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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
2The intent of this file is to give a brief summary of hugetlbpage support in
3the Linux kernel. This support is built on top of multiple page size support
4that is provided by most modern architectures. For example, i386
5architecture supports 4K and 4M (2M in PAE mode) page sizes, ia64
6architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
7256M and ppc64 supports 4K and 16M. A TLB is a cache of virtual-to-physical
8translations. Typically this is a very scarce resource on processor.
9Operating systems try to make best use of limited number of TLB resources.
10This optimization is more critical now as bigger and bigger physical memories
11(several GBs) are more readily available.
12
13Users can use the huge page support in Linux kernel by either using the mmap
14system call or standard SYSv shared memory system calls (shmget, shmat).
15
Muli Ben-Yehuda5c7ad512005-11-07 00:59:42 -080016First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
17(present under "File systems") and CONFIG_HUGETLB_PAGE (selected
18automatically when CONFIG_HUGETLBFS is selected) configuration
19options.
Linus Torvalds1da177e2005-04-16 15:20:36 -070020
21The kernel built with hugepage support should show the number of configured
Muli Ben-Yehuda5c7ad512005-11-07 00:59:42 -080022hugepages in the system by running the "cat /proc/meminfo" command.
Linus Torvalds1da177e2005-04-16 15:20:36 -070023
24/proc/meminfo also provides information about the total number of hugetlb
25pages configured in the kernel. It also displays information about the
26number of free hugetlb pages at any time. It also displays information about
27the configured hugepage size - this is needed for generating the proper
28alignment and size of the arguments to the above system calls.
29
Randy Dunlap21a26d42006-04-10 22:53:04 -070030The output of "cat /proc/meminfo" will have lines like:
Linus Torvalds1da177e2005-04-16 15:20:36 -070031
32.....
33HugePages_Total: xxx
34HugePages_Free: yyy
Randy Dunlap5e122272006-04-18 22:21:51 -070035HugePages_Rsvd: www
36Hugepagesize: zzz kB
37
38where:
39HugePages_Total is the size of the pool of hugepages.
40HugePages_Free is the number of hugepages in the pool that are not yet
41allocated.
42HugePages_Rsvd is short for "reserved," and is the number of hugepages
43for which a commitment to allocate from the pool has been made, but no
44allocation has yet been made. It's vaguely analogous to overcommit.
Linus Torvalds1da177e2005-04-16 15:20:36 -070045
46/proc/filesystems should also show a filesystem of type "hugetlbfs" configured
47in the kernel.
48
49/proc/sys/vm/nr_hugepages indicates the current number of configured hugetlb
50pages in the kernel. Super user can dynamically request more (or free some
Muli Ben-Yehuda5c7ad512005-11-07 00:59:42 -080051pre-configured) hugepages.
52The allocation (or deallocation) of hugetlb pages is possible only if there are
Linus Torvalds1da177e2005-04-16 15:20:36 -070053enough physically contiguous free pages in system (freeing of hugepages is
Randy Dunlap21a26d42006-04-10 22:53:04 -070054possible only if there are enough hugetlb pages free that can be transferred
Linus Torvalds1da177e2005-04-16 15:20:36 -070055back to regular memory pool).
56
Randy Dunlap21a26d42006-04-10 22:53:04 -070057Pages that are used as hugetlb pages are reserved inside the kernel and cannot
58be used for other purposes.
Linus Torvalds1da177e2005-04-16 15:20:36 -070059
60Once the kernel with Hugetlb page support is built and running, a user can
61use either the mmap system call or shared memory system calls to start using
62the huge pages. It is required that the system administrator preallocate
Muli Ben-Yehuda5c7ad512005-11-07 00:59:42 -080063enough memory for huge page purposes.
Linus Torvalds1da177e2005-04-16 15:20:36 -070064
65Use the following command to dynamically allocate/deallocate hugepages:
66
67 echo 20 > /proc/sys/vm/nr_hugepages
68
69This command will try to configure 20 hugepages in the system. The success
70or failure of allocation depends on the amount of physically contiguous
71memory that is preset in system at this time. System administrators may want
Randy Dunlap21a26d42006-04-10 22:53:04 -070072to put this command in one of the local rc init files. This will enable the
Linus Torvalds1da177e2005-04-16 15:20:36 -070073kernel to request huge pages early in the boot process (when the possibility
74of getting physical contiguous pages is still very high).
75
76If the user applications are going to request hugepages using mmap system
77call, then it is required that system administrator mount a file system of
78type hugetlbfs:
79
Randy Dunlape73a75f2007-07-15 23:40:52 -070080 mount -t hugetlbfs \
81 -o uid=<value>,gid=<value>,mode=<value>,size=<value>,nr_inodes=<value> \
82 none /mnt/huge
Linus Torvalds1da177e2005-04-16 15:20:36 -070083
84This command mounts a (pseudo) filesystem of type hugetlbfs on the directory
85/mnt/huge. Any files created on /mnt/huge uses hugepages. The uid and gid
86options sets the owner and group of the root of the file system. By default
87the uid and gid of the current process are taken. The mode option sets the
88mode of root of file system to value & 0777. This value is given in octal.
