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

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

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@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.
> 
> 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.

-- 
 Kirill A. Shutemov

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