lkml.org 
[lkml]   [2014]   [Jun]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] mm, thp: move invariant bug check out of loop in __split_huge_page_map
On Mon, Jun 16, 2014 at 11:45:42PM -0400, Waiman Long wrote:
> On 06/16/2014 04:59 PM, Kirill A. Shutemov wrote:
> >On Mon, Jun 16, 2014 at 11:49:34PM +0300, Kirill A. Shutemov wrote:
> >>On Mon, Jun 16, 2014 at 03:35:48PM -0400, Waiman Long wrote:
> >>>In the __split_huge_page_map() function, the check for
> >>>page_mapcount(page) is invariant within the for loop. Because of the
> >>>fact that the macro is implemented using atomic_read(), the redundant
> >>>check cannot be optimized away by the compiler leading to unnecessary
> >>>read to the page structure.
> >And atomic_read() is *not* atomic operation. It's implemented as
> >dereferencing though cast to volatile, which suppress compiler
> >optimization, but doesn't affect what CPU can do with the variable.
> >
> >So I doubt difference will be measurable anywhere.
> >
>
> Because it is treated as an volatile object, the compiler will have to
> reread the value of the relevant page structure field in every iteration of
> the loop (512 for x86) when pmd_write(*pmd) is true. I saw some slight
> improvement (about 2%) of a microbench that I wrote to break up 1000 THPs
> with 1000 forked processes.

Then bring patch with performance data.

--
Kirill A. Shutemov


\
 
 \ /
  Last update: 2014-06-17 10:01    [W:0.131 / U:0.180 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site