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 :)

Thanks,
Fengguang