Messages in this thread | | | Date | Mon, 19 Nov 2012 19:29:23 +0800 | From | Jaegeuk Hanse <> | Subject | Re: [PATCH v6 10/12] thp: implement refcounting for huge zero page |
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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@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.
Oh, I see. Thanks! :-)
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