On Wed, Nov 24, 2010 at 08:46:51PM +0800, Peter Zijlstra wrote:
> On Wed, 2010-11-24 at 20:30 +0800, Wu Fengguang wrote:
> >
> > For the 1-dd case, it looks better to lower the break threshold to
> > 125ms. After all, it's not easy for the dirty pages to drop by 250ms
> > worth of data when you only slept 200ms (note: the max pause time has
> > been doubled mainly for servers).
> >
> > - if (nr_dirty < dirty_thresh &&
> > - bdi_prev_dirty - bdi_dirty > (long)bdi->write_bandwidth / 4)
> > + if (nr_dirty <= dirty_thresh &&
> > + bdi_prev_dirty - bdi_dirty > (long)bdi->write_bandwidth / 8)
> > break;
>
> Hrm, but 125ms worth in 200ms is rather easy, you'd want to keep that
> limit above what the pause should give you, right?
Yeah, I exactly mean to quit the loop after sleeping 200ms.
200ms pause already seem large..
I'll leave the safeguard to the "nr_dirty <= dirty_thresh" test :)
This trace shows the problem it tries to solve. Here on fresh boot,
bdi_dirty=106600 which is much larger than bdi_limit=13525.
The same situation may happen if some task quickly eats lots of
anonymous memory, then the global/bdi limits will be knock down
suddenly and it takes time to bring down bdi_dirty.
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
<...>-2722 [005] 39.623216: balance_dirty_pages: bdi=8:0 bdi_dirty=106600 bdi_limit=13525 task_limit=13423 task_weight=12% task_gap=-11022% bdi_gap=-5504% dirtied=268 bw=0 ratelimit=0 period=0 think=0 pause=200
<...>-2720 [004] 39.623222: balance_dirty_pages: bdi=8:0 bdi_dirty=106600 bdi_limit=13513 task_limit=13441 task_weight=9% task_gap=-11029% bdi_gap=-5510% dirtied=267 bw=0 ratelimit=0 period=0 think=0 pause=200
<...>-2717 [003] 39.623666: balance_dirty_pages: bdi=8:0 bdi_dirty=106560 bdi_limit=13742 task_limit=13659 task_weight=10% task_gap=-10815% bdi_gap=-5402% dirtied=268 bw=0 ratelimit=0 period=0 think=0 pause=200
<...>-2718 [004] 39.623990: balance_dirty_pages: bdi=8:0 bdi_dirty=106560 bdi_limit=13979 task_limit=13913 task_weight=8% task_gap=-10603% bdi_gap=-5297% dirtied=272 bw=0 ratelimit=0 period=0 think=0 pause=200
<...>-2715 [007] 39.626472: balance_dirty_pages: bdi=8:0 bdi_dirty=106360 bdi_limit=15312 task_limit=15211 task_weight=11% task_gap=-9523% bdi_gap=-4756% dirtied=268 bw=0 ratelimit=0 period=0 think=0 pause=200
<...>-2723 [001] 39.628951: balance_dirty_pages: bdi=8:0 bdi_dirty=106560 bdi_limit=15749 task_limit=15640 task_weight=11% task_gap=-9236% bdi_gap=-4612% dirtied=269 bw=0 ratelimit=0 period=0 think=0 pause=200
Thanks,
Fengguang
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