From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
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
Subject: Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page
Date: Fri, 19 Oct 2012 02:59:41 +0300 [thread overview]
Message-ID: <20121018235941.GA32397@shutemov.name> (raw)
In-Reply-To: <20121018164502.b32791e7.akpm@linux-foundation.org>
On Thu, Oct 18, 2012 at 04:45:02PM -0700, Andrew Morton wrote:
> On Mon, 15 Oct 2012 09:00:59 +0300
> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
>
> > 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.
>
> I'd like more details on this please. The cost of freeing then
> reinstantiating that page is tremendous, because it has to be zeroed
> out again. If there is any way at all in which the kernel can be made
> to enter a high-frequency free/reinstantiate pattern then I expect the
> effects would be quite bad.
>
> Do we have sufficient mechanisms in there to prevent this from
> happening in all cases? If so, what are they, because I'm not seeing
> them?
We only free huge zero page in shrinker callback if nobody in the system
uses it. Never on put_huge_zero_page(). Shrinker runs only under memory
pressure or if user asks (drop_caches).
Do you think we need an additional protection mechanism?
>
> > 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.
--
Kirill A. Shutemov
next prev parent reply other threads:[~2012-10-18 23:58 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-15 6:00 [PATCH v4 00/10, REBASED] Introduce huge zero page Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 01/10] thp: huge zero page: basic preparation Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 02/10] thp: zap_huge_pmd(): zap huge zero pmd Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 03/10] thp: copy_huge_pmd(): copy huge zero page Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 04/10] thp: do_huge_pmd_wp_page(): handle " Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 05/10] thp: change_huge_pmd(): keep huge zero page write-protected Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 06/10] thp: change split_huge_page_pmd() interface Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 07/10] thp: implement splitting pmd for huge zero page Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 08/10] thp: setup huge zero page on non-write page fault Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 09/10] thp: lazy huge zero page allocation Kirill A. Shutemov
2012-10-15 6:00 ` [PATCH v4 10/10] thp: implement refcounting for huge zero page Kirill A. Shutemov
2012-10-18 23:45 ` Andrew Morton
2012-10-18 23:59 ` Kirill A. Shutemov [this message]
2012-10-23 6:35 ` Kirill A. Shutemov
2012-10-23 6:43 ` Andrew Morton
2012-10-23 7:00 ` Kirill A. Shutemov
2012-10-23 22:59 ` Andrew Morton
2012-10-23 23:38 ` Kirill A. Shutemov
2012-10-24 19:22 ` Andrew Morton
2012-10-24 19:45 ` Kirill A. Shutemov
2012-10-24 20:25 ` Andrew Morton
2012-10-24 20:33 ` Kirill A. Shutemov
2012-10-24 20:44 ` Andi Kleen
2012-10-25 20:49 ` Kirill A. Shutemov
2012-10-25 21:05 ` Andrew Morton
2012-10-25 21:22 ` Kirill A. Shutemov
2012-10-25 21:37 ` Andrew Morton
2012-10-25 22:10 ` Kirill A. Shutemov
2012-10-16 9:53 ` [PATCH v4 00/10, REBASED] Introduce " Ni zhan Chen
2012-10-16 10:54 ` Kirill A. Shutemov
2012-10-16 11:13 ` Ni zhan Chen
2012-10-16 11:28 ` Kirill A. Shutemov
2012-10-16 11:37 ` Ni zhan Chen
2012-10-26 15:14 ` [PATCH] thp, vmstat: implement HZP_ALLOC and HZP_ALLOC_FAILED events Kirill A. Shutemov
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