On Sun, Nov 18, 2012 at 02:23:44PM +0800, Jaegeuk Hanse wrote:On 11/16/2012 03:27 AM, Kirill A. Shutemov wrote:http://www.spinics.net/lists/linux-mm/attachments/gtarazbJaHPaAT.gtarFrom: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>Hi Kirill,
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.
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?
It's hard for me toshrink_slab() calls the callback with nr_to_scan > 0 if system is under
find nr_to_scan > 0 in every callset, how can nr_to_scan > 0 in your
scenario?
pressure -- look for do_shrinker_shrink().