linux-kernel - Re: [PATCH] ksm: add vm_stat and meminfo entry to reflect pte mapping to ksm pages

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Message-Id: <201103181516.37671.nai.xia@gmail.com>
Date:	Fri, 18 Mar 2011 15:16:37 +0800
From:	Nai Xia <nai.xia@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Izik Eidus <ieidus@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] ksm: add vm_stat and meminfo entry to reflect pte mapping to ksm pages


>On Wednesday 02 March 2011, at 07:41:00, <Andrew Morton <akpm@...ux-foundation.org>> wrote
> On Sat, 26 Feb 2011 22:56:31 +0800
> Nai Xia <nai.xia@...il.com> wrote:
> 
> > ksm_pages_sharing is updated by ksmd periodically.  In some cases, it cannot 
> > reflect the actual savings and makes the benchmarks on volatile VMAs very 
> > inaccurate.
> > 
> > This patch add a vm_stat entry and let the /proc/meminfo show information 
> > about how much virutal address pte is being mapped to ksm pages.  With default 
> > ksm paramters (pages_to_scan==100 && sleep_millisecs==20), this can result in 
> > 50% more accurate averaged savings result for the following test program. 
> > Bigger sleep_millisecs values will increase this deviation. 
> 
> So I think you're saying that the existing ksm_pages_sharing sysfs file
> is no good.
> 
> You added a new entry to /proc/meminfo and left ksm_pages_sharing
> as-is.  Why not leave /proc/meminfo alone, and fix up the existing
> ksm_pages_sharing?


The ksm_pages_sharing is really a count for how many "rmap_item"s is currently
linked in stable_nodes. ksmd updates ksm_pages_sharing whenever it's waken up.
However, just during the time ksmd is sleeping, many shared KSM pages can be 
broken because of page writes. So ksm_pages_sharing means much more an internal
state for ksm than a real count for how much pages is being shared at some time
point. Since the state of the internal data structures of ksm is only updated 
by ksmd privately. I think it's hard to make it correctly reflect the real time
memory saving. So I just added another count and let ksm_pages_sharing still 
focus on it original role.

> 
> Also, the patch accumulates the NR_KSM_PAGES_SHARING counts on a
> per-zone basis as well as on a global basis, but only provides the
> global count to userspace.  The per-zone counts are potentially
> interesting?  If not, maintaining the per-zone counters is wasted
> overhead.

Yes, I will make it to the zoneinfo, soon. 

> 
> > 
> > --- test.c-----
> >
> 
> The "^---" token conventionally means "end of changelog".  Please avoid
> inserting it into the middle of the changelog.

OK, I understand now. :)

> 
> > +++ b/mm/ksm.c
> > @@ -897,6 +897,7 @@ static int try_to_merge_one_page(struct vm_area_struct 
> > *vma,
> 
> Your email client wordwraps the patches.

Oops, sorry for the crappy client it is also responsible for my late reply.
I will fix it soon.

Nai

> 
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