Enable defer+madvise defragmention for THP by default

[ventureoo]

This should improve the performance of applications that use the tcmalloc memory allocator, which is used by Chromium/Electron and Valve's native games, among others.
Ref: https://github.com/google/tcmalloc/blob/master/docs/tuning.md#system-level-optimizations

[Boria138]

I guess the settings could be a little more customized

w- /sys/kernel/mm/transparent_hugepage/enabled - - - - - madvise
w- /sys/kernel/mm/transparent_hugepage/khugepaged/defrag - - - - - - 1
w- /sys/kernel/mm/transparent_hugepage/defrag - - - - - defer+madvise

[Boria138]

@ventureoo
Failed to write file "/sys/kernel/mm/transparent_hugepage/defrag": Invalid argument

[ptr1337]

@ventureoo Failed to write file "/sys/kernel/mm/transparent_hugepage/defrag": Invalid argument

b26ae7b

Should fix it

[Torston420]

Kernel docs imply that setting defrag to 'madvise' would perform the stalling behavior seen in 'always', wouldn't 'defer' be more advantageous than 'defer+madvise' for interactivity?

Update: Team Fortress 2 does not seem to allocate any hugepages at all if /transparent_hugepage/enabled is set to 'madvise' instead of 'always', this appears to be a global issue with TCMalloc: google/tcmalloc#85

[ventureoo]

Update: Team Fortress 2 does not seem to allocate any hugepages at all if /transparent_hugepage/enabled is set to 'madvise' instead of 'always', this appears to be a global issue with TCMalloc: google/tcmalloc#85

Thanks for notice. I'll remove madvise for transparent_hugepage/enabled in that case.

[ventureoo]

@ptr1337 I've signed the last commit. Are there any objections to this PR?