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translate done: 20171102 What is huge pages in Linux.md
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What is huge pages in Linux?
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======
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Learn about huge pages in Linux. Understand what is hugepages, how to configure it, how to check current state and how to disable it.
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![Huge Pages in Linux][1]
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In this article, we will walk you though details about huge pages so that you will be able to answer : what is huge pages in Linux? How to enable/disable huge pages? How to determine huge page value? in Linux like RHEL6, RHEL7, Ubuntu etc.
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Lets start with Huge pages basics.
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### What is Huge page in Linux?
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Huge pages are helpful in virtual memory management in Linux system. As name suggests, they help is managing huge size pages in memory in addition to standard 4KB page size. You can define as huge as 1GB page size using huge pages.
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During system boot, you reserve your memory portion with huge pages for your application. This memory portion i.e. these memory occupied by huge pages is never swapped out of memory. It will stick there until you change your configuration. This increases application performance to great extent like Oracle database with pretty large memory requirement.
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### Why use huge page?
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In virtual memory management, kernel maintains table in which it has mapping of virtual memory address to physical address. For every page transaction, kernel needs to load related mapping. If you have small size pages then you need to load more numbers of pages resulting kernel to load more mapping tables. This decreases performance.
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Using huge pages, means you will need fewer pages. This decreases number of mapping tables to load by kernel to great extent. This increases your kernel level performance which ultimately benefits your application.
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In short, by enabling huge pages, system has fewer page tables to deal with and hence less overhead to access / maintain them!
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### How to configure huge pages?
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Run below command to check current huge pages details.
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```
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root@kerneltalks # grep Huge /proc/meminfo
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AnonHugePages: 0 kB
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HugePages_Total: 0
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HugePages_Free: 0
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HugePages_Rsvd: 0
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HugePages_Surp: 0
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Hugepagesize: 2048 kB
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```
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In above output you can see one page size is 2MB `Hugepagesize` and total of 0 pages on system `HugePages_Total`. This huge page size can be increased from 2MB to max 1GB.
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Run below script to get how much huge pages your system needs currently . Script is from Oracle and can be found.
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```
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#!/bin/bash
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#
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# hugepages_settings.sh
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#
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# Linux bash script to compute values for the
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# recommended HugePages/HugeTLB configuration
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#
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# Note: This script does calculation for all shared memory
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# segments available when the script is run, no matter it
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# is an Oracle RDBMS shared memory segment or not.
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# Check for the kernel version
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KERN=`uname -r | awk -F. '{ printf("%d.%d\n",$1,$2); }'`
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# Find out the HugePage size
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HPG_SZ=`grep Hugepagesize /proc/meminfo | awk {'print $2'}`
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# Start from 1 pages to be on the safe side and guarantee 1 free HugePage
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NUM_PG=1
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# Cumulative number of pages required to handle the running shared memory segments
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for SEG_BYTES in `ipcs -m | awk {'print $5'} | grep "[0-9][0-9]*"`
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do
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MIN_PG=`echo "$SEG_BYTES/($HPG_SZ*1024)" | bc -q`
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if [ $MIN_PG -gt 0 ]; then
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NUM_PG=`echo "$NUM_PG+$MIN_PG+1" | bc -q`
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fi
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done
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# Finish with results
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case $KERN in
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'2.4') HUGETLB_POOL=`echo "$NUM_PG*$HPG_SZ/1024" | bc -q`;
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echo "Recommended setting: vm.hugetlb_pool = $HUGETLB_POOL" ;;
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'2.6' | '3.8' | '3.10' | '4.1' ) echo "Recommended setting: vm.nr_hugepages = $NUM_PG" ;;
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*) echo "Unrecognized kernel version $KERN. Exiting." ;;
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esac
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# End
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```
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You can save it in `/tmp` as `hugepages_settings.sh` and then run it like below :
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```
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root@kerneltalks # sh /tmp/hugepages_settings.sh
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Recommended setting: vm.nr_hugepages = 124
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```
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Output will be similar to some number as shown in above sample output.
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This means your system needs 124 huge pages of 2MB each! If you have set 4MB as page size then output would have been 62. You got the point, right?
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### Configure hugepages in kernel
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Now last part is to configure above stated [kernel parameter][2] and reload it. Add below value in `/etc/sysctl.conf` and reload configuration by issuing `sysctl -p` command.
