On Sun, Jun 10, 2012 at 12:54:03PM +0800, Wanpeng Li wrote:
> On Sun, Jun 10, 2012 at 12:36:41PM +0800, Fengguang Wu wrote:
> >Wanpeng,
> >
> >Sorry this I won't take this: it don't really improve anything. Even
> >with the changed test, the real intervals are still some random values
> >above (and not far away from) 200ms.. We are saying about 200ms
> >intervals just for convenience.
> >
> But some parts like:
>
> __bdi_update_bandwidth which bdi_update_bandwidth will call:
>
> if(elapsed < BANDWIDTH_INTERVAL)
> return;
>
> or
>
> global_update_bandwidth:
>
> if(time_before(now, update_time + BANDWIDTH_INTERVAL))
> return;
>
> You me just ignore this disunion ?
Not a problem for me. But if that consistency makes you feel happy,
you might revise the changelog and resend. But it's not that simple
for the below reason..
> >On Sun, Jun 10, 2012 at 12:20:05PM +0800, Wanpeng Li wrote:
> >> From: Wanpneg Li <liwp@xxxxxxxxxxxxxxxxxx>
> >>
> >> Since bdi_update_bandwidth function should estimate write bandwidth at 200ms intervals,
The above line represents a wrong assumption. It's normal for the
re-estimate intervals to be >= 200ms.
> >> so the time is bdi->bw_time_stamp + BANDWIDTH_INTERVAL == jiffies, but
> >> if use time_is_after_eq_jiffies intervals will be bdi->bw_time_stamp +
> >> BANDWIDTH_INTERVAL + 1.
Strictly speaking, to ensure that ">= 200ms" is true, we'll have to
skip the "jiffies == bw_time_stamp + BANDWIDTH_INTERVAL" case. For
example, when HZ=100, the bw_time_stamp may actually be recorded in
the very last ms of a 10ms range, and jiffies may be in the very first
ms of the current 10ms range. So if using ">=" comparisons, it may
actually let less than 200ms intervals go though.
We can only reliably ensure "> 200ms", but no way for ">= 200ms".
Thanks,
Fengguang
> >> Signed-off-by: Wanpeng Li <liwp@xxxxxxxxxxxxxxxxxx>
> >> ---
> >> mm/page-writeback.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> >> index c833bf0..099e225 100644
> >> --- a/mm/page-writeback.c
> >> +++ b/mm/page-writeback.c
> >> @@ -1032,7 +1032,7 @@ static void bdi_update_bandwidth(struct backing_dev_info *bdi,
> >> unsigned long bdi_dirty,
> >> unsigned long start_time)
> >> {
> >> - if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
> >> + if (time_is_after_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
> >> return;
> >> spin_lock(&bdi->wb.list_lock);
> >> __bdi_update_bandwidth(bdi, thresh, bg_thresh, dirty,
> >> --
> >> 1.7.9.5
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