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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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@linux.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.

-- 
 Kirill A. Shutemov

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