linux-kernel - Re: [RFC mm][PATCH 2/5] percpu cached mm counter

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Date:	Fri, 11 Dec 2009 10:25:03 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	cl@...ux-foundation.org,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	mingo@...e.hu
Subject: Re: [RFC mm][PATCH 2/5] percpu cached mm counter

On Fri, Dec 11, 2009 at 9:51 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Fri, 11 Dec 2009 09:40:07 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
>> > static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
>> >  {
>> > -       return (unsigned long)atomic_long_read(&(mm)->counters[member]);
>> > +       long ret;
>> > +       /*
>> > +        * Because this counter is loosely synchronized with percpu cached
>> > +        * information, it's possible that value gets to be minus. For user's
>> > +        * convenience/sanity, avoid returning minus.
>> > +        */
>> > +       ret = atomic_long_read(&(mm)->counters[member]);
>> > +       if (unlikely(ret < 0))
>> > +               return 0;
>> > +       return (unsigned long)ret;
>> >  }
>>
>> Now, your sync point is only task switching time.
>> So we can't show exact number if many counting of mm happens
>> in short time.(ie, before context switching).
>> It isn't matter?
>>
> I think it's not a matter from 2 reasons.
>
> 1. Now, considering servers which requires continuous memory usage monitoring
> as ps/top, when there are 2000 processes, "ps -elf" takes 0.8sec.
> Because system admins know that gathering process information consumes
> some amount of cpu resource, they will not do that so frequently.(I hope)
>
> 2. When chains of page faults occur continously in a period, the monitor
> of memory usage just see a snapshot of current numbers and "snapshot of what
> moment" is at random, always. No one can get precise number in that kind of situation.
>

Yes. I understand that.

But we did rss updating as batch until now.
It was also stale. Just only your patch make stale period longer.
Hmm. I hope people don't expect mm count is precise.

I saw the many people believed sanpshot of mm counting is real in
embedded system.
They want to know the exact memory usage in system.
Maybe embedded system doesn't use SPLIT_LOCK so that there is no regression.

At least, I would like to add comment "It's not precise value." on
statm's Documentation.
Of course, It's off topic.  :)

Thanks for commenting. Kame.
-- 
Kind regards,
Minchan Kim
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