hugepage compaction causes performance drop

[Aaron Lu <aaron.lu@intel.com>]

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Hi,

One vm related test case run by LKP on a Haswell EP with 128GiB memory
showed that compaction code would cause performance drop about 30%. To
illustrate the problem, I've simplified the test with a program called
usemem(see attached). The test goes like this:
1 Boot up the server;
2 modprobe scsi_debug(a module that could use memory as SCSI device),
  dev_size set to 4/5 free memory, i.e. about 100GiB. Use it as swap.
3 run the usemem test, which use mmap to map a MAP_PRIVATE | MAP_ANON
  region with the size set to 3/4 of (remaining_free_memory + swap), and
  then write to that region sequentially to trigger page fault and swap
  out.

The above test runs with two configs regarding the below two sysfs files:
/sys/kernel/mm/transparent_hugepage/enabled
/sys/kernel/mm/transparent_hugepage/defrag
1 transparent hugepage and defrag are both set to always, let's call it
  always-always case;
2 transparent hugepage is set to always while defrag is set to never,
  let's call it always-never case.

The output from the always-always case is:
Setting up swapspace version 1, size = 104627196 KiB
no label, UUID=aafa53ae-af9e-46c9-acb9-8b3d4f57f610
cmdline: /lkp/aaron/src/bin/usemem 99994672128
99994672128 transferred in 95 seconds, throughput: 1003 MB/s

And the output from the always-never case is:
etting up swapspace version 1, size = 104629244 KiB
no label, UUID=60563c82-d1c6-4d86-b9fa-b52f208097e9
cmdline: /lkp/aaron/src/bin/usemem 99995965440
99995965440 transferred in 67 seconds, throughput: 1423 MB/s

The vmstat and perf-profile are also attached, please let me know if you
need any more information, thanks.

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