[RFC 00/11] THP swap: Delay splitting THP during swapping out

["Huang, Ying" <ying.huang@intel.com>]

From: Huang Ying <ying.huang@intel.com>

This patchset is based on 8/4 head of mmotm/master.

This is the first step for Transparent Huge Page (THP) swap support.
The plan is to delaying splitting THP step by step and avoid splitting
THP finally during THP swapping out and swapping in.

The advantages of THP swap support are:

- Batch swap operations for THP to reduce lock acquiring/releasing,
  including allocating/freeing swap space, adding/deleting to/from swap
  cache, and writing/reading swap space, etc.

- THP swap space read/write will be 2M sequence IO.  It is particularly
  helpful for swap read, which usually are 4k random IO.

- It will help memory fragmentation, especially when THP is heavily used
  by the applications.  2M continuous pages will be free up after THP
  swapping out.

As the first step, in this patchset, the splitting huge page is
delayed from almost the first step of swapping out to after allocating
the swap space for THP and adding the THP into swap cache.  This will
reduce lock acquiring/releasing for locks used for swap space and swap
cache management.

With the patchset, the swap out bandwidth improved 12.1% in
vm-scalability swap-w-seq test case with 16 processes on a Xeon E5 v3
system.  To test sequence swap out, the test case uses 16 processes
sequentially allocate and write to anonymous pages until RAM and part of
the swap device is used up.

The detailed compare result is as follow,

base             base+patchset
---------------- -------------------------- 
         %stddev     %change         %stddev
             \          |                \  
   1118821 A+-  0%     +12.1%    1254241 A+-  1%  vmstat.swap.so
   2460636 A+-  1%     +10.6%    2720983 A+-  1%  vm-scalability.throughput
    308.79 A+-  1%      -7.9%     284.53 A+-  1%  vm-scalability.time.elapsed_time
      1639 A+-  4%    +232.3%       5446 A+-  1%  meminfo.SwapCached
      0.70 A+-  3%      +8.7%       0.77 A+-  5%  perf-stat.ipc
      9.82 A+-  8%     -31.6%       6.72 A+-  2%  perf-profile.cycles-pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_swap.shrink_page_list

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