Date: Mon, 20 Jun 2011 13:58:59 -0400
From: Eric B Munson <emunson@mgebm.net>
To: Cong Wang <amwang@redhat.com>
Cc: Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        Andrea Arcangeli <aarcange@redhat.com>,
        Johannes Weiner <jweiner@redhat.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, linux-mm@kvack.org
Subject: Re: [PATCH 1/3] mm: completely disable THP by
 transparent_hugepage=never
Message-ID: <20110620175859.GB9697@mgebm.net>
References: <1308587683-2555-1-git-send-email-amwang@redhat.com>
 <20110620165844.GA9396@suse.de>
 <4DFF7E3B.1040404@redhat.com>
 <4DFF7F0A.8090604@redhat.com>
 <4DFF8106.8090702@redhat.com>
 <4DFF8327.1090203@redhat.com>
 <4DFF84BB.3050209@redhat.com>
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On Tue, 21 Jun 2011, Cong Wang wrote:

> =E4=BA=8E 2011=E5=B9=B406=E6=9C=8821=E6=97=A5 01:28, Rik van Riel =E5=86=
=99=E9=81=93:
> >On 06/20/2011 01:19 PM, Cong Wang wrote:
> >>=E4=BA=8E 2011=E5=B9=B406=E6=9C=8821=E6=97=A5 01:10, Rik van Riel =E5=
=86=99=E9=81=93:
> >>>On 06/20/2011 01:07 PM, Cong Wang wrote:
> >>>>=E4=BA=8E 2011=E5=B9=B406=E6=9C=8821=E6=97=A5 00:58, Mel Gorman =E5=
=86=99=E9=81=93:
> >>>>>On Tue, Jun 21, 2011 at 12:34:28AM +0800, Amerigo Wang wrote:
> >>>>>>transparent_hugepage=3Dnever should mean to disable THP completely,
> >>>>>>otherwise we don't have a way to disable THP completely.
> >>>>>>The design is broken.
> >>>>>>
> >>>>>
> >>>>>I don't get why it's broken. Why would the user be prevented from
> >>>>>enabling it at runtime?
> >>>>>
> >>>>
> >>>>We need to a way to totally disable it, right? Otherwise, when I
> >>>>configure
> >>>>THP in .config, I always have THP initialized even when I pass "=3Dne=
ver".
> >>>>
> >>>>For me, if you don't provide such way to disable it, it is not flexib=
le.
> >>>>
> >>>>I meet this problem when I try to disable THP in kdump kernel, there =
is
> >>>>no user of THP in kdump kernel, THP is a waste for kdump kernel. This=
 is
> >>>>why I need to find a way to totally disable it.
> >>>
> >>>What you have not explained yet is why having THP
> >>>halfway initialized (but not used, and without a
> >>>khugepaged thread) is a problem at all.
> >>>
> >>>Why is it a problem for you?
> >>
> >>It occupies some memory, memory is valuable in kdump kernel (usually
> >>only 128M). :) Since I am sure no one will use it, why do I still need
> >>to initialize it at all?
> >
> >Lets take a look at how much memory your patches end
> >up saving.
> >
> >By bailing out earlier in hugepage_init, you end up
> >saving 3 sysfs objects, one slab cache and a hash
> >table with 1024 pointers. That's a total of maybe
> >10kB of memory on a 64 bit system.
> >
> >I'm not convinced that a 10kB memory reduction is
> >worth the price of never being able to enable
> >transparent hugepages when a system is booted with
> >THP disabled...
> >
>=20
> Even if it is really 10K, why not save it since it doesn't
> much effort to make this. ;) Not only memory, but also time,
> this could also save a little time to initialize the kernel.
>=20
> For me, the more serious thing is the logic, there is
> no way to totally disable it as long as I have THP in .config
> currently. This is why I said the design is broken.
>=20
> Thanks.
>=20

If memory is this scarce, why not set CONFIG_TRANSPARENT_HUGEPAGE=3Dn and b=
e done
with it?  If the config option is enabled, the admin should be able to turn=
 the
functionality back on if desired.  If you really don't _ever_ want THP then
disable the config.

Eric

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