linux-kernel - Re: [PATCH]mmap: improve scalability for updating vm_committed

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Message-Id: <20110330193404.9525b4e9.akpm@linux-foundation.org>
Date:	Wed, 30 Mar 2011 19:34:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	linux-mm <linux-mm@...ck.org>, lkml <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH]mmap: improve scalability for updating vm_committed_as

On Thu, 31 Mar 2011 08:56:43 +0800 Shaohua Li <shaohua.li@...el.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.
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