THPTransparentHugePages相关知识与关闭

最近遇到个LINUX系统内存比较大，未开 HugePages，业务有变化导致ORACLE连接数剧增至上千个，PageTables达到上百G，导致内存不足系统HANG住的案例。 因此需要开启 HugePages，操作系统是OEL6以上的。LINUX的以下版本:Red Hat Enterprise Linux 6, SUSE Linux

最近遇到个LINUX系统内存比较大，未开 HugePages，业务有变化导致ORACLE连接数剧增至上千个，PageTables达到上百G，导致内存不足系统HANG住的案例。

因此需要开启 HugePages，操作系统是OEL6以上的。LINUX的以下版本:Red Hat Enterprise Linux 6, SUSE Linux Enterprise Server 11, and Oracle Linux 6 with earlier releases of Oracle Linux Unbreakable Enterprise Kernel 2 (UEK2) kernels. Transparent HugePages是默认开启的，因此在这些版本的LINUX系统中，ORACLE强烈建议开启HugePages需要关闭Transparent HugePages。

在以下版本的LINUX中Transparent HugePages默认是关闭--从内核移除的。

Transparent HugePages memory is disabled in later releases of Oracle Linux UEK2 kernels.

验证方法是：如果以下文件不存在，则是THP已经从内核中移除。

/sys/kernel/mm/transparent_hugepage or <code dir="ltr">/sys/kernel/mm/redhat_transparent_hugepage

<code dir="ltr">C.3 Disabling Transparent HugePages

<code dir="ltr">Transparent HugePages memory is enabled by default with Red Hat Enterprise Linux 6, SUSE Linux Enterprise Server 11, and Oracle Linux 6 with earlier releases of Oracle Linux Unbreakable Enterprise Kernel 2 (UEK2) kernels. <strong>Transparent HugePages memory is disabled in later releases of Oracle Linux UEK2 kernels.</strong>

<code dir="ltr">Transparent HugePages can cause memory allocation delays during runtime. To avoid performance issues, <strong>Oracle recommends that you disable Transparent HugePages on all Oracle Database servers. </strong>Oracle recommends that you instead use standard HugePages for enhanced performance.

<code dir="ltr">Transparent HugePages memory differs from standard HugePages memory because the kernel khugepaged thread allocates memory dynamically during runtime. Standard HugePages memory is pre-allocated at startup, and does not change during runtime.

<code dir="ltr">o check if Transparent HugePages memory is enabled, run one of the following commands as the <code dir="ltr">root user:

<code dir="ltr"><code dir="ltr">Red Hat Enterprise Linux kernels:

<code dir="ltr"><code dir="ltr"><code dir="ltr"># cat /sys/kernel/mm/redhat_transparent_hugepage/enabled
</code></code></code>

<code dir="ltr"><code dir="ltr">Other kernels:

<code dir="ltr"><code dir="ltr"><code dir="ltr"># cat /sys/kernel/mm/transparent_hugepage/enabled
</code></code></code>

<code dir="ltr"><code dir="ltr">The following is a sample output that shows Transparent HugePages memory being used as the <code dir="ltr">[always] flag is enabled.

<code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr">[always] never
</code></code></code></code>

<code dir="ltr"><code dir="ltr"><code dir="ltr"><strong>Note:</strong>

If Transparent HugePages is removed from the kernel, then the /sys/kernel/mm/transparent_hugepage or <code dir="ltr">/sys/kernel/mm/redhat_transparent_hugepage files do not exist.<code dir="ltr">

<code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr">To disable Transparent HugePages, perform the following steps:

<code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr">Add the following entry to the kernel boot line in the <code dir="ltr">/etc/grub.conf file:

<code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr">transparent_hugepage=never
</code></code></code></code></code></code></code>

<code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr">For example:

<code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr">title Oracle Linux Server (2.6.32-300.25.1.el6uek.x86_64)
        root (hd0,0)
        kernel /vmlinuz-2.6.32-300.25.1.el6uek.x86_64 ro root=LABEL=/   transparent_hugepage=never
        initrd /initramfs-2.6.32-300.25.1.el6uek.x86_64.img
</code></code></code></code></code></code></code>

<code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr"><code dir="ltr">Restart the system to make the changes permanent.