Huge pages are a way to enhance the performances of the applications by reducing the number of TLB misses. The mechanism coalesces contiguous standard physical pages (typical size of 4 KB) into a big one (e.g. 2 MB). Linux implements this feature in two flavors: Transparent Huge pages and explicit huge pages.
Transparent Huge Pages
Transparent huge pages (THP) are managed transparently by the kernel. The user space applications have no control on them. The kernel makes its best to allocate huge pages whenever it is possible but it is not guaranteed. Moreover, THP may introduce overhead as an underlying "garbage collector" kernel daemon named khugepaged is in charge of the coalescing of the physical pages to make huge pages. This may consume CPU time with undesirable effects on the performances of the running applications. In systems with time critical applications, it is generally advised to deactivate THP.
THP can be disabled on the boot command line (cf. the end of this answer) or from the shell in sysfs:
$ cat /sys/kernel/mm/transparent_hugepage/enabled
always [madvise] never
$ sudo sh -c "echo never > /sys/kernel/mm/transparent_hugepage/enabled"
$ cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]
N.B.: Some interesting papers exist on the performance evaluation/issues of the THP:
Explicit huge pages
If the huge pages are required at application level (i.e. from user space). HUGETLBFS kernel configuration must be set to activate the hugetlbfs pseudo-filesystem (the menu in the kernel configurator is something like: "File systems" --> "Pseudo filesystems" --> "HugeTLB file system support"). In the kernel source tree this parameter is in fs/Kconfig:
config HUGETLBFS
bool "HugeTLB file system support"
depends on X86 || IA64 || SPARC64 || (S390 && 64BIT) ||
SYS_SUPPORTS_HUGETLBFS || BROKEN
help
hugetlbfs is a filesystem backing for HugeTLB pages, based on
ramfs. For architectures that support it, say Y here and read
<file:Documentation/admin-guide/mm/hugetlbpage.rst> for details.
If unsure, say N.
For example, on an Ubuntu system, we can check:
$ cat /boot/config-5.4.0-53-generic | grep HUGETLBFS
CONFIG_HUGETLBFS=y
N.B.: On Raspberry Pi, it is possible to configure the apparition of /proc/config.gz and do the same with zcat to check the parameter. To make it, the configuration menu is: "General setup" --> "Kernel .config support" + "Enable access to .config through /proc/config.gz"
When this parameter is set, hugetlbfs pseudo-filesystem is added into the kernel build (cf. fs/Makefile):
obj-$(CONFIG_HUGETLBFS) += hugetlbfs/
The source code of hugetlbfs is located in fs/hugetlbfs/inode.c. At startup, the kernel will mount internal hugetlbfs file systems to support all the available huge page sizes for the architecture it is running on:
static int __init init_hugetlbfs_fs(void)
{
struct vfsmount *mnt;
struct hstate *h;
int error;
int i;
if (!hugepages_supported()) {
pr_info("disabling because there are no supported hugepage sizes
");
return -ENOTSUPP;
}
error = -ENOMEM;
hugetlbfs_inode_cachep = kmem_cache_create("hugetlbfs_inode_cache",
sizeof(struct hugetlbfs_inode_info),
0, SLAB_ACCOUNT, init_once);
if (hugetlbfs_inode_cachep == NULL)
goto out;
error = register_filesystem(&hugetlbfs_fs_type);
if (error)
goto out_free;
/* default hstate mount is required */
mnt = mount_one_hugetlbfs(&hstates[default_hstate_idx]);
if (IS_ERR(mnt)) {
error = PTR_ERR(mnt);
goto out_unreg;
}
hugetlbfs_vfsmount[default_hstate_idx] = mnt;
/* other hstates are optional */
i = 0;
for_each_hstate(h) {
if (i == default_hstate_idx) {
i++;
continue;
}
mnt = mount_one_hugetlbfs(h);
if (IS_ERR(mnt))
hugetlbfs_vfsmount[i] = NULL;
else
hugetlbfs_vfsmount[i] = mnt;
i++;
}
return 0;
out_unreg:
(void)unregister_filesystem(&hugetlbfs_fs_type);
out_free:
kmem_cache_destroy(hugetlbfs_inode_cachep);
out:
return error;
}
A hugetlbfs file system is a sort of RAM file system into which the kernel creates files to back the memory regions mapped by the applications.
The amount of needed huge pages can be reserved by writing the number of needed huge pages into /sys/kernel/mm/hugepages/hugepages-hugepagesize/nr_hugepages.
