[PATCH] writeback: account IO throttling wait as iowait
From: Wu Fengguang
Date: Thu Oct 08 2009 - 22:55:04 EST
On Thu, Oct 08, 2009 at 04:36:09PM +0800, Jens Axboe wrote:
> On Thu, Oct 08 2009, Peter Zijlstra wrote:
> > On Thu, 2009-10-08 at 09:58 +0800, Wu Fengguang wrote:
> > >
> > > > How this runqueue->nr_iowait is handled now ?
> > >
> > > Good question. io_schedule() has an old comment for throttling IO wait:
> > >
> > > * But don't do that if it is a deliberate, throttling IO wait (this task
> > > * has set its backing_dev_info: the queue against which it should throttle)
> > > */
> > > void __sched io_schedule(void)
> > >
> > > So it looks both Jens' and this patch behaves right in ignoring the
> > > iowait accounting for balance_dirty_pages() :)
> >
> > Well it is a change in behaviour, and I think IOWAIT makes sense when
> > we're blocked due to io throttle..
> >
> > Hmm?
>
> Yep agree, if we're deliberately waiting on IO, it should count as
> iowait time.
Then let's revert to the old behavior :)
For one single cp, it increases iowait from 29% to 56%.
Before patch:
----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai hiq siq| read writ| recv send| in out | int csw
0 4 64 28 0 3| 0 0 | 272k 10M| 0 0 |1854 863
0 6 69 23 0 3| 0 0 | 249k 11M| 0 0 |1709 865
0 6 64 27 0 4| 0 0 | 235k 10M| 0 0 |1807 788
0 4 61 30 0 4| 0 0 | 271k 12M| 0 0 |1910 898
0 4 72 21 0 4| 0 0 | 289k 13M| 0 0 |1832 905
0 6 58 35 0 2| 0 0 | 252k 11M| 0 0 |1713 900
0 4 54 38 0 4| 0 0 | 257k 11M| 0 0 |1777 841
0 5 59 30 0 7| 0 0 | 270k 12M| 0 0 |1758 836
After patch:
----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai hiq siq| read writ| recv send| in out | int csw
0 5 35 57 0 4| 0 0 | 255k 11M| 0 0 |1705 879
0 4 38 53 0 4| 0 0 | 326k 14M| 0 0 |1940 980
0 3 36 59 0 2| 0 0 | 291k 13M| 0 0 |1970 970
0 4 28 66 0 2| 0 0 | 290k 13M| 0 0 |1805 928
0 6 38 54 0 3| 0 0 | 230k 10M| 0 0 |1866 842
0 5 44 49 0 4| 0 0 | 278k 12M| 0 0 |1808 868
Thanks,
Fengguang
---
writeback: account IO throttling wait as iowait
It makes sense to do IOWAIT when someone is blocked
due to IO throttle, as suggested by Kame and Peter.
There is an old comment for not doing IOWAIT on throttle,
however it has been mismatching the code for a long time.
If we stop accounting IOWAIT for 2.6.32, it could be an
undesirable behavior change. So restore the io_schedule.
CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
CC: Jens Axboe <jens.axboe@xxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
kernel/sched.c | 3 ---
mm/page-writeback.c | 3 ++-
2 files changed, 2 insertions(+), 4 deletions(-)
--- linux.orig/mm/page-writeback.c 2009-10-09 10:40:19.000000000 +0800
+++ linux/mm/page-writeback.c 2009-10-09 10:40:20.000000000 +0800
@@ -566,7 +566,8 @@ static void balance_dirty_pages(struct a
if (pages_written >= write_chunk)
break; /* We've done our duty */
- schedule_timeout_interruptible(pause);
+ __set_current_state(TASK_INTERRUPTIBLE);
+ io_schedule_timeout(pause);
/*
* Increase the delay for each loop, up to our previous
--- linux.orig/kernel/sched.c 2009-10-09 10:40:30.000000000 +0800
+++ linux/kernel/sched.c 2009-10-09 10:40:51.000000000 +0800
@@ -6720,9 +6720,6 @@ EXPORT_SYMBOL(yield);
/*
* This task is about to go to sleep on IO. Increment rq->nr_iowait so
* that process accounting knows that this is a task in IO wait state.
- *
- * But don't do that if it is a deliberate, throttling IO wait (this task
- * has set its backing_dev_info: the queue against which it should throttle)
*/
void __sched io_schedule(void)
{
--
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