Transparent Hugepage impact on memcpy

[Jianguo Wu <wujianguo@huawei.com>]

Hi all,

I tested memcpy with perf bench, and found that in prefault case, When Transparent Hugepage is on,
memcpy has worse performance.

When THP on is 3.672879 GB/Sec (with prefault), while THP off is 6.190187 GB/Sec (with prefault).

I think THP will improve performance, but the test result obviously not the case. 
Andrea mentioned THP cause "clear_page/copy_page less cache friendly" in
http://events.linuxfoundation.org/slides/2011/lfcs/lfcs2011_hpc_arcangeli.pdf.

I am not quite understand this, could you please give me some comments, Thanks!

I test in Linux-3.4-stable, and my machine info is:
Intel(R) Xeon(R) CPU           E5520  @ 2.27GHz

available: 2 nodes (0-1)
node 0 cpus: 0 1 2 3 8 9 10 11
node 0 size: 24567 MB
node 0 free: 23550 MB
node 1 cpus: 4 5 6 7 12 13 14 15
node 1 size: 24576 MB
node 1 free: 23767 MB
node distances:
node   0   1 
  0:  10  20 
  1:  20  10

Below is test result:
---with THP---
#cat /sys/kernel/mm/transparent_hugepage/enabled
[always] madvise never
#./perf bench mem memcpy -l 1gb -o
# Running mem/memcpy benchmark...
# Copying 1gb Bytes ...

       3.672879 GB/Sec (with prefault)

#./perf stat ...
Performance counter stats for './perf bench mem memcpy -l 1gb -o':

          35455940 cache-misses              #   53.504 % of all cache refs     [49.45%]
          66267785 cache-references                                             [49.78%]
              2409 page-faults                                                 
         450768651 dTLB-loads
                                                  [50.78%]
             24580 dTLB-misses
              #    0.01% of all dTLB cache hits  [51.01%]
        1338974202 dTLB-stores
                                                 [50.63%]
             77943 dTLB-misses
                                                 [50.24%]
         697404997 iTLB-loads
                                                  [49.77%]
               274 iTLB-misses
              #    0.00% of all iTLB cache hits  [49.30%]

       0.855041819 seconds time elapsed

---no THP---
#cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]

#./perf bench mem memcpy -l 1gb -o
# Running mem/memcpy benchmark...
# Copying 1gb Bytes ...

       6.190187 GB/Sec (with prefault)

#./perf stat ...
Performance counter stats for './perf bench mem memcpy -l 1gb -o':

          16920763 cache-misses              #   98.377 % of all cache refs     [50.01%]
          17200000 cache-references                                             [50.04%]
            524652 page-faults                                                 
         734365659 dTLB-loads
                                                  [50.04%]
           4986387 dTLB-misses
              #    0.68% of all dTLB cache hits  [50.04%]
        1013408298 dTLB-stores
                                                 [50.04%]
           8180817 dTLB-misses
                                                 [49.97%]
        1526642351 iTLB-loads
                                                  [50.41%]
                56 iTLB-misses
              #    0.00% of all iTLB cache hits  [50.21%]

       1.025425847 seconds time elapsed

Thanks,
Jianguo Wu.

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