Beware the transparent HugePages in Linux 6

Beware the transparent HugePages in Linux 6

I came across this one earlier this week on a client site which had me scratching my head.

[root@someServer ~]# grep Huge /proc/meminfo
AnonHugePages	458752 kB
HugePages_Total	0
HugePages_Free	0
HugePages_Rsvd	0
HugePages_Surp	0
Hugepagesize	2048 kB

All looks good, except what are Anonymous HugePages?

Turns out that new with Linux 6 amongst others, comes this weird set of dynamic HugePages that are different from the static ones defined and created at boot time.

Digging into Oracle Support, I see the lovely note 1557478.1 that indicates that these are generally not good things to have around on Oracle servers and that they should be disabled.

For your edification, this is the important part of that note and describes how to tell if it's enabled and if so, how to disable it.

ALERT: Disable Transparent HugePages on SLES11, RHEL6, OL6 and UEK2 Kernels (Doc ID 1557478.1)
Starting with RedHat6, OL6, SLES11 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.

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

To disable Transparent HugePages boot time either one of the following 2 methods may be used:
Add the following to the kernel boot line in /etc/grub.conf (a symlink to /boot/grub/grub.conf) and reboot the server (this is the preferred method):
transparent_hugepage=never

Once modified the line will read similar to the following example:
title Oracle Linux Server (2.6.32-300.25.1.el6uek.x86_64)
        root (hd0,0)
        kernel /vmlinuz-2.6.32-300.25.1.el6uek.x86_64 ro root=LABEL=/ transparent_hugepage=never
        initrd /initramfs-2.6.32-300.25.1.el6uek.x86_64.img

OR
Add the following lines in /etc/rc.local and reboot the server:
if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
  echo never > /sys/kernel/mm/transparent_hugepage/enabledfiif test -f /sys/kernel/mm/transparent_hugepage/defrag; then  echo never > /sys/kernel/mm/transparent_hugepage/defragfiPlease change the file path for RHEL kernel to /sys/kernel/mm/redhat_transparent_hugepage/ accordingly.