Re: [RFC PATCH] mm: thp: Add per-mm_struct flag to control THP

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Jan 10, 2014 at 01:55:18PM -0600, Alex Thorlton wrote:
> This patch adds an mm flag (MMF_THP_DISABLE) to disable transparent
> hugepages using prctl.  It is based on my original patch to add a
> per-task_struct flag to disable THP:
> 
> v1 - https://lkml.org/lkml/2013/8/2/671
> v2 - https://lkml.org/lkml/2013/8/2/703
> 
> After looking at alternate methods of modifying how THPs are handed out,
> it sounds like people might be more in favor of this type of approach,
> so I'm re-introducing the patch.
> 
> It seemed that everyone was in favor of moving this control over to the
> mm_struct, if it is to be implemented.  That's the only major change
> here, aside from the added ability to both set and clear the flag from
> prctl.
> 
> The main motivation behind this patch is to provide a way to disable THP
> for jobs where the code cannot be modified and using a malloc hook with
> madvise is not an option (i.e. statically allocated data).  This patch
> allows us to do just that, without affecting other jobs running on the
> system.
> 
> Here are some results showing the improvement that my test case gets
> when the MMF_THP_DISABLE flag is clear vs. set:
> 
> MMF_THP_DISABLE clear:
> 
> # perf stat -a -r 3 ./prctl_wrapper_mm 0 ./thp_pthread -C 0 -m 0 -c 512 -b 256g
> 
>  Performance counter stats for './prctl_wrapper_mm 0 ./thp_pthread -C 0 -m 0 -c 512 -b 256g' (3 runs):
> 
>   267694862.049279 task-clock                #  641.100 CPUs utilized            ( +-  0.23% ) [100.00%]
>            908,846 context-switches          #    0.000 M/sec                    ( +-  0.23% ) [100.00%]
>                874 CPU-migrations            #    0.000 M/sec                    ( +-  4.01% ) [100.00%]
>            131,966 page-faults               #    0.000 M/sec                    ( +-  2.75% )
> 351,127,909,744,906 cycles                    #    1.312 GHz                      ( +-  0.27% ) [100.00%]
> 523,537,415,562,692 stalled-cycles-frontend   #  149.10% frontend cycles idle     ( +-  0.26% ) [100.00%]
> 392,400,753,609,156 stalled-cycles-backend    #  111.75% backend  cycles idle     ( +-  0.29% ) [100.00%]
> 147,467,956,557,895 instructions              #    0.42  insns per cycle
>                                              #    3.55  stalled cycles per insn  ( +-  0.09% ) [100.00%]
> 26,922,737,309,021 branches                  #  100.572 M/sec                    ( +-  0.24% ) [100.00%]
>      1,308,714,545 branch-misses             #    0.00% of all branches          ( +-  0.18% )
> 
>      417.555688399 seconds time elapsed                                          ( +-  0.23% )
> 
> 
> MMF_THP_DISABLE set:
> 
> # perf stat -a -r 3 ./prctl_wrapper_mm 1 ./thp_pthread -C 0 -m 0 -c 512 -b 256g
> 
>  Performance counter stats for './prctl_wrapper_mm 1 ./thp_pthread -C 0 -m 0 -c 512 -b 256g' (3 runs):
> 
>   141674994.160138 task-clock                #  642.107 CPUs utilized            ( +-  0.23% ) [100.00%]
>          1,190,415 context-switches          #    0.000 M/sec                    ( +- 42.87% ) [100.00%]
>                688 CPU-migrations            #    0.000 M/sec                    ( +-  2.47% ) [100.00%]
>         62,394,646 page-faults               #    0.000 M/sec                    ( +-  0.00% )
> 156,748,225,096,919 cycles                    #    1.106 GHz                      ( +-  0.20% ) [100.00%]
> 211,440,354,290,433 stalled-cycles-frontend   #  134.89% frontend cycles idle     ( +-  0.40% ) [100.00%]
> 114,304,536,881,102 stalled-cycles-backend    #   72.92% backend  cycles idle     ( +-  0.88% ) [100.00%]
> 179,939,084,230,732 instructions              #    1.15  insns per cycle
>                                              #    1.18  stalled cycles per insn  ( +-  0.26% ) [100.00%]
> 26,659,099,949,509 branches                  #  188.171 M/sec                    ( +-  0.72% ) [100.00%]
>        762,772,361 branch-misses             #    0.00% of all branches          ( +-  0.97% )
> 
>      220.640905073 seconds time elapsed                                          ( +-  0.23% )
> 
> As you can see, this particular test gets about a 2x performance boost
> when THP is turned off. 

Do you know what cause the difference? I prefer to fix THP instead of
adding new knob to disable it.

-- 
 Kirill A. Shutemov

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