Then, mmap() is able to map some part of the application address space onto huge pages. Here is an example showing how to do it:
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>
#define HP_SIZE (2 * 1024 * 1024) // <-- Adjust with size of the supported HP size on your system
int main(void)
{
char *addr, *addr1;
// Map a Huge page
addr = mmap(NULL, HP_SIZE, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED| MAP_HUGETLB, -1, 0);
if (addr == MAP_FAILED) {
perror("mmap()");
return 1;
}
printf("Mapping located at address: %p
", addr);
pause();
return 0;
}
In the preceding program, the memory pointed by addr is based on huge pages. Example of usage:
$ gcc alloc_hp.c -o alloc_hp
$ ./alloc_hp
mmap(): Cannot allocate memory
$ cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
0
$ sudo sh -c "echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages"
$ cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
1
$ ./alloc_hp
Mapping located at address: 0x7f7ef6c00000
In another terminal, the process map can be observed to verify the size of the memory page (it is blocked in pause() system call):
$ pidof alloc_hp
13009
$ cat /proc/13009/smaps
[...]
7f7ef6c00000-7f7ef6e00000 rw-s 00000000 00:0f 331939 /anon_hugepage (deleted)
Size: 2048 kB
KernelPageSize: 2048 kB <----- The page size is 2MB
MMUPageSize: 2048 kB
[...]
In the preceding map, the file name /anon_hugepage for the huge page region is made internally by the kernel. It is marked deleted because the kernel removes the associated memory file which will make the file disappear as soon as there are no longer references on it (e.g. when the calling process ends, the underlying file is closed upon exit(), the reference counter on the file drops to 0 and the remove operation finishes to make it disappear).
Allocation of other huge page sizes
On Raspberry Pi 4B, the default huge page size is 2MB but the card supports several other huge page sizes:
$ ls -l /sys/kernel/mm/hugepages
total 0
drwxr-xr-x 2 root root 0 Nov 23 14:58 hugepages-1048576kB
drwxr-xr-x 2 root root 0 Nov 23 14:58 hugepages-2048kB
drwxr-xr-x 2 root root 0 Nov 23 14:58 hugepages-32768kB
drwxr-xr-x 2 root root 0 Nov 23 14:58 hugepages-64kB
To use them, it is necessary to mount a hugetlbfs type file system corresponding to the size of the desired huge page. The kernel documentation provides details on the available mount options. For example, to mount a hugetlbfs file system on /mnt/huge with 8 Huge Pages of size 64KB, the command is:
mount -t hugetlbfs -o pagesize=64K,size=512K,min_size=512K none /mnt/huge
Then it is possible to map huge pages of 64KB in a user program. The following program creates the /tmp/hpfs directory on which it mounts a hugetlbfs file system with a size of 4 huge pages of 64KB. A file named /memfile_01 is created and extended to the size of 2 huge pages. The file is mapped into memory thanks to mmap() system call. It is not passed MAP_HUGETLB flag as the provided file descriptor is for a file created on a hugetlbfs filesystem. Then, the program calls pause() to suspend its execution in order to make some observations in another terminal:
#include <sys/types.h>
#include <errno.h>
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <fcntl.h>
#define ERR(fmt, ...) do {
fprintf(stderr,
"ERROR@%s#%d: "fmt,
__FUNCTION__, __LINE__, ## __VA_ARGS__);
} while(0)
#define HP_SIZE (64 * 1024)
#define HPFS_DIR "/tmp/hpfs"
#define HPFS_SIZE (4 * HP_SIZE)
int main(void)
{
void *addr;
char cmd[256];
int status;
int rc;
char mount_opts[256];
int fd;
rc = mkdir(HPFS_DIR, 0777);
if (0 != rc && EEXIST != errno) {
ERR("mkdir(): %m (%d)
", errno);
return 1;
}
snprintf(mount_opts, sizeof(mount_opts), "pagesize=%d,size=%d,min_size=%d", HP_SIZE, 2*HP_SIZE, HP_SIZE);
rc = mount("none", HPFS_DIR, "hugetlbfs", 0, mount_opts);
if (0 != rc) {
ERR("mount(): %m (%d)
", errno);
return 1;
}
fd = open(HPFS_DIR"/memfile_01", O_RDWR|O_CREAT, 0777);
if (fd < 0) {
ERR("open(%s): %m (%d)
", "memfile_01", errno);
return 1;
}
rc = ftruncate(fd, 2 * HP_SIZE);
if (0 != rc) {
ERR("ftruncate(): %m (%d)
", errno);
return 1;
}
addr = mmap(NULL, 2 * HP_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
if (MAP_FAILED == addr) {
ERR("mmap(): %m (%d)
", errno);
return 1;
}
// The file can be closed
rc = close(fd);
if (0 != rc) {
ERR("close(%d): %m (%d)
", fd, errno);
return 1;
}
pause();
return 0;
} // main
The preceding program must be run as root as it calls mount():
$ gcc mount_tlbfs.c -o mount_tlbfs
$ cat /sys/kernel/mm/hugepages/hugepages-64kB/nr_hugepages
0
$ sudo sh -c "echo 8 > /sys/kernel/mm/hugepages/hugepages-64kB/nr_hugepages"
$ cat /sys/kernel/mm/hugepages/hugepages-64kB/nr_hugepages
8
$ sudo ./mount_tlbfs
In another terminal, the /proc/[pid]/smaps file can be displayed to check the huge page allocation. As soon as the program writes into the huge pages, the Lazy allocation mechanism triggers the effe
