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

[Jaegeuk Hanse]

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@xxxxxxxxxxxxxxx>
> > > > > > >
> > > > > > > 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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