Check HugePages CPU Support and Change Default Size on RHEL

Static HugePages (HugeTLB) use larger memory pages than the normal 4 KB pages. Workloads that need predictable huge pages use a managed pool; that is separate from Transparent HugePages, where the kernel may promote memory automatically.

At boot the kernel picks a default hugepage size, usually 2 MB on typical x86_64 systems. You can change that default with grubby and a reboot when the CPU exposes the size you need.

The checks and 2 MB outputs below were tested on RHEL 10.2 after a reboot. This test CPU does not expose 1 GB HugePages (pdpe1gb is missing), so the 1 GB grubby workflow is for systems where that flag is present and hugepages-1048576kB appears under /sys/kernel/mm/hugepages/.

Quick answer: check support and read the default size

default_hugepagesz= chooses which HugeTLB page size the kernel treats as the default. It does not create or reserve a HugePages pool. To reserve pages, use hugepages= (often with hugepagesz=) or configure nr_hugepages separately.

On x86_64, CPU flags are a quick hardware check:

grep -qw pse /proc/cpuinfo && echo '2MB hugepages: supported' || echo '2MB hugepages: not supported'

Check 1 GB support the same way:

grep -qw pdpe1gb /proc/cpuinfo && echo '1GB hugepages: supported' || echo '1GB hugepages: not supported'

The running kernel must also expose that size. List registered HugeTLB sizes before you change boot parameters:

ls /sys/kernel/mm/hugepages/

See which default size is active today:

grep Hugepagesize /proc/meminfo

To persist a new default on RHEL 8–10, add a kernel argument and reboot only when the CPU supports that size and the running kernel exposes it under /sys/kernel/mm/hugepages/:

sudo grubby --update-kernel=ALL --args="default_hugepagesz=1G"

On x86_64, do not set default_hugepagesz=1G unless pdpe1gb is present and you see hugepages-1048576kB under /sys/kernel/mm/hugepages/ after boot on that platform (or you have confirmed the same on identical hardware).

What “default HugePage size” means

A system can support more than one HugeTLB page size at the same time. On x86_64, 2 MB and 1 GB are the usual choices. default_hugepagesz= picks which supported size is the default for HugeTLB operations. It does not remove the other size; you can still add another pool with hugepagesz= and hugepages= at boot if the kernel exposes both sizes.

The same size shows up in different forms depending on where you look:

Boot parameters: default vs reservation

Example from kernel documentation (multiple pools at boot): hugepagesz=2M hugepages=10 hugepagesz=1G hugepages=1 alongside default_hugepagesz= when you need a specific default. Pool sizing is covered in Configure static HugePages on RHEL 7.

Static HugePages vs Transparent HugePages

THP is a different knob. For always, madvise, or never, see Configure Transparent HugePages on RHEL 8–10.

Check if the CPU supports 2 MB HugePages

On x86_64, the pse flag indicates support for 2 MB pages. A simple yes/no check is:

grep -qw pse /proc/cpuinfo && echo '2MB hugepages: supported' || echo '2MB hugepages: not supported'

2MB hugepages: supported

Check if the CPU supports 1 GB HugePages

The pdpe1gb flag means 1 GB huge pages are supported on x86_64. Many laptops and VMs do not expose it to the guest.

grep -qw pdpe1gb /proc/cpuinfo && echo '1GB hugepages: supported' || echo '1GB hugepages: not supported'

1GB hugepages: not supported

Check which HugeTLB sizes the kernel exposes

Sysfs lists every hugepage size the running kernel registered, not just the default:

ls /sys/kernel/mm/hugepages/

hugepages-2048kB

When pdpe1gb is absent, sysfs often lists only hugepages-2048kB. CPUs with 1 GB support can show both hugepages-2048kB and hugepages-1048576kB after boot.

Hugepagesize in /proc/meminfo shows the default HugeTLB size, not every size the system might support. Use sysfs when you need the full list.

grep Hugepagesize /proc/meminfo

Hugepagesize:       2048 kB

2048 kB is the default on this boot (2 MB). That does not by itself mean 1 GB pages are impossible on every machine; it means the default is 2 MB here and sysfs only registered 2 MB pools on this host.

Check whether default_hugepagesz is already on the boot command line:

grubby --info=DEFAULT | grep '^args='

args="ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet crashkernel=2G-64G:256M,64G-:512M $tuned_params"

No default_hugepagesz entry is present, which matches the 2 MB default above.

Change the default hugepage size with grubby (RHEL 8–10)

On current RHEL releases, use grubby to update kernel arguments on the installed BLS boot entries. Do not hand-edit generated grub.cfg files. For the wider GRUB workflow, see Configure GRUB2 on RHEL.

Add default_hugepagesz for all installed kernels

When pdpe1gb is present and you want 1 GB as the default after the next reboot:

sudo grubby --update-kernel=ALL --args="default_hugepagesz=1G"

To change only the kernel you boot by default:

sudo grubby --update-kernel="$(grubby --default-kernel)" --args="default_hugepagesz=1G"

Before you reboot, make sure the argument landed on the boot entry:

grubby --info=DEFAULT | grep -E '^args=|default_hugepagesz'

Reboot when you can afford a short outage; the new default applies only after the system comes back up:

sudo systemctl reboot

Verify after reboot

After rebooting, check all three values so you know the new setting actually took effect:

Read the default HugeTLB size:

grep Hugepagesize /proc/meminfo

Hugepagesize:       2048 kB

Check whether default_hugepagesz is on the active kernel command line:

grep -o 'default_hugepagesz=[^ ]*' /proc/cmdline || echo 'default_hugepagesz not in cmdline'

default_hugepagesz not in cmdline

List the hugepage sizes the kernel registered:

ls /sys/kernel/mm/hugepages/

hugepages-2048kB

When you set default_hugepagesz=1G on a CPU with pdpe1gb, the same checks should show Hugepagesize: 1048576 kB, default_hugepagesz=1G in /proc/cmdline, and a hugepages-1048576kB directory (often alongside hugepages-2048kB).

Remove or change the boot parameter

When you need to drop a previous value from every boot entry:

sudo grubby --update-kernel=ALL --remove-args="default_hugepagesz=1G"

Replace 1G with the value you previously set. Reboot again for the running kernel to drop the override.

Troubleshooting

Summary

default_hugepagesz= sets which HugeTLB page size is the default; it does not reserve memory. On x86_64, use pse and pdpe1gb as quick CPU checks, then confirm what the kernel actually registered under /sys/kernel/mm/hugepages/. Hugepagesize in /proc/meminfo shows the default for this boot, not the only size your hardware class might ever support.

To change the default on future boots, append default_hugepagesz= with grubby, verify the boot entry, reboot, and run the checks above. On x86_64, choose a 1 GB default only when pdpe1gb is set and the kernel exposes hugepages-1048576kB. Reserving how many pages to allocate remains a separate step from picking the default size.

References

• HugeTLB Pages (kernel documentation)
• kernel-parameters.txt — hugepagesz and default_hugepagesz