concurrent dd's, I observed very bumpy commits submission > > (partly because the dd's are started at the same time, and hence reached > > 4MB to-commit pages at the same time). Basically we rely on the server > > to complete and return write/commit requests, and want both to progress > > smoothly and not consume too many pages. The write request wait queue is > > not enough as it's mainly network bounded. So add another commit request > > wait queue. Only async writes need to sleep on this queue. > > > > I'm not understanding the above reasoning. Why should we serialise > commits at the per-filesystem level (and only for non-blocking flushes > at that)? I did the commit wait queue after seeing this graph, where there is very bursty pattern of commit submission and hence completion: http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/nfs-100dd-1M-8p-2953M-2.6.37-rc3+-2010-12-03-01/nfs-commit-1000.png leading to big fluctuations, eg. the almost straight up/straight down lines below http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/nfs-100dd-1M-8p-2953M-2.6.37-rc3+-2010-12-03-01/vmstat-dirty-300.png http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/nfs-100dd-1M-8p-2953M-2.6.37-rc3+-2010-12-03-01/dirty-pages.png http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/nfs-100dd-1M-8p-2953M-2.6.37-rc3+-2010-12-03-01/dirty-pages-200.png A commit wait queue will help wipe out the "peaks". The "fixed" graph is http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/nfs-100dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-03-23/vmstat-dirty-300.png http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/nfs-100dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-03-23/dirty-pages.png Blocking flushes don't need to wait on this queue because they already throttle themselves by waiting on the inode commit lock before/after the commit. They actually should not wait on this queue, to prevent sync requests being unnecessarily blocked by async ones. Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
