Chapter 36. Configuring huge pages Physical memory is managed in fixed-size chunks called pages. On the x86_64 architecture, supported by Red Hat Enterprise Linux 8, the default size of a memory page is 4 KB . This default page size has proved to be suitable for general-purpose operating systems, such as Red Hat Enterprise Linux, which supports many different kinds of workloads. However, specific applications can benefit from using larger page sizes in certain cases. For example, an application that works with a large and relatively fixed data set of hundreds of megabytes or even dozens of gigabytes can have performance issues when using 4 KB pages. Such data sets can require a huge amount of 4 KB pages, which can lead to overhead in the operating system and the CPU. This section provides information about huge pages available in RHEL 8 and how you can configure them. 36.1. Available huge page features With Red Hat Enterprise Linux 8, you can use huge pages for applications that work with big data sets, and improve the performance of such applications. The following are the huge page methods, which are supported in RHEL 8: HugeTLB pages HugeTLB pages are also called static huge pages. There are two ways of reserving HugeTLB pages: At boot time: It increases the possibility of success because the memory has not yet been significantly fragmented. However, on NUMA machines, the number of pages is automatically split among the NUMA nodes. For more information about parameters that influence HugeTLB page behavior at boot time, see Parameters for reserving HugeTLB pages at boot time and how to use these parameters to configure HugeTLB pages at boot time, see Configuring HugeTLB at boot time . At run time: It allows you to reserve the huge pages per NUMA node. If the run-time reservation is done as early as possible in the boot process, the probability of memory fragmentation is lower. For more information about parameters that influence HugeTLB page behavior at run time, see Parameters for reserving HugeTLB pages at run time and how to use these parameters to configure HugeTLB pages at run time, see Configuring HugeTLB at run time . Transparent HugePages (THP) With THP, the kernel automatically assigns huge pages to processes, and therefore there is no need to manually reserve the static huge pages. The following are the two modes of operation in THP: system-wide : Here, the kernel tries to assign huge pages to a process whenever it is possible to allocate the huge pages and the process is using a large contiguous virtual memory area. per-process : Here, the kernel only assigns huge pages to the memory areas of individual processes which you can specify using the madvise () system call. Note The THP feature only supports 2 MB pages. For more information about parameters that influence HugeTLB page behavior at boot time, see Enabling transparent hugepages and Disabling transparent hugepages . 36.2. Parameters for reserving HugeTLB pages at boot time Use the following parameters to influence HugeTLB page behavior at boot time. For more infomration on how to use these parameters to configure HugeTLB pages at boot time, see Configuring HugeTLB at boot time . Table 36.1. Parameters used to configure HugeTLB pages at boot time Parameter Description Default value hugepages Defines the number of persistent huge pages configured in the kernel at boot time. In a NUMA system, huge pages, that have this parameter defined, are divided equally between nodes. You can assign huge pages to specific nodes at runtime by changing the value of the nodes in the /sys/devices/system/node/node_id/hugepages/hugepages-size/nr_hugepages file. The default value is 0 . To update this value at boot, change the value of this parameter in the /proc/sys/vm/nr_hugepages file. hugepagesz Defines the size of persistent huge pages configured in the kernel at boot time. Valid values are 2 MB and 1 GB . The default value is 2 MB . default_hugepagesz Defines the default size of persistent huge pages configured in the kernel at boot time. Valid values are 2 MB and 1 GB . The default value is 2 MB . 36.3. Configuring HugeTLB at boot time The page size, which the HugeTLB subsystem supports, depends on the architecture. The x86_64 architecture supports 2 MB huge pages and 1 GB gigantic pages. This procedure describes how to reserve a 1 GB page at boot time. Procedure To create a HugeTLB pool for 1 GB pages, enable the default_hugepagesz=1G and hugepagesz=1G kernel options: Create a new file called hugetlb-gigantic-pages.service in the /usr/lib/systemd/system/ directory and add the following content: Create a new file called hugetlb-reserve-pages.sh in the /usr/lib/systemd/ directory and add the following content: While adding the following content, replace number_of_pages with the number of 1GB pages you want to reserve, and node with the name of the node on which to reserve these pages. For example, to reserve two 1 GB pages on node0 and one 1GB page on node1 , replace the number_of_pages with 2 for node0 and 1 for node1 : Create an executable script: Enable early boot reservation: Note You can try reserving more 1 GB pages at runtime by writing to nr_hugepages at any time. However, to prevent failures due to memory fragmentation, reserve 1 GB pages early during the boot process. Reserving static huge pages can effectively reduce the amount of memory available to the system, and prevents it from properly utilizing its full memory capacity. Although a properly sized pool of reserved huge pages can be beneficial to applications that utilize it, an oversized or unused pool of reserved huge pages will eventually be detrimental to overall system performance. When setting a reserved huge page pool, ensure that the system can properly utilize its full memory capacity. Additional resources systemd.service(5) man page on your system /usr/share/doc/kernel-doc-kernel_version/Documentation/vm/hugetlbpage.txt file 36.4. Parameters for reserving HugeTLB pages at run time Use the following parameters to influence HugeTLB page behavior at run time. For more information about how to use these parameters to configure HugeTLB pages at run time, see Configuring HugeTLB at run time . Table 36.2. Parameters used to