Linux 标准大页和透明大页

Huge Pages

Transparent Huge Pages

标准大页的页面大小

[root@localhost ~]# grep Hugepagesize /proc/meminfo
Hugepagesize: 2048 kB

注：THP  目前只能映射异步内存区域，比如堆和栈空间

使用Huge pages优点

Larger Page Size and Less # ofPages: Default page size is 4K whereas the HugeTLB size is 2048K. That meansthe system would need to handle 512 times less pages.

Reduced Page Table Walking:Since a HugePage covers greater contiguous virtual address range than a regularsized page, a probability of getting a TLB hit per TLB entry with HugePages arehigher than with regular pages. This reduces the number of times page tablesare walked to obtain physical address from a virtual address.

Less Overhead for MemoryOperations: On virtual memory systems (any modern OS) each memory operation isactually two abstract memory operations. With HugePages, since there are lessnumber of pages to work on, the possible bottleneck on page table access isclearly avoided.

Less Memory Usage: From theOracle Database perspective, with HugePages, the Linux kernel will use lessmemory to create pagetables to maintain virtual to physical mappings for SGAaddress range, in comparison to regular size pages. This makes more memory tobe available for process-private computations or PGA usage.

No Swapping: We must avoidswapping to happen on Linux OS at all Document 1295478.1. HugePages are notswappable (whereas regular pages are). Therefore there is no page replacementmechanism overhead. HugePages are universally regarded as pinned.

No 'kswapd' Operations: kswapdwill get very busy if there is a very large area to be paged (i.e. 13 millionpage table entries for 50GB memory) and will use an incredible amount of CPUresource. When HugePages are used, kswapd is not involved in managing them. Seealso Document 361670.1

使用  Huge pages缺点

Transparent Huge pages存在的问题

1. 在 RAC 环境下   透明大页（ TransparentHugePages  ）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（ TransparentHugePages  ） 也会导致一些异常的性能问题；

如何关闭 Transparent Huge pages ？#

[root@localhost ~]# grep Huge /proc/meminfo
AnonHugePages: 7956480 kB      －－－－－＞＞返回值不为０，表示开启了THP
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB

# cat /sys/kernel/mm/transparent_hugepage/enabled
[always] madvise never
# cat /sys/kernel/mm/transparent_hugepage/defrag
[always] madvise never

方法一

1. 手动临时关闭

echo never > /sys/kernel/mm/transparent_hugepage/enabled
echo never > /sys/kernel/mm/transparent_hugepage/defrag

参数说明

1. 开机自动关闭

vim /etc/rc.d/rc.local

if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
echo never > /sys/kernel/mm/transparent_hugepage/enabled
fi

if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
echo never > /sys/kernel/mm/transparent_hugepage/defrag
fi

chmod +x /etc/rc.d/rc.local

方法二

cp /etc/default/grub /etc/default/grub.date +%F
.bak

cp /boot/grub2/grub.cfg  /boot/grub2/grub.cfg.date +%F
.bak

#cat /etc/sysconfig/grub
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto spectre_v2=retpoline rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rd.lvm.lv=centos/usr rhgb quiet transparent_hugepage=never"
GRUB_DISABLE_RECOVERY="true"

# grub2-mkconfig -o /boot/grub2/grub.cfg
# init 6 -------->>>>重启生效

# cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-3.10.0-1127.el7.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rd.lvm.lv=rhel/usr rhgb quiet transparent_hugepage=never

# cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]    ---->>>[never]表示成功禁用THP
#cat /sys/kernel/mm/transparent_hugepage/defrag   ------>>> 这个应该是内存碎片整理
[always] madvise never

# grep Hug /proc/meminfo        
AnonHugePages:         0 kB      ---------->>>返回值若是零，代表成功禁用THP
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB

扩展内容

出处：https://www.cnblogs.com/my-show-time/p/14237757.html