linux-kernel - Re: [PATCH v5 10/11] thp: implement refcounting for huge zero page

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Date:	Thu, 15 Nov 2012 11:50:20 +0200
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	David Rientjes <rientjes@...gle.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>
Subject: Re: [PATCH v5 10/11] thp: implement refcounting for huge zero page

On Wed, Nov 14, 2012 at 03:40:37PM -0800, David Rientjes wrote:
> On Wed, 7 Nov 2012, 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.
> > 
> 
> Eek, this is disappointing that we need to check a refcount before 
> referencing the zero huge page

No we don't. It's parallel *read* page fault benchmark meaning we
map/unmap huge zero page all the time. So it's pure synthetic test to show
refcounting overhead.

If we see only 1% overhead on the synthetic test we will not see it in
real world workloads.

> and it obviously shows in your benchmark 
> (which I consider 1% to be significant given the alternative is 2MB of 
> memory for a system where thp was enabled to be on).  I think it would be 
> much better to simply allocate and reference the zero huge page locklessly 
> when thp is enabled to be either "madvise" or "always", i.e. allocate it 
> when enabled.

-- 
 Kirill A. Shutemov

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