configure HugeTLB pages at run time Parameter Description File name nr_hugepages Defines the number of huge pages of a specified size assigned to a specified NUMA node. /sys/devices/system/node/node_id/hugepages/hugepages-size/nr_hugepages nr_overcommit_hugepages Defines the maximum number of additional huge pages that can be created and used by the system through overcommitting memory. Writing any non-zero value into this file indicates that the system obtains that number of huge pages from the kernel's normal page pool if the persistent huge page pool is exhausted. As these surplus huge pages become unused, they are then freed and returned to the kernel's normal page pool. /proc/sys/vm/nr_overcommit_hugepages 36.5. Configuring HugeTLB at run time This procedure describes how to add 20 2048 kB huge pages to node2 . To reserve pages based on your requirements, replace: 20 with the number of huge pages you wish to reserve, 2048kB with the size of the huge pages, node2 with the node on which you wish to reserve the pages. Procedure Display the memory statistics: Add the number of huge pages of a specified size to the node: Verification Ensure that the number of huge pages are added: Additional resources numastat(8) man page on your system 36.6. Managing transparent hugepages Transparent hugepages (THP) are enabled by default in Red Hat Enterprise Linux 8. However, you can enable, disable, or set the transparent hugepages to madvise with runtime configuration, TuneD profiles, kernel command line parameters, or systemd unit file. 36.6.1. Managing transparent hugepages with runtime configuration Transparent hugepages (THP) can be managed at runtime to optimize memory usage. The runtime configuration is not persistent across system reboots. Procedure Check the status of THP: Configure THP. Enabling THP: Disabling THP: Setting THP to madvise : To prevent applications from allocating more memory resources than necessary, disable the system-wide transparent hugepages and only enable them for the applications that explicitly request it through the madvise system call. Note Sometimes, providing low latency to short-lived allocations has higher priority than immediately achieving the best performance with long-lived allocations. In such cases, you can disable direct compaction while leaving THP enabled. Direct compaction is a synchronous memory compaction during the huge page allocation. Disabling direct compaction provides no guarantee of saving memory, but can decrease the risk of higher latencies during frequent page faults. Also, disabling direct compaction allows synchronous compaction of Virtual Memory Areas (VMAs) highlighted in madvise only. Note that if the workload benefits significantly from THP, the performance decreases. Disable direct compaction: USD echo never > /sys/kernel/mm/transparent_hugepage/defrag Additional resources madvise(2) man page on your system. 36.6.2. Managing transparent hugepages with TuneD profiles You can manage transparent hugepages (THP) by using TuneD profiles. The tuned.conf file provides the configuration of TuneD profiles. This configuration is persistent across system reboots. Prerequisites TuneD package is installed. TuneD service is enabled. Procedure Copy the active profile file to the same directory: Edit the tune.conf file: To enable THP, add the line: To disable THP, add the line: To set THP to madvise , add the line: Restart the TuneD service: Set the new profile active: Verification Verify that the new profile is active: Verify that the required mode of THP is set: 36.6.3. Managing transparent hugepages with kernel command line parameters You can manage transparent hugepages (THP) at boot time by modifying kernel parameters. This configuration is persistent across system reboots. Prerequisite You have root permissions on the system. Procedure Get the current kernel command line parameters: Configure THP by adding kernel parameters. To enable THP: To disable THP: To set THP to madvise : Reboot the system for changes to take effect: Verification To verify the status of THP, view the following files: 36.6.4. Managing transparent hugepages with a systemd unit file You can manage transparent hugepages (THP) at system startup by using systemd unit files. By creating a systemd service, you get consistent THP configuration across system reboots. Prerequisite You have root permissions on the system. Procedure Create new systemd service files for enabling, disabling and setting THP to madvise . For example, /etc/systemd/system/disable-thp.service . Configure THP by adding the following contents to a new systemd service file. To enable THP, add the following content to <new_thp_file>.service file: To disable THP, add the following content to <new_thp_file>.service file: To set THP to madvise , add the following content to <new_thp_file>.service file: Enable and start the service: Verification To verify the status of THP, view the following files: 36.6.5. Additional resources You can also disable Transparent Huge Pages (THP) by setting up TuneD profile or using predefined TuneD profiles. See TuneD profiles distributed with RHEL and Available TuneD plug-ins . 36.7. Impact of page size on translation lookaside buffer size Reading address mappings from the page table is time-consuming and resource-expensive, so CPUs are built with a cache for recently-used addresses, called the Translation Lookaside Buffer (TLB). However, the default TLB can only cache a certain number of address mappings. If a requested address mapping is not in the TLB, called a TLB miss, the system still needs to read the page table to determine the physical to virtual address mapping. Because of the relationship between application memory requirements and the size of pages used to cache address mappings, applications with large memory requirements are more likely to suffer performance degradation from TLB misses than applications with minimal memory requirements. It is therefore important to avoid TLB misses wherever possible. Both HugeTLB and Transparent Huge Page features allow applications to use pages larger than 4 KB . This allows addresses stored in the TLB to reference more memory, which reduces TLB misses and improves application performance.

