¿Qué son las páginas enormes transparentes de Linux?

What is transparent huge pages Linux?

Transparent Huge Pages (THP) is a Linux memory management system that reduces the overhead of Translation Lookaside Buffer (TLB) lookups on machines with large amounts of memory by using larger memory pages. When running MongoDB on Linux, THP should be disabled for best performance.

Enable Transparent Hugepages on Red Hat 7/CentOS 7 and Amazon Linux 2.0

1. cat /sys/kernel/mm/transparent_hugepage/enabled [always] madvise never.
2. if test -f /sys/kernel/mm/transparent_hugepage/enabled; then echo always > /sys/kernel/mm/transparent_hugepage/enabled fi.

Transparent Huge Pages (THP) are enabled by default in RHEL 6 for all applications.

To disable Transparent HugePages:

1. For Oracle Linux 7 and Red Hat Enterprise Linux 7, add or modify the transparent_hugepage=never parameter in the /etc/default/grub file: Copy transparent_hugepage=never.
2. Run the grub2–mkconfig command to regenerate the grub.
3. Restart the system to make the changes permanent.

The AnonHugePages entry lists the number of pages that the newer Transparent Huge Page mechanism currently has in use. For this machine there are 309472kB, 1489 huge pages each 2048kB in size. The number of HugePages in use can be computed as HugePages_Total – HugePagesFree .

always means transparent hugepages are always enabled for each process. This usually increases performance, but if you have a usecase with many processes that only consume a small amount of memory each, your overall memory usage could grow drastically.

Enabling HugePages makes it possible for the operating system to support memory pages greater than the default (usually 4 KB). Using very large page sizes can improve system performance by reducing the amount of system resources required to access page table entries.

Complete the following steps to configure HugePages on the computer:

1. Run the following command to determine if the kernel supports HugePages: $ grep Huge /proc/meminfo.
2. Some Linux systems do not support HugePages by default.
3. Edit the memlock setting in the /etc/security/limits.conf file.

In userland, no modifications to the applications are necessary (hence transparent). But there are ways to optimize its use. 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.

Transparent Huge Pages (THP) are enabled by default in RHEL 6 for all applications. The kernel attempts to allocate hugepages whenever possible and any Linux process will receive 2MB pages if the mmap region is 2MB naturally aligned. The main kernel address space itself is mapped with hugepages, reducing TLB pressure from kernel code.

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: Note: Red Hat Enterprise Linux 6.2 or later publishes additional THP monitoring via /proc/vmstat:

In certain cases when hugepages are enabled system wide, application may end up allocating more memory resources. An application may mmap a large region but only touch 1 byte of it, in that case a 2M page might be allocated instead of a 4k page for no good.