Re: [PATCH, RFC 0/9] Introduce huge zero page

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

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On Fri, Aug 10, 2012 at 11:49:12AM +0800, Wanpeng Li wrote:
> On Thu, Aug 09, 2012 at 12:08:11PM +0300, Kirill A. Shutemov wrote:
> >From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> >
> >During testing I noticed big (up to 2.5 times) memory consumption overhead
> >on some workloads (e.g. ft.A from NPB) if THP is enabled.
> >
> >The main reason for that big difference is lacking zero page in THP case.
> >We have to allocate a real page on read page fault.
> >
> >A program to demonstrate the issue:
> >#include <assert.h>
> >#include <stdlib.h>
> >#include <unistd.h>
> >
> >#define MB 1024*1024
> >
> >int main(int argc, char **argv)
> >{
> >        char *p;
> >        int i;
> >
> >        posix_memalign((void **)&p, 2 * MB, 200 * MB);
> >        for (i = 0; i < 200 * MB; i+= 4096)
> >                assert(p[i] == 0);
> >        pause();
> >        return 0;
> >}
> >
> >With thp-never RSS is about 400k, but with thp-always it's 200M.
> >After the patcheset thp-always RSS is 400k too.
> >
> Hi Kirill, 
> 
> Thank you for your patchset, I have some questions to ask.
> 
> 1. In your patchset, if read page fault, the pmd will be populated by huge
> zero page, IIUC, assert(p[i] == 0) is a read operation, so why thp-always
> RSS is 400K ? You allocate 100 pages, why each cost 4K? I think the
> right overhead should be 2MB for the huge zero page instead of 400K, where
> I missing ?

400k comes not from the allocation, but from libc runtime. The test
program consumes about the same without any allocation at all.

Zero page is a global resource. System owns it. It's not accounted to any
process.

> 
> 2. If the user hope to allocate 200MB, total 100 pages needed. The codes 
> will allocate one 2MB huge zero page and populate to all associated pmd
> in your patchset logic. When the user attempt to write pages, wp will be 
> triggered, and if allocate huge page failed will fallback to
> do_huge_pmd_wp_zero_page_fallback in your patch logic, but you just
> create a new table and set pte around fault address to the newly
> allocated page, all other ptes set to normal zero page. In this scene 
> user only get one 4K page and all other zero pages, how the codes can
> cotinue to work? Why not fallback to allocate normal page even if not 
> physical continuous.

Since we allocate 4k page around the fault address the fault is handled.
Userspace can use it.

If the process will try to write to any other 4k page of this area a new
fault will be triggered and do_wp_page() will allocate a real page.

It's not reasonable to allocate all 4k pages in the fallback path. We can
postpone it until userspace will really want to use them. This way we reduce
memory pressure in fallback path.

> 3. In your patchset logic:
> "In fallback path we create a new table and set pte around fault address
> to the newly allocated page. All other ptes set to normal zero page."
> When these zero pages will be replaced by real pages and add memcg charge?

I guess I've answered the question above.

> Look forward to your detail response, thank you! :)

Thanks for your questions.

-- 
 Kirill A. Shutemov

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