Commands:
grubby --update-kernel=ALL --args="default_hugepagesz=1G hugepagesz=1G"
[Unit] Description=HugeTLB Gigantic Pages Reservation DefaultDependencies=no Before=dev-hugepages.mount ConditionPathExists=/sys/devices/system/node ConditionKernelCommandLine=hugepagesz=1G [Service] Type=oneshot RemainAfterExit=yes ExecStart=/usr/lib/systemd/hugetlb-reserve-pages.sh [Install] WantedBy=sysinit.target
#!/bin/sh nodes_path=/sys/devices/system/node/ if [ ! -d USDnodes_path ]; then echo "ERROR: USDnodes_path does not exist" exit 1 fi reserve_pages() { echo USD1 > USDnodes_path/USD2/hugepages/hugepages-1048576kB/nr_hugepages } reserve_pages number_of_pages node
reserve_pages 2 node0 reserve_pages 1 node1
chmod +x /usr/lib/systemd/hugetlb-reserve-pages.sh
systemctl enable hugetlb-gigantic-pages
numastat -cm | egrep 'Node|Huge' Node 0 Node 1 Node 2 Node 3 Total add AnonHugePages 0 2 0 8 10 HugePages_Total 0 0 0 0 0 HugePages_Free 0 0 0 0 0 HugePages_Surp 0 0 0 0 0
echo 20 > /sys/devices/system/node/node2/hugepages/hugepages- 2048kB /nr_hugepages
numastat -cm | egrep 'Node|Huge' Node 0 Node 1 Node 2 Node 3 Total AnonHugePages 0 2 0 8 10 HugePages_Total 0 0 40 0 40 HugePages_Free 0 0 40 0 40 HugePages_Surp 0 0 0 0 0
cat /sys/kernel/mm/transparent_hugepage/enabled
echo always > /sys/kernel/mm/transparent_hugepage/enabled
echo never > /sys/kernel/mm/transparent_hugepage/enabled
echo madvise > /sys/kernel/mm/transparent_hugepage/enabled
sudo cp -R /usr/lib/tuned/ my_profile /usr/lib/tuned/ my_copied_profile
sudo vi /usr/lib/tuned/ my_copied_profile /tuned.conf
[bootloader] cmdline = transparent_hugepage=always
[bootloader] cmdline = transparent_hugepage=never
[bootloader] cmdline = transparent_hugepage=madvise
sudo systemctl restart tuned
sudo tuned-adm profile my_copied_profile
sudo tuned-adm active
cat /sys/kernel/mm/transparent_hugepage/enabled
grubby --info=USD(grubby --default-kernel) kernel="/boot/vmlinuz-4.18.0-553.el8_10.x86_64" args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=UUID= XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXXX console=tty0 console=ttyS0" root="UUID= XXXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX " initrd="/boot/initramfs-4.18.0-553.el8_10.x86_64.img" title="Red Hat Enterprise Linux (4.18.0-553.el8_10.x86_64) 8.10 (Ootpa)" id=" XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX -4.18.0-553.el8_10.x86_64"
grubby --args="transparent_hugepage=always" --update-kernel=DEFAULT
grubby --args="transparent_hugepage=never" --update-kernel=DEFAULT
grubby --args="transparent_hugepage=madvise" --update-kernel=DEFAULT
reboot
cat /sys/kernel/mm/transparent_hugepage/enabled always madvise [never]
grep AnonHugePages: /proc/meminfo AnonHugePages: 0 kB
grep nr_anon_transparent_hugepages /proc/vmstat nr_anon_transparent_hugepages 0
[Unit] Description=Enable Transparent Hugepages After=local-fs.target Before=sysinit.target [Service] Type=oneshot RemainAfterExit=yes ExecStart=/bin/sh -c 'echo always > /sys/kernel/mm/transparent_hugepage/enabled [Install] WantedBy=multi-user.target
[Unit] Description=Disable Transparent Hugepages After=local-fs.target Before=sysinit.target [Service] Type=oneshot RemainAfterExit=yes ExecStart=/bin/sh -c 'echo never > /sys/kernel/mm/transparent_hugepage/enabled [Install] WantedBy=multi-user.target
[Unit] Description=Madvise Transparent Hugepages After=local-fs.target Before=sysinit.target [Service] Type=oneshot RemainAfterExit=yes ExecStart=/bin/sh -c 'echo madvise > /sys/kernel/mm/transparent_hugepage/enabled [Install] WantedBy=multi-user.target
systemctl enable <new_thp_file>.service
systemctl start <new_thp_file>.service
cat /sys/kernel/mm/transparent_hugepage/enabled