Subject: Re: [PATCH]mmap: improve scalability for updating vm_committed_as
From: Shaohua Li <shaohua.li@intel.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm <linux-mm@kvack.org>, lkml <linux-kernel@vger.kernel.org>,
        Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
        Hugh Dickins <hughd@google.com>
In-Reply-To: <20110330193404.9525b4e9.akpm@linux-foundation.org>
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	 <20110330155114.fa47dd9d.akpm@linux-foundation.org>
	 <1301533003.3981.75.camel@sli10-conroe>
	 <20110330193404.9525b4e9.akpm@linux-foundation.org>
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Date: Thu, 31 Mar 2011 10:57:19 +0800
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On Thu, 2011-03-31 at 10:34 +0800, Andrew Morton wrote:
> On Thu, 31 Mar 2011 08:56:43 +0800 Shaohua Li <shaohua.li@intel.com> wrote:
> 
> > > This is a big change, and it wasn't even changelogged.  It's
> > > potentially a tremendous increase in the expense of a read from
> > > /proc/meminfo, which is a file that lots of tools will be polling. 
> > > Many of those tools we don't even know about or have access to.
> > Assume we don't read /proc/meminfo too often.
> 
> That's a poor assumption.  top(1) and vmstat(8) read it, for a start. 
> There will be zillions of locally-developed monitoring tools which read
> meminfo.
> 
> Now, it could be that something under meminfo reads _already_ does a
> massive walk across all CPUs.  If so then we'll have already trained
> people to avoid reading /proc/meminfo and this change might be
> acceptable.
> 
> But if this isn't the case then it's quite likely that this change will
> hurt some people quite a lot.  And, unfortunately, the sort of people
> who we will hurt tend to be people who don't run our stuff until a long
> time (years) after we wrote it.  By which time it's going to be quite
> expensive to get a fix down the chain and into their hands.
Just looked at the code. nr_blockdev_pages() of si_meminfo iterate all
block devices. For people who care about the time, their system must
have more block devices than CPUs. so this isn't a big issue?

Thanks,
Shaohua

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