How to use, monitor, and disable transparent hugepages in red hat enterprise linux 6_ – red hat customer portal linux kernel programming guide

How to use, monitor, and disable transparent hugepages in red hat enterprise linux 6_ – red hat customer portal linux kernel programming guide If you are a new customer, register now for access to product evaluations and purchasing capabilities. Linux kernel source browser Need access to an account? If your company has an existing Red Hat account, your organization administrator can grant you access. To help you use Red Hat products to their full potential, our engineers have developed a powerful set of tools to support your ongoing success.

Engage with our Red Hat Product Security team, access security updates, and ensure your environments are not exposed to any known security vulnerabilities. • Do we still


need to preallocate some amount of memory for use as (transparent) hugepages? • How can I see the number of transparent hugepages that are actually in use on the system (either globally or by individual processes)?

• Freezing issue while streaming out from RHEL 6.2 server, We are providing a streaming platform (RTSP, HLS) to our customers, based on RHEL 6.2 x86_64 While streaming out, we experiencing delays (more than one to ten seconds) because of a process freeze and at the same point in time a bunch of memory is being freed Note: Transparent Huge Pages are not available on the 32-bit version of RHEL 6. Transparent Huge Pages (THP) are enabled by default in RHEL 6 for all applications. Linux kernel learning The kernel attempts to allocate hugepages whenever possible and any Linux process will receive 2MB pages if the mmap region is 2MB naturally aligned. How to learn linux kernel The main kernel address space itself is mapped with hugepages, reducing TLB pressure from kernel code. Linux kernel mailing list For general information on Hugepages, see: What are Huge Pages and what are the advantages of using them?

The kernel will always attempt to satisfy a memory allocation using hugepages. How linux kernel works If no hugepages are available (due to non availability of physically continuous memory for example) the kernel will fall back to the regular 4KB pages. Lts linux kernel THP are also swappable (unlike hugetlbfs). Linux kernel types This is achieved by breaking the huge page to smaller 4KB pages, which are then swapped out normally.

But to use hugepages effectively, the kernel must find physically continuous areas of memory big enough to satisfy the request, and also properly aligned. Smallest linux kernel For this, a khugepaged kernel thread has been added. Linux kernel structure This thread will occasionally attempt to substitute smaller pages being used currently with a hugepage allocation, thus maximizing THP usage. In userland, no modifications to the applications are necessary (hence transparent).

Linux kernel community But there are ways to optimize its use. What is kernel in linux For applications that want to use hugepages, use of posix_memalign() can also help ensure that large allocations are aligned to huge page (2MB) boundaries. Also, THP is only enabled for anonymous memory regions. Linux latest kernel There are plans to add support for tmpfs and page cache.

View kernel version linux THP tunables are found in the /sys tree under /sys/kernel/mm/redhat_transparent_hugepage. The values for /sys/kernel/mm/redhat_transparent_hugepage/enabled can be one of the following: always – always use THP khugepaged will be automatically started when transparent_hugepage/enabled is set to “always” or “madvise, and it’ll be automatically shutdown if it’s set to “never”. Linux kernel list The redhat_transparent_hugepage/defrag parameter takes the same values and it controls whether the kernel should make aggressive use of memory compaction to make more hugepages available. Run the following command to check system-wide THP usage: # grep AnonHugePages /proc/meminfo Note: Red Hat Enterprise Linux 6.2 or later publishes additional THP monitoring via /proc/vmstat: # egrep ‘trans|thp’ /proc/vmstat Run the following command to monitor which processes are using THP: # grep -e AnonHugePages /proc/*/smaps | awk ‘{ if($2>4) print $0} ‘ | awk -F “/” ‘{print $0; system(“ps -fp ” $3)} ‘ qemu 7519 1 1 08:53 ? 00:00:48 /usr/bin/qemu-system-x86_64 -machine accel=kvm -name rhel7 -S -machine pc-i440fx-1.6,accel=kvm,usb=of qemu 7610 1 2 08:53 ? 00:01:30 /usr/bin/qemu-system-x86_64 -machine accel=kvm -name util6vm -S -machine pc-i440fx-1.6,accel=kvm,usb= qemu 7788 1 1 08:54 ? 00:00:55 /usr/bin/qemu-system-x86_64 -machine accel=kvm -name rhel64eus -S -machine pc-i440fx-1.6,accel=kvm,us Append the following to the kernel command line in grub.

