On Sun, Aug 07, 2011 at 03:18:57PM +0800, Wu Fengguang wrote: > Andrea, > > On Sun, Aug 07, 2011 at 12:46:56AM +0800, Andrea Righi wrote: > > On Sat, Aug 06, 2011 at 04:44:52PM +0800, Wu Fengguang wrote: > > > > So here is a pause time oriented approach, which tries to control the > > > pause time in each balance_dirty_pages() invocations, by controlling > > > the number of pages dirtied before calling balance_dirty_pages(), for > > > smooth and efficient dirty throttling: > > > > > > - avoid useless (eg. zero pause time) balance_dirty_pages() calls > > > - avoid too small pause time (less than 4ms, which burns CPU power) > > > - avoid too large pause time (more than 200ms, which hurts responsiveness) > > > - avoid big fluctuations of pause times > > > > I definitely agree that too small pauses must be avoided. However, I > > don't understand very well from the code how the minimum sleep time is > > regulated. > > Thanks for pointing this out. Yes, the sleep time regulation is not > here and I should have mentioned that above. Since this is only the > core bits, there will be some followup patches to fix the rough edges. > (attached the two relevant patches) > > > I've added a simple tracepoint (see below) to monitor the pause times in > > balance_dirty_pages(). > > > > Sometimes I see very small pause time if I set a low dirty threshold > > (<=32MB). > > Yeah, it's definitely possible. > > > Example: > > > > # echo $((16 * 1024 * 1024)) > /proc/sys/vm/dirty_bytes > > # iozone -A >/dev/null & > > # cat /sys/kernel/debug/tracing/trace_pipe > > ... > > iozone-2075 [001] 380.604961: writeback_dirty_throttle: 1 > > iozone-2075 [001] 380.605966: writeback_dirty_throttle: 2 > > iozone-2075 [001] 380.608405: writeback_dirty_throttle: 0 > > iozone-2075 [001] 380.608980: writeback_dirty_throttle: 1 > > iozone-2075 [001] 380.609952: writeback_dirty_throttle: 1 > > iozone-2075 [001] 380.610952: writeback_dirty_throttle: 2 > > iozone-2075 [001] 380.612662: writeback_dirty_throttle: 0 > > iozone-2075 [000] 380.613799: writeback_dirty_throttle: 1 > > iozone-2075 [000] 380.614771: writeback_dirty_throttle: 1 > > iozone-2075 [000] 380.615767: writeback_dirty_throttle: 2 > > ... > > > > BTW, I can see this behavior only in the first minute while iozone is > > running. Ater ~1min things seem to get stable (sleeps are usually > > between 50ms and 200ms). > > > > Yeah, it's roughly in line with this graph, where the red dots are the > pause time: > > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v8/512M/xfs-1dd-4k-8p-438M-20:10-3.0.0-next-20110802+-2011-08-06.11:03/balance_dirty_pages-pause.png > > Note that the big change of pattern in the middle is due to a > deliberate disturb: a dd will be started at 100s _reading_ 1GB data, > which effectively livelocked the other dd dirtier task with the CFQ io > scheduler. > > > I wonder if we shouldn't add an explicit check also for the minimum > > sleep time. > > With the more complete patchset including the pause time regulation, > the pause time distribution should look much better, falling nicely > into the range (5ms, 20ms): > > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v8/3G/xfs-1dd-4k-8p-2948M-20:10-3.0.0-rc2-next-20110610+-2011-06-12.21:51/balance_dirty_pages-pause.png > > > +TRACE_EVENT(writeback_dirty_throttle, > > + TP_PROTO(unsigned long sleep), > > + TP_ARGS(sleep), > > btw, I've just pushed two more tracing patches to the git tree. > Hope it helps :) Perfect. Thanks for the clarification and the additional patches, I'm going to test them right now. -Andrea -- 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
