封闭transparent_hugepage

关闭transparent_hugepage

好多地方都提示关闭这个，MongoDB 3.0提示

2015-10-21T09:03:24.256+0800 I CONTROL  [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/enabled is 'always'.
2015-10-21T09:03:24.256+0800 I CONTROL  [initandlisten] **        We suggest setting it to 'never'
2015-10-21T09:03:24.256+0800 I CONTROL  [initandlisten] 
2015-10-21T09:03:24.256+0800 I CONTROL  [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is 'always'.
2015-10-21T09:03:24.256+0800 I CONTROL  [initandlisten] **        We suggest setting it to 'never'

sponge安装文档要求关闭，mapr也需要关闭，oracle也要求关闭，HDFS会因为这个性能严重受影响。

究竟是何方妖孽？

看看oracle网站如何说：

Starting from RHEL6/OL6, Transparent HugePages are implemented and enabled by default. They are meant to improve memory management by allowing HugePages to be allocated dynamically by the "khugepaged" kernel thread, rather than at boot time like conventional HugePages. That sounds like a good idea, but unfortunately Transparent HugePages don't play well with Oracle databases and are associated with node reboots in RAC installations and performance problems on both single instance and RAC installations. As a result Oracle recommends disabling Transparent HugePages on all servers running Oracle databases, as described in this MOS note.

就是允许hugepage可以动态分配，而不是系统启动时预先分配，看上去对内存消耗很大的服务都不喜欢它。感觉这是一个lazy loading的设计思想。

如何关闭呢。有几种方法，centos上的方法是

echo never > /sys/kernel/mm/redhat_transparent_hugepage/enabled

永久关闭，就将上面的命令写入到/etc/rc.local文件中

ubuntu上的方法是(针对mongodb的需求)

echo never > /sys/kernel/mm/transparent_hugepage/enabled
echo never> /sys/kernel/mm/transparent_hugepage/defrag

也放到/etc/rc.local文件中作为永久关闭。

写的好点的命令如下：

if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
   echo never > /sys/kernel/mm/transparent_hugepage/enabled
fi
if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
   echo never > /sys/kernel/mm/transparent_hugepage/defrag
fi

注意，各系统可能文件路径（名称）有点区别。