关于大内存页面 transparent_hugepage

Transparent Huge Pages (THP) are enabled by default in RHEL 6 for all applications. The kernel attempts to allocate hugepages whenever possible and any Linux process will receive 2MB pages if the mmap region is 2MB naturally aligned. The main kernel address space itself is mapped with hugepages, reducing TLB pressure from kernel code. For general information on Hugepages, see: What are Huge Pages and what are the advantages of using them?

The kernel will always attempt to satisfy a memory allocation using hugepages. If no hugepages are available (due to non availability of physically continuous memory for example) the kernel will fall back to the regular 4KB pages. THP are also swappable (unlike hugetlbfs). This is achieved by breaking the huge page to smaller 4KB pages, which are then swapped out normally.

But to use hugepages effectively, the kernel must find physically continuous areas of memory big enough to satisfy the request, and also properly aligned. For this, a khugepaged kernel thread has been added. This thread will occasionally attempt to substitute smaller pages being used currently with a hugepage allocation, thus maximizing THP usage.

In userland, no modifications to the applications are necessary (hence transparent). But there are ways to optimize its use. For applications that want to use hugepages, use of posix_memalign() can also help ensure that large allocations are aligned to huge page (2MB) boundaries.

Also, THP is only enabled for anonymous memory regions. There are plans to add support for tmpfs and page cache. THP tunables are found in the /sys tree under /sys/kernel/mm/redhat_transparent_hugepage.






从redhat 6(centos,sl,ol)开始,操作系统开始支持 Huge Pages,也就是大页。

简单来说, Huge Pages就是大小为2M到1GB的内存page,主要用于管理数千兆的内存,比如1GB的page对于1TB的内存来说是相对比较合适的。

THP(Transparent Huge Pages)是一个使管理Huge Pages自动化的抽象层。


操作系统后台有一个叫做khugepaged的进程,它会一直扫描所有进程占用的内存,在可能的情况下会把4kpage交换为Huge Pages,在这个过程中,对于操作的内存的各种分配活动都需要各种内存锁,直接影响程序的内存访问性能,并且,这个过程对于应用是透明的,在应用层面不可控制,对于专门为4k page优化的程序来说,可能会造成随机的性能下降现象。


1:从RedHat 6, OEL 6, SLES 11 and UEK2 kernels 开始,系统缺省会启用 Transparent HugePages :用来提高内存管理的性能透明大页(Transparent HugePages )和之前版本中的大页功能上类似。主要的区别是:Transparent HugePages 可以实时配置,不需要重启才能生效配置;

2:Transparent Huge Pages在32位的RHEL 6中是不支持的。

Transparent Huge Pages are not available on the 32-bit version of RHEL 6.

3: ORACLE官方不建议我们使用RedHat 6, OEL 6, SLES 11 and UEK2 kernels 时的开启透明大页(Transparent HugePages ), 因为透明大页(Transparent HugePages ) 存在一些问题:

        1.在RAC环境下 透明大页(Transparent HugePages )会导致异常节点重启,和性能问题;

        2.在单机环境中,透明大页(Transparent HugePages ) 也会导致一些异常的性能问题;

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. Transparent HugePages memory is disabled in later releases of Oracle Linux UEK2 kernels.Transparent HugePages can cause memory allocation delays during runtime. To avoid performance issues, Oracle recommends that you disable Transparent HugePages on all Oracle Database servers. Oracle recommends that you instead use standard HugePages for enhanced performance.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.

Starting with RedHat 6, OEL 6, SLES 11 and UEK2 kernels, Transparent HugePages are implemented and enabled (default) in an attempt to improve the memory management. Transparent HugePages are similar to the HugePages that have been available in previous Linux releases. The main difference is that the Transparent HugePages are set up dynamically at run time by the khugepaged thread in kernel while the regular HugePages had to be preallocated at the boot up time. Because Transparent HugePages are known to cause unexpected node reboots and performance problems with RAC, Oracle strongly advises to disable the use of Transparent HugePages. In addition, Transparent Hugepages may cause problems even in a single-instance database environment with unexpected performance problems or delays. As such, Oracle recommends disabling Transparent HugePages on all Database servers running Oracle.

4:安装Vertica Analytic Database时也必须关闭透明大页功能。





除了手动修改运行时参数之外,还可以修改 /etc/grub.conf 里内核的启动参数,追加“transparent_hugepage=never”(此选项只对 /sys/kernel/mm/redhat_transparent_hugepage/enabled 有效)。

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