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```
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vm .nr_hugepages=126
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```
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Notice that we added 2 extra pages in kernel since we want to keep couple of pages spare than actual required number.
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Now, huge pages has been configured in kernel but to allow your application to use them you need to increase memory limits as well. New memory limit should be 126 pages x 2 MB each = 252 MB i.e. 258048 KB.
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You need to edit below settings in `/etc/security/limits.conf`
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```
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soft memlock 258048
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hard memlock 258048
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```
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Sometimes these settings are configured in app specific files like for Oracle DB its in `/etc/security/limits.d/99-grid-oracle-limits.conf`
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Thats it! You might want to restart your application to make use of these new huge pages.
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### How to disable hugepages?
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HugePages are generally enabled by default. Use below command to check current state of hugepages.
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```
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root@kerneltalks# cat /sys/kernel/mm/transparent_hugepage/enabled
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[always] madvise never
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```
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`[always]` flag in output shows that hugepages are enabled on system.
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For RedHat base systems file path is `/sys/kernel/mm/redhat_transparent_hugepage/enabled`
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If you want to disable huge pages then add `transparent_hugepage=never` at the end of `kernel` line in `/etc/grub.conf` and reboot the system.
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--------------------------------------------------------------------------------
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via: https://kerneltalks.com/services/what-is-huge-pages-in-linux/
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作者：[Shrikant Lavhate][a]
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译者：[lujun9972](https://github.com/lujun9972)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://kerneltalks.com
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[1]:https://c1.kerneltalks.com/wp-content/uploads/2017/11/hugepages-in-linux.png
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[2]:https://kerneltalks.com/linux/how-to-tune-kernel-parameters-in-linux/
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Linux 中的 huge pages 是个什么玩意？
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======
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学习 Linux 中的 huge pages( 巨大页)。理解什么是 hugepages，如何进行配置，如何查看当前状态以及如何禁用它。
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![Huge Pages in Linux][1]
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本文，我们会详细介绍 huge page，让你能够回答：Linux 中的 huge page 是什么玩意？在 RHEL6，RHEL7，Ubuntu 等 Linux 中，如何启用/禁用 huge pages？如何查看 huge page 的当前值？
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首先让我们从 Huge page 的基础知识开始讲起。
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### Linux 中的 Huge page 是个什么玩意？
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Huge pages 有助于 Linux 系统进行虚拟内存管理。顾名思义，除了标准的 4KB 大小的页面外，他们还能帮助管理内存中的巨大页面。使用 huge pages，你最大可以定义 1GB 的页面大小。
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在系统启动期间，huge pages 会为应用程序预留一部分内存。这部分内存，即被 huge pages 占用的这些存储器永远不会被交换出内存。它会一直保留其中除非你修改了配置。这会极大地提高像 Orcle 数据库这样的需要海量内存的应用程序的性能。
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### 为什么使用巨大的页？
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在虚拟内存管理中，内核维护一个将虚拟内存地址映射到物理地址的表，对于每个页面操作，内核都需要加载相关的映射标。如果你的内存页很小，那么你需要加载的页就会很多，导致内核加载更多的映射表。而这会降低性能。
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使用巨大的页，意味着所需要的页变少了。从而大大减少由内核加载的映射表的数量。这提高了内核级别的性能最终有利于应用程序的性能。
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简而言之，通过启用 huge pages，系统具只需要处理较少的页面映射表，从而减少访问/维护它们的开销！
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### 如何配置 huge pages？
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运行下面命令来查看当前 huge pages 的详细内容。
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```
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root@kerneltalks # grep Huge /proc/meminfo
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AnonHugePages: 0 kB
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HugePages_Total: 0
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HugePages_Free: 0
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HugePages_Rsvd: 0
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HugePages_Surp: 0
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Hugepagesize: 2048 kB
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```
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从上面输出可以看到，每个页的大小为 2MB(`Hugepagesize`) 并且系统中目前有 0 个页 (`HugePages_Total`)。这里巨大页的大小可以从 2MB 增加到 1GB。
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运行下面的脚本可以获取系统当前需要多少个巨大页。该脚本取之于 Oracle。
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```
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#!/bin/bash
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#
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# hugepages_settings.sh
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#
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# Linux bash script to compute values for the
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# recommended HugePages/HugeTLB configuration
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#
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# Note: This script does calculation for all shared memory
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# segments available when the script is run, no matter it
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# is an Oracle RDBMS shared memory segment or not.
