From: Andrew Morton <akpm@linux-foundation.org>
To: Shaohua Li <shaohua.li@intel.com>
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>
Subject: Re: [PATCH]mmap: improve scalability for updating vm_committed_as
Date: Wed, 30 Mar 2011 20:06:50 -0700 [thread overview]
Message-ID: <20110330200650.d93a1aec.akpm@linux-foundation.org> (raw)
In-Reply-To: <1301540239.3981.80.camel@sli10-conroe>
On Thu, 31 Mar 2011 10:57:19 +0800 Shaohua Li <shaohua.li@intel.com> wrote:
> 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.
How can we be sure of that?
> so this isn't a big issue?
Well it might be. Experience tells us that some people are likely to
get bitten by this.
It's far safer and saner to find a solution which doesn't have big fat
failure modes!
Also, we don't (yet) know what we're *gaining* for this big fat failure
mode.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
prev parent reply other threads:[~2011-03-31 3:05 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-30 1:17 [PATCH]mmap: improve scalability for updating vm_committed_as Shaohua Li
2011-03-30 22:51 ` Andrew Morton
2011-03-31 0:56 ` Shaohua Li
2011-03-31 2:34 ` Andrew Morton
2011-03-31 2:57 ` Shaohua Li
2011-03-31 3:06 ` Andrew Morton [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20110330200650.d93a1aec.akpm@linux-foundation.org \
--to=akpm@linux-foundation.org \
--cc=andi@firstfloor.org \
--cc=hughd@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=riel@redhat.com \
--cc=shaohua.li@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).