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Linux 关于透明大页的介绍

2017-3-22 12:42| 发布者: CODETC| 查看: 2000| 评论: 0

Redis在Linux中启动时通常会报“WARNING you have Transparent Huge Pages (THP) support enabled in your kernel”的警告信息,意思是:你使用的是透明大页,可能导致redis延迟和内存使用问题。

关于透明大页,我们看看官方的介绍

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.

查看是否启用透明大页

1:命令cat /sys/kernel/mm/redhat_transparent_hugepage/enabled 该命令适用于Red Hat Enterprise Linux系统

[root@getlnx06 ~]# more /etc/issue
 
Red Hat Enterprise Linux Server release 6.6 (Santiago)
 
Kernel \r on an \m
 
[root@getlnx06 ~]# cat /sys/kernel/mm/redhat_transparent_hugepage/enabled
 
[always] madvise never

2:命令cat /sys/kernel/mm/transparent_hugepage/enabled 该命令适用于其它Linux系统

[root@getlnx06 ~]# cat /sys/kernel/mm/transparent_hugepage/enabled
 
always madvise [never]
 
[root@getlnx06 ~]# 

使用命令查看时,如果输出结果为[always]表示透明大页启用了。[never]表示透明大页禁用、[madvise]表示

3:如何HugePages_Total返回0,也意味着透明大页禁用了

[root@getlnx06 ~]# grep -i HugePages_Total /proc/meminfo 
 
HugePages_Total: 0

4:cat /proc/sys/vm/nr_hugepages返回0也意味着透明大页禁用了。

[root@getlnx06 ~]# cat /proc/sys/vm/nr_hugepages 
 
0

禁用、启用透明大页功能

方法1:设置/etc/grub.conf文件,在系统启动是禁用。

[root@getlnx06 ~]# vi /etc/grub.conf
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/mapper/VolGroup--LogVol0-LogVol01
#          initrd /initrd-[generic-]version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux 6 (2.6.32-504.el6.x86_64)
        root (hd0,0)
        kernel /vmlinuz-2.6.32-504.el6.x86_64 ro root=/dev/mapper/VolGroup--LogVol0-LogVol01 rd_NO_LUKS LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto rd_LVM_LV=VolGroup-LogVol0/LogVol01 rd_LVM_LV=VolGroup-LogVol0/LogVol00  KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
        initrd /initramfs-2.6.32-504.el6.x86_64.img
transparent_hugepage=never
方法2:设置/etc/rc.local文件

[root@getlnx06 ~]# vi /etc/rc.local
#!/bin/sh
#
# This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don't
# want to do the full Sys V style init stuff.
 
touch /var/lock/subsys/local
 
if test -f /sys/kernel/mm/redhat_transparent_hugepage/enabled; then
   echo never > /sys/kernel/mm/redhat_transparent_hugepage/enabled
fi

使用上面的配置后必须重启操作系统才能生效,你也可以运行下面命令不用重启操作系统。

[root@getlnx06 ~]# echo never > /sys/kernel/mm/redhat_transparent_hugepage/enabled
[root@getlnx06 ~]# cat /sys/kernel/mm/redhat_transparent_hugepage/enabled
always madvise [never]
[root@getlnx06 ~]# 

小知识点:

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。

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:
  • 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
  • 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
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