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

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Date:	Mon, 19 Nov 2012 18:20:01 +0800
From:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 11/19/2012 05:56 PM, Kirill A. Shutemov wrote:
> 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

Thanks for your response.  But how to use it, I even can't compile the 
files.

# ./case-lru-file-mmap-read
./case-lru-file-mmap-read: line 3: hw_vars: No such file or directory
./case-lru-file-mmap-read: line 7: 10 * mem / nr_cpu: division by 0 
(error token is "nr_cpu")

# gcc usemem.c -o usemem
/tmp/ccFkIDWk.o: In function `do_task':
usemem.c:(.text+0x9f2): undefined reference to `pthread_create'
usemem.c:(.text+0xa44): undefined reference to `pthread_join'
collect2: ld returned 1 exit status

>
>> 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().

Why Andrew's example(dd if=/fast-disk/large-file) doesn't call this 
path? I think it also can add memory pressure, where I miss?


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