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Date: Fri, 11 Dec 2009 10:26:29 +0900 From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> To: Minchan Kim <minchan.kim@...il.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, 11 Dec 2009 10:25:03 +0900 Minchan Kim <minchan.kim@...il.com> wrote: > 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 hope so, too... > 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. Ok, I'll will do. > Of course, It's off topic. :) > > Thanks for commenting. Kame. Thank you for review. Regards, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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