In my last article I shared the steps to improve disk IO performance in Linux. Now another important part of optimization of databases are the Transparent HugePages. So in this article I will share the steps to disable transparent hugepages with examples for runtime and to disable them permanently (reboot persistent).
Huge pages are pages with a default size of two megabytes or more specific. Red Hat is using two different huge page sizes, the defaults are two megabytes and there’s an option to use bigger huge pages as well with the size of one gigabyte.
Why to use Hugepages?
- One of the reasons can be huge pages help optimizing TLB.
- TLB stands for Translation Lookaside Buffer.
- A TLB is a memory cache that is used to reduce the time taken to access a user memory location.
- This buffer is kept to keep information about huge memory pages into cache
- This buffer isn’t very big and it needs precious memory so you want it to be as efficient as possible.
- And that is why if you want to optimize the TLB usage using huge pages might be an option.
How to monitor Transparent Hugepages?
The number of anonymous transparent huge pages currently used by the
system is available by reading the AnonHugePages field in
/proc/meminfo
# grep -i AnonHugePages /proc/meminfo
AnonHugePages: 1216512 kB
To identify what applications are using anonymous transparent huge
pages, it is necessary to read /proc/PID/smaps and count the
AnonHugePages fields for each mapping.
# grep -e AnonHugePages /proc/$(pgrep test.sh)/smaps | awk '{ if($2>0) print $0} '
AnonHugePages: 120832 kB
How to allocate HugePages?
You can allocate hugepages on runtime from the command line using
“sysctl -w”. Now before making the reservation let us validate our
hugepage reservation
# grep -i huge /proc/meminfo
AnonHugePages: 10240 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
So there are no reservation for hugepages, below are the available and used memory details
# free -m
total used free shared buff/cache available
Mem: 3790 194 3318 46 277 3314
Swap: 759 0 759
~]# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
# grub2-mkconfig -o /boot/grub2/grub.cfg
Reboot the node for the changes to take affect.
# shutdown -r now
Once the node is up make sure the newly added entry exists in your loaded grub configuration
# grep transparent_hugepage /proc/cmdline
BOOT_IMAGE=/vmlinuz-3.10.0-862.6.3.el7.x86_64 root=/dev/mapper/os-root ro novga console=ttyS0,115200 panic=1 numa=off elevator=cfq rd.md.uuid=d265dd3d:9ee4d53a:597b8c08:8201b9af rd.lvm.lv=os/root rd.md.uuid=5398452a:ab1b8e91:4307b53b:5c3cccbd rd.md.uuid=131bc1e7:7c9087c3:03f3ad4a:7cde170c noht biosdevname=0 net.ifnames=0 rhgb quiet transparent_hugepage=never
So the configuration is correctly loaded.
Next you can verify the status of transparent hugepages (AnonHugePages)
How to disable Explicit Transparent Hugepages (nr_hugepages) on runtime?
nr_hugepages indicates the current number of “persistent” huge pages
in the kernel’s huge page pool. “Persistent” huge pages will be returned
to the huge page pool when freed by a task. A user with root privileges
can dynamically allocate more or free some persistent huge pages by
increasing or decreasing the value of ‘nr_hugepages’.
When multiple huge page sizes are supported, /proc/sys/vm/nr_hugepages
indicates the current number of pre-allocated huge pages of the default
size.
Thus, one can use the following command to dynamically
allocate/deallocate default sized persistent huge pages. If the value in
/proc/sys/vm/nr_hugepages file or vm.nr_hugepagesin sysctl.conf
is “0” it means HugePages is disabled on the system
To disable explicit transparent hugepages for the current session execute below command:
# echo 0 > /proc/sys/vm/nr_hugepages
# cat /proc/sys/vm/nr_hugepages
0
# sysctl vm.nr_hugepages
vm.nr_hugepages = 0
Next you can verify the status of explicit transparent hugepages (nr_hugepages) on your system.
How to disable Explicit Transparent Hugepages (nr_hugepages) permanently using sysctl?
Now let me guide you with the steps to disable explicit transparent
hugepages permanently. I will
create a new sysctl
file and add vm.nr_hugepages=0
# cat /etc/sysctl.d/disable_hugepage.conf
vm.nr_hugepages=0
Now before I refresh dracut entry, let us verify the initramfs content
to make sure it does not has hugepage entry from sysctl.
# lsinitrd /boot/initramfs-`uname -r`.img | grep hugepage
Now let us refresh our dracut entry
# dracut -f
Now re-verify the initramfs content. As you see our new sysctl content is available now.
# lsinitrd /boot/initramfs-`uname -r`.img | grep hugepage
-rw-r----- 1 root root 18 Jul 31 18:46 etc/sysctl.d/disable_hugepage.conf
Verify the status of explicit transparent hugepage
# cat /sys/devices/system/node/node*/meminfo | fgrep Huge
Node 0 AnonHugePages: 0 kB
Node 0 HugePages_Total: 0
Node 0 HugePages_Free: 0
Node 0 HugePages_Surp: 0
Next you can verify the status of explicit transparent hugepages (nr_hugepages) on your system.
Lastly I hope the steps from the article to disable transparent hugepages and allocate hugepages on Linux was helpful. So, let me know your suggestions and feedback using the comment section.
