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

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Message-ID: <50AA1813.80408@gmail.com>
Date:	Mon, 19 Nov 2012 19:29:23 +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 07:09 PM, Kirill A. Shutemov wrote:
> On Mon, Nov 19, 2012 at 07:02:22PM +0800, Jaegeuk Hanse wrote:
>> 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?
> The only what I see is slabs_scanned in vmstat.

Oh, I see. Thanks! :-)


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