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Documentation/vm/hugepage-shm.c
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/*
* hugepage-shm:
*
* Example of using huge page memory in a user application using Sys V shared
* memory system calls. In this example the app is requesting 256MB of
* memory that is backed by huge pages. The application uses the flag
* SHM_HUGETLB in the shmget system call to inform the kernel that it is
* requesting huge pages.
*
* For the ia64 architecture, the Linux kernel reserves Region number 4 for
* huge pages. That means that if one requires a fixed address, a huge page
* aligned address starting with 0x800000... will be required. If a fixed
* address is not required, the kernel will select an address in the proper
* range.
* Other architectures, such as ppc64, i386 or x86_64 are not so constrained.
*
* Note: The default shared memory limit is quite low on many kernels,
* you may need to increase it via:
*
* echo 268435456 > /proc/sys/kernel/shmmax
*
* This will increase the maximum size per shared memory segment to 256MB.
* The other limit that you will hit eventually is shmall which is the
* total amount of shared memory in pages. To set it to 16GB on a system
* with a 4kB pagesize do:
*
* echo 4194304 > /proc/sys/kernel/shmall
*/
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/mman.h>
#ifndef SHM_HUGETLB
#define SHM_HUGETLB 04000
#endif
#define LENGTH (256UL*1024*1024)
#define dprintf(x) printf(x)
/* Only ia64 requires this */
#ifdef __ia64__
#define ADDR (void *)(0x8000000000000000UL)
#define SHMAT_FLAGS (SHM_RND)
#else
#define ADDR (void *)(0x0UL)
#define SHMAT_FLAGS (0)
#endif
int main(void)
{
int shmid;
unsigned long i;
char *shmaddr;
if ((shmid = shmget(2, LENGTH,
SHM_HUGETLB | IPC_CREAT | SHM_R | SHM_W)) < 0) {
perror("shmget");
exit(1);
}
printf("shmid: 0x%x
", shmid);
shmaddr = shmat(shmid, ADDR, SHMAT_FLAGS);
if (shmaddr == (char *)-1) {
perror("Shared memory attach failure");
shmctl(shmid, IPC_RMID, NULL);
exit(2);
}
printf("shmaddr: %p
", shmaddr);
dprintf("Starting the writes:
");
for (i = 0; i < LENGTH; i++) {
shmaddr[i] = (char)(i);
if (!(i % (1024 * 1024)))
dprintf(".");
}
dprintf("
");
dprintf("Starting the Check...");
for (i = 0; i < LENGTH; i++)
if (shmaddr[i] != (char)i)
printf("
Index %lu mismatched
", i);
dprintf("Done.
");
if (shmdt((const void *)shmaddr) != 0) {
perror("Detach failure");
shmctl(shmid, IPC_RMID, NULL);
exit(3);
}
shmctl(shmid, IPC_RMID, NULL);
return 0;
}
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