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Subject[PATCH v2 2/4] mm: make the threshold of enabling THP configurable
Date
Don't hard-code 512M as the threshold in kernel, make it configruable,
and set 512M by default.

And print info when THP is disabled automatically on small systems.

V2: Add more description in help messages, correct some typos,
print the mini threshold too.

Cc: dave@linux.vnet.ibm.com
Signed-off-by: WANG Cong <amwang@redhat.com>
---
mm/Kconfig | 20 ++++++++++++++++++++
mm/huge_memory.c | 7 ++++++-
2 files changed, 26 insertions(+), 1 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 8ca47a5..f1b3175f 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -340,6 +340,26 @@ choice
benefit.
endchoice

+config TRANSPARENT_HUGEPAGE_THRESHOLD
+ depends on TRANSPARENT_HUGEPAGE
+ int "The minimal threshold of enabling Transparent Hugepage"
+ range 512 8192
+ default "512"
+ help
+ The threshold of enabling Transparent Huagepage automatically,
+ in Mbytes, below this value Transparent Hugepage will be disabled
+ by default during boot. You can still enable it via /sys after
+ boot.
+
+ Transparent hugepages are created by moving other pages out of
+ the way to create large, contiguous swaths of free memory.
+ However, some memory on a system can not be easily moved. It is
+ likely on small systems that this unmovable memory will occupy a
+ large portion of total memory, which makes even attempting to
+ create transparent hugepages very expensive.
+
+ If you are unsure, set this to the smallest possible value.
+
#
# UP and nommu archs use km based percpu allocator
#
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e4a4f2b..126c96b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -546,8 +546,13 @@ static int __init hugepage_init(void)
* where the extra memory used could hurt more than TLB overhead
* is likely to save. The admin can still enable it through /sys.
*/
- if (totalram_pages < (512 << (20 - PAGE_SHIFT)))
+ if (totalram_pages < (CONFIG_TRANSPARENT_HUGEPAGE_THRESHOLD
+ << (20 - PAGE_SHIFT))) {
+ printk(KERN_INFO "THP: disabled automatically"
+ "with less than %dMB of RAM\n",
+ CONFIG_TRANSPARENT_HUGEPAGE_THRESHOLD);
transparent_hugepage_flags = 0;
+ }

start_khugepaged();

--
1.7.4.4


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