linux-kernel - Re: [PATCH v6 10/12] thp: implement refcounting for huge zero page

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Date:	Mon, 19 Nov 2012 11:56:15 +0200
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v6 10/12] thp: implement refcounting for huge zero page

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@...ux.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.
> 
> Hi Kirill,
> 
> I see your and Andew's hot discussion in v4 resend thread.
> 
> "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. "
> 
> 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().

-- 
 Kirill A. Shutemov

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