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# Check for the kernel version
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KERN=`uname -r | awk -F. '{ printf("%d.%d\n",$1,$2); }'`
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# Find out the HugePage size
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HPG_SZ=`grep Hugepagesize /proc/meminfo | awk {'print $2'}`
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# Start from 1 pages to be on the safe side and guarantee 1 free HugePage
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NUM_PG=1
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# Cumulative number of pages required to handle the running shared memory segments
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for SEG_BYTES in `ipcs -m | awk {'print $5'} | grep "[0-9][0-9]*"`
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do
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MIN_PG=`echo "$SEG_BYTES/($HPG_SZ*1024)" | bc -q`
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if [ $MIN_PG -gt 0 ]; then
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NUM_PG=`echo "$NUM_PG+$MIN_PG+1" | bc -q`
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fi
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done
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# Finish with results
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case $KERN in
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'2.4') HUGETLB_POOL=`echo "$NUM_PG*$HPG_SZ/1024" | bc -q`;
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echo "Recommended setting: vm.hugetlb_pool = $HUGETLB_POOL" ;;
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'2.6' | '3.8' | '3.10' | '4.1' ) echo "Recommended setting: vm.nr_hugepages = $NUM_PG" ;;
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*) echo "Unrecognized kernel version $KERN. Exiting." ;;
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esac
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# End
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```
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将它以 `hugepages_settings.sh` 为名保存到 `/tmp` 中，然后运行之：
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```
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root@kerneltalks # sh /tmp/hugepages_settings.sh
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Recommended setting: vm.nr_hugepages = 124
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```
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输出如上结果，只是数字会有一些出入。
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这意味着，你系统需要 124 个每个 2MB 的巨大页！若你设置页面大小为 4MB，则结果就变成了 62。你明白了吧？
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### 配置内核中的 hugepages
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本文最后一部分内容是配置上面提到的 [内核参数 ][2] 然后重新加载。将下面内容添加到 `/etc/sysctl.conf` 中，然后输入 `sysctl -p` 命令重新加载配置。
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```
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vm .nr_hugepages=126
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```
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注意我们这里多加了两个额外的页，因为我们希望在实际需要的页面数量外多一些额外的空闲页。
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现在，内核已经配置好了，但是要让应用能够使用这些巨大页还需要提高内存的使用阀值。新的内存阀值应该为 126 个页 x 每个页 2 MB = 252 MB，也就是 258048 KB。
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你需要编辑 `/etc/security/limits.conf` 中的如下配置
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```
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soft memlock 258048
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hard memlock 258048
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```
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某些情况下，这些设置是在指定应用的文件中配置的，比如 Oracle DB 就是在 `/etc/security/limits.d/99-grid-oracle-limits.conf` 中配置的。
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这就完成了！你可能还需要重启应用来让应用来使用这些新的巨大页。
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### 如何禁用 hugepages？
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HugePages 默认是开启的。使用下面命令来查看 hugepages 的当前状态。
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```
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root@kerneltalks# cat /sys/kernel/mm/transparent_hugepage/enabled
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[always] madvise never
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```
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输出中的 `[always]` 标志说明系统启用了 hugepages。
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若使用的是基于 RedHat 的系统，则应该要查看的文件路径为 `/sys/kernel/mm/redhat_transparent_hugepage/enabled`。
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若想禁用巨大页，则在 `/etc/grub.conf` 中的 `kernel` 行后面加上 `transparent_hugepage=never`，然后重启系统。
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--------------------------------------------------------------------------------
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via: https://kerneltalks.com/services/what-is-huge-pages-in-linux/
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作者：[Shrikant Lavhate][a]
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译者：[lujun9972](https://github.com/lujun9972)
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校对：[校对者ID](https://github.com/校对者ID)
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||||
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://kerneltalks.com
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[1]:https://c1.kerneltalks.com/wp-content/uploads/2017/11/hugepages-in-linux.png
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[2]:https://kerneltalks.com/linux/how-to-tune-kernel-parameters-in-linux/
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