LinuxHugePages配置与Oracle性能关系说明

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Linux HugePages 配置 与 Oracle 性能关系说明

一. HugePages 说明

1.1 HugePages 介绍

HugePages is afeature integrated into the Linux kernel with release 2.6. This featurebasically provides the alternative to the 4K page size (16Kfor IA64) providing bigger pages.

关于HugePages，有一些相关的专业术语，具体如下：

（1） Page Table: A page table is thedata structure of a virtual memory system in an operating system to store themapping between virtual addresses and physical addresses. This means that on avirtual memory system, the memory is accessed by first accessing a page tableand then accessing the actual memory location implicitly.

--Page Table 是操作系统上的虚拟内存系统的数据结构，其用来存储虚拟内存地址和物理内存地址之间的映射关系。这就意味着在虚拟内存系统上，，我们访问内存时，是先访问Page Table，然后根据Page Table 中的映射关系，隐式的转移到物理的内存位置。

（2） TLB: A Translation LookasideBuffer (TLB) is a buffer (or cache) in a CPU that contains parts ofthe page table. This is a fixed size buffer being used to do virtual addresstranslation faster.

--TLB(Translation Lookaside Buffer) 是CPU 中的一块buffer 或者cache，其大小的固定的， TLB中包含了部分Page Table，用来快速进行虚拟地址的转换。

（3） hugetlb: This is an entryin the TLB that points to a HugePage (a large/big page larger than regular 4Kand predefined in size). HugePages are implemented via hugetlb entries, i.e. wecan say that a HugePage is handled by a "hugetlb page entry". The'hugetlb" term is also (and mostly) used synonymously with a HugePage(See Note261889.1). In this document the term "HugePage" is going to beused but keep in mind that mostly "hugetlb" refers to the sameconcept.

--hugetlb 是TLB中的一个entry，其指向HugePage（大于4k或预定义的一个large page）。 HugePage 通过hugetlb entries来实现，我们也可以说HugePage 是hugetlb page entry的一个句柄。 在MOS 文档：Note 261889.1中，二者是几乎是相同的概念。

（4） hugetlbfs: This is a newin-memory filesystem like tmpfs and is presented by 2.6 kernel. Pages allocatedon hugetlbfs type filesystem are allocated in HugePages.

--hugetlbfs 是2.6内核中提出的一个新的in-memory filesystem，就像tmpfs一样。

1.2 常见的错误概念

WRONG: HugePages is a method to be able to use large SGA on 32-bit VLM systems

RIGHT: HugePages is a method to have larger pages where it is useful for working with very large memory. It is both useful in 32- and 64-bit configurations

WRONG: HugePages cannot be used without USE_INDIRECT_DATA_BUFFERS

RIGHT: HugePages can be used without indirect buffers. 64-bit systems does not need to use indirect buffers to have a large buffer cache for the RDBMS instance and HugePages can be used there too.

WRONG: hugetlbfs means hugetlb

RIGHT: hugetlbfs is a filesystem type **BUT** hugetlb is the mechanism employed in the back where hugetlb can be employed WITHOUT hugetlbfs

WRONG: hugetlbfs means hugepages

RIGHT: hugetlbfs is a filesystem type **BUT** HugePages is the mechanism employed in the back (synonymously with hugetlb) where HugePages can be employed WITHOUT hugetlbfs.

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