Re: [PATCH v4 2/2] mm: allow to set overcommit ratio more precisely

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 6 Nov 2013 03:42:20 -0500 (EST) Jerome Marchand <jmarchan@redhat.com> wrote:

> 
> 
> ----- Original Message -----
> > From: "Andrew Morton" <akpm@linux-foundation.org>
> > To: "Jerome Marchand" <jmarchan@redhat.com>
> > Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, "dave hansen" <dave.hansen@intel.com>
> > Sent: Wednesday, November 6, 2013 12:53:19 AM
> > Subject: Re: [PATCH v4 2/2] mm: allow to set overcommit ratio more precisely
> > 
> > On Fri, 18 Oct 2013 14:56:59 +0200 Jerome Marchand <jmarchan@redhat.com>
> > wrote:
> > 
> > > Some applications that run on HPC clusters are designed around the
> > > availability of RAM and the overcommit ratio is fine tuned to get the
> > > maximum usage of memory without swapping. With growing memory, the 1%
> > > of all RAM grain provided by overcommit_ratio has become too coarse
> > > for these workload (on a 2TB machine it represents no less than
> > > 20GB).
> > > 
> > > This patch adds the new overcommit_ratio_ppm sysctl variable that
> > > allow to set overcommit ratio with a part per million precision.
> > > The old overcommit_ratio variable can still be used to set and read
> > > the ratio with a 1% precision. That way, overcommit_ratio interface
> > > isn't broken in any way that I can imagine.
> > 
> > The way we've permanently squished this mistake in the past is to
> > switch to "bytes".  See /proc/sys/vm/*bytes.
> > 
> > Would that approach work in this case?
> > 
> 
> That was my first version of this patch (actually "kbytes" to avoid
> overflow).
> Dave raised the issue that it silently breaks the user interface:
> overcommit_ratio is zero while the system behaves differently.

I don't understand that at all.  We keep overcommit_ratio as-is, with
the same default values and add a different way of altering it.  That
should be back-compatible?

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