conf: transparent_hugepage=never Note: Certain ktune and/or tuned profiles specify to enable THP when they are applied. Types of kernel in linux If the transparent_hugepage=never parameter is set at boot time, but THP does not appear to be disabled after the system is fully booted. Linux kernel options Refer to the following article: Disabling transparent hugepages (THP) on Red Hat Enterprise Linux 6 is not taking effect Run the following commands to disable THP on-the-fly: # echo never > /sys/kernel/mm/redhat_transparent_hugepage/enabled # echo never > /sys/kernel/mm/redhat_transparent_hugepage/defrag • NOTE: Running the above commands will stop only creation and usage of the new THP. Kernel of linux The THP which were created and used at the moment the above commands were run would not be disassembled into the regular memory pages.

Linux kernel hacking To get rid of THP completely the system should be rebooted with THP disabled at boot time. • NOTE: Some third party application install scripts check value of above files and complain even if THP is disabled at boot time using transparent_hugepage=never, this is due to the fact when THP is disabled at boot time, the value of /sys/kernel/mm/redhat_transparent_hugepage/defrag will not be changed, however this is expected and system will never go in THP defragmentation code path when it is disabled at boot and THP defrag need not to be disabled separately. There can be two types of HugePages in the system: Explicit Huge Pages which are allocated explicitly by vm.nr_hugepages sysctl parameter and Tranparent Huge Pages which are allocated automatically by the kernel. Linux kernel contributors See below on how to tell if Explicit HugePages is enabled or disabled.

If the value of HugePages_Total is “0” it means HugePages is disabled on the system. Linux kernel development by robert love # grep -i HugePages_Total /proc/meminfo Similarly, if the value in /proc/sys/vm/nr_hugepages file or vm.nr_hugepages sysctl parameter is “0” it means HugePages is disabled on the system: # cat /proc/sys/vm/nr_hugepages If the value of HugePages_Total is greater than “0”, it means HugePages is enabled on the system: # grep -i HugePages_Total /proc/meminfo Similarly if the value in /proc/sys/vm/nr_hugepages file or vm.nr_hugepages sysctl parameter is greater than “0”, it means HugePages is enabled on the system: # cat /proc/sys/vm/nr_hugepages • RHEL 6 disables THP on systems with < 1G of ram. What is a kernel in linux Refer to Red Hat Bug 618444 - disable transparent hugepages by default on small systems for more information. • Disadvantages of using the explicit hugepages (libhugetlbfs): Using hugetlbfs requires significant work from both application developers and system administrators; explicit hugepages must be set aside at boot time, and applications must map them explicitly. Linux kernel upgrade The process is fiddly enough that use of hugetlbfs is restricted to those who really care and who have the time to mess with it. Linux kernel newbies Hugetlbfs is often seen as a feature for large, proprietary database management systems and little else. Links contained herein to external website(s) are provided for convenience only. Linux kernel entry point Red Hat has not reviewed the links and is not responsible for the content or its availability. Linux kernel fundamentals The inclusion of any link to an external website does not imply endorsement by Red Hat of the website or their entities, products or services. Linux kernel names You agree that Red Hat is not responsible or liable for any loss or expenses that may result due to your use of (or reliance on) the external site or content. • Andrea Arcangeli's presentation "Transparent Hugepage Support" : KVM Forum 2010 - Presentations This solution is part of Red Hat’s fast-track publication program, providing a huge library of solutions that Red Hat engineers have created while supporting our customers. Build kernel linux To give you the knowledge you need the instant it becomes available, these articles may be presented in a raw and unedited form. Site: https://access.redhat.com/solutions/46111

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