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

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Date:	Mon, 19 Nov 2012 19:02:22 +0800
From:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	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, David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v6 10/12] thp: implement refcounting for huge zero page

On 11/19/2012 06:23 PM, Kirill A. Shutemov wrote:
> On Mon, Nov 19, 2012 at 06:20:01PM +0800, Jaegeuk Hanse wrote:
>> 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
> -lpthread
>
>> /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?
> dd if=large-file only fills pagecache -- easy reclaimable memory.
> Pagecache will be dropped first, before shrinking slabs.

How could I confirm page reclaim working hard and slabs are reclaimed at 
this time?


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