Date: Mon, 19 Nov 2012 11:56:15 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Jaegeuk Hanse <jaegeuk.hanse@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>, linux-mm@kvack.org,
        Andi Kleen <ak@linux.intel.com>,
        "H. Peter Anvin" <hpa@linux.intel.com>, linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        David Rientjes <rientjes@google.com>
Subject: Re: [PATCH v6 10/12] thp: implement refcounting for huge zero page
Message-ID: <20121119095615.GA23869@otc-wbsnb-06>
References: <1353007622-18393-1-git-send-email-kirill.shutemov@linux.intel.com>
 <1353007622-18393-11-git-send-email-kirill.shutemov@linux.intel.com>
 <50A87EF0.3060706@gmail.com>
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On Sun, Nov 18, 2012 at 02:23:44PM +0800, Jaegeuk Hanse wrote:
> On 11/16/2012 03:27 AM, Kirill A. Shutemov wrote:
> >From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> >
> >H. Peter Anvin doesn't like huge zero page which sticks in memory forever
> >after the first allocation. Here's implementation of lockless refcounting
> >for huge zero page.
> >
> >We have two basic primitives: {get,put}_huge_zero_page(). They
> >manipulate reference counter.
> >
> >If counter is 0, get_huge_zero_page() allocates a new huge page and
> >takes two references: one for caller and one for shrinker. We free the
> >page only in shrinker callback if counter is 1 (only shrinker has the
> >reference).
> >
> >put_huge_zero_page() only decrements counter. Counter is never zero
> >in put_huge_zero_page() since shrinker holds on reference.
> >
> >Freeing huge zero page in shrinker callback helps to avoid frequent
> >allocate-free.
> >
> >Refcounting has cost. On 4 socket machine I observe ~1% slowdown on
> >parallel (40 processes) read page faulting comparing to lazy huge page
> >allocation.  I think it's pretty reasonable for synthetic benchmark.
>=20
> Hi Kirill,
>=20
> I see your and Andew's hot discussion in v4 resend thread.
>=20
> "I also tried another scenario: usemem -n16 100M -r 1000. It creates
> real memory pressure - no easy reclaimable memory. This time
> callback called with nr_to_scan > 0 and we freed hzp. "
>=20
> What's "usemem"? Is it a tool and how to get it?

http://www.spinics.net/lists/linux-mm/attachments/gtarazbJaHPaAT.gtar

> It's hard for me to
> find nr_to_scan > 0 in every callset, how can nr_to_scan > 0 in your
> scenario?

shrink_slab() calls the callback with nr_to_scan > 0 if system is under
pressure -- look for do_shrinker_shrink().

--=20
 Kirill A. Shutemov

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