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SubjectRe: + mm-align-vmstat_works-timer.patch added to -mm tree
Date
(swich to lkml and linux-mm)

Hi Anton,

Do you have any mesurement data?

Honestly, I made the same patch few week ago.
but I found two problems.

1)
work queue tracer (in -tip) reported it isn't proper rounded.

The fact is, schedule_delayed_work(work, round_jiffies_relative()) is
a bit ill.

it mean
- round_jiffies_relative() calculate rounded-time - jiffies
- schedule_delayed_work() calculate argument + jiffies

it assume no jiffies change at above two place. IOW it assume
non preempt kernel.


2)
> - schedule_delayed_work_on(cpu, vmstat_work, HZ + cpu);
> + schedule_delayed_work_on(cpu, vmstat_work,
> + __round_jiffies_relative(HZ, cpu));

isn't same meaning.

vmstat_work mean to move per-cpu stastics to global stastics.
Then, (HZ + cpu) mean to avoid to touch the same global variable at the same time.

Oh well, this patch have performance regression risk on _very_ big server.
(perhaps, only sgi?)

but I agree vmstat_work is one of most work queue heavy user.
For power consumption view, it isn't proper behavior.

I still think improving another way.

>
> The patch titled
> mm: align vmstat_work's timer
> has been added to the -mm tree. Its filename is
> mm-align-vmstat_works-timer.patch
>
> Before you just go and hit "reply", please:
> a) Consider who else should be cc'ed
> b) Prefer to cc a suitable mailing list as well
> c) Ideally: find the original patch on the mailing list and do a
> reply-to-all to that, adding suitable additional cc's
>
> *** Remember to use Documentation/SubmitChecklist when testing your code ***
>
> See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find
> out what to do about this
>
> The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
>
> ------------------------------------------------------
> Subject: mm: align vmstat_work's timer
> From: Anton Blanchard <anton@samba.org>
>
> Even though vmstat_work is marked deferrable, there are still benefits to
> aligning it. For certain applications we want to keep OS jitter as low as
> possible and aligning timers and work so they occur together can reduce
> their overall impact.
>
> Signed-off-by: Anton Blanchard <anton@samba.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> mm/vmstat.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff -puN mm/vmstat.c~mm-align-vmstat_works-timer mm/vmstat.c
> --- a/mm/vmstat.c~mm-align-vmstat_works-timer
> +++ a/mm/vmstat.c
> @@ -984,7 +984,7 @@ static void vmstat_update(struct work_st
> {
> refresh_cpu_vm_stats(smp_processor_id());
> schedule_delayed_work(&__get_cpu_var(vmstat_work),
> - sysctl_stat_interval);
> + round_jiffies_relative(sysctl_stat_interval));
> }
>
> static void __cpuinit start_cpu_timer(int cpu)
> @@ -992,7 +992,8 @@ static void __cpuinit start_cpu_timer(in
> struct delayed_work *vmstat_work = &per_cpu(vmstat_work, cpu);
>
> INIT_DELAYED_WORK_DEFERRABLE(vmstat_work, vmstat_update);
> - schedule_delayed_work_on(cpu, vmstat_work, HZ + cpu);
> + schedule_delayed_work_on(cpu, vmstat_work,
> + __round_jiffies_relative(HZ, cpu));
> }
>
> /*
> _
>
> Patches currently in -mm which might be from anton@samba.org are
>
> origin.patch
> mm-align-vmstat_works-timer.patch
> random-align-rekey_works-timer.patch
> sunrpc-align-cache_clean-works-timer.patch
>
> --
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