89By default the value 0755 is picked. The size option sets the maximum value of
90memory (huge pages) allowed for that filesystem (/mnt/huge). The size is
Randy Dunlap21a26d42006-04-10 22:53:04 -070091rounded down to HPAGE_SIZE. The option nr_inodes sets the maximum number of
Randy Dunlape73a75f2007-07-15 23:40:52 -070092inodes that /mnt/huge can use. If the size or nr_inodes option is not
Linus Torvalds1da177e2005-04-16 15:20:36 -070093provided on command line then no limits are set. For size and nr_inodes
Muli Ben-Yehuda5c7ad512005-11-07 00:59:42 -080094options, you can use [G|g]/[M|m]/[K|k] to represent giga/mega/kilo. For
Randy Dunlape73a75f2007-07-15 23:40:52 -070095example, size=2K has the same meaning as size=2048.
Linus Torvalds1da177e2005-04-16 15:20:36 -070096
97read and write system calls are not supported on files that reside on hugetlb
98file systems.
99
Randy Dunlap21a26d42006-04-10 22:53:04 -0700100Regular chown, chgrp, and chmod commands (with right permissions) could be
Linus Torvalds1da177e2005-04-16 15:20:36 -0700101used to change the file attributes on hugetlbfs.
102
103Also, it is important to note that no such mount command is required if the
104applications are going to use only shmat/shmget system calls. Users who
105wish to use hugetlb page via shared memory segment should be a member of
106a supplementary group and system admin needs to configure that gid into
107/proc/sys/vm/hugetlb_shm_group. It is possible for same or different
Randy Dunlap21a26d42006-04-10 22:53:04 -0700108applications to use any combination of mmaps and shm* calls, though the
109mount of filesystem will be required for using mmap calls.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700110
111*******************************************************************
112
113/*
114 * Example of using hugepage memory in a user application using Sys V shared
115 * memory system calls. In this example the app is requesting 256MB of
116 * memory that is backed by huge pages. The application uses the flag
117 * SHM_HUGETLB in the shmget system call to inform the kernel that it is
118 * requesting hugepages.
119 *
120 * For the ia64 architecture, the Linux kernel reserves Region number 4 for
121 * hugepages. That means the addresses starting with 0x800000... will need
122 * to be specified. Specifying a fixed address is not required on ppc64,
123 * i386 or x86_64.
124 *
125 * Note: The default shared memory limit is quite low on many kernels,
126 * you may need to increase it via:
127 *
128 * echo 268435456 > /proc/sys/kernel/shmmax
129 *
130 * This will increase the maximum size per shared memory segment to 256MB.
131 * The other limit that you will hit eventually is shmall which is the
132 * total amount of shared memory in pages. To set it to 16GB on a system
133 * with a 4kB pagesize do:
134 *
135 * echo 4194304 > /proc/sys/kernel/shmall
136 */
137#include <stdlib.h>
138#include <stdio.h>
139#include <sys/types.h>
140#include <sys/ipc.h>
141#include <sys/shm.h>
142#include <sys/mman.h>
143
144#ifndef SHM_HUGETLB
145#define SHM_HUGETLB 04000
146#endif
147
148#define LENGTH (256UL*1024*1024)
149
150#define dprintf(x) printf(x)
151
152/* Only ia64 requires this */
153#ifdef __ia64__
154#define ADDR (void *)(0x8000000000000000UL)
155#define SHMAT_FLAGS (SHM_RND)
156#else
157#define ADDR (void *)(0x0UL)
158#define SHMAT_FLAGS (0)
159#endif
160
161int main(void)
162{
163 int shmid;
164 unsigned long i;
165 char *shmaddr;
166
167 if ((shmid = shmget(2, LENGTH,
168 SHM_HUGETLB | IPC_CREAT | SHM_R | SHM_W)) < 0) {
169 perror("shmget");
170 exit(1);
171 }
172 printf("shmid: 0x%x\n", shmid);
173
174 shmaddr = shmat(shmid, ADDR, SHMAT_FLAGS);
175 if (shmaddr == (char *)-1) {
176 perror("Shared memory attach failure");
177 shmctl(shmid, IPC_RMID, NULL);
178 exit(2);
179 }
180 printf("shmaddr: %p\n", shmaddr);
181
182 dprintf("Starting the writes:\n");
183 for (i = 0; i < LENGTH; i++) {
184 shmaddr[i] = (char)(i);
185 if (!(i % (1024 * 1024)))
186 dprintf(".");
187 }
188 dprintf("\n");
189
190 dprintf("Starting the Check...");
191 for (i = 0; i < LENGTH; i++)
192 if (shmaddr[i] != (char)i)
193 printf("\nIndex %lu mismatched\n", i);
194 dprintf("Done.\n");
195
196 if (shmdt((const void *)shmaddr) != 0) {
197 perror("Detach failure");
198 shmctl(shmid, IPC_RMID, NULL);
199 exit(3);
200 }
201
202 shmctl(shmid, IPC_RMID, NULL);
203
204 return 0;
205}
206
207*******************************************************************
208
209/*
210 * Example of using hugepage memory in a user application using the mmap
211 * system call. Before running this application, make sure that the
212 * administrator has mounted the hugetlbfs filesystem (on some directory
213 * like /mnt) using the command mount -t hugetlbfs nodev /mnt. In this
214 * example, the app is requesting memory of size 256MB that is backed by
215 * huge pages.
216 *
217 * For ia64 architecture, Linux kernel reserves Region number 4 for hugepages.
218 * That means the addresses starting with 0x800000... will need to be
219 * specified. Specifying a fixed address is not required on ppc64, i386
220 * or x86_64.
221 */
222#include <stdlib.h>
223#include <stdio.h>
224#include <unistd.h>
225#include <sys/mman.h>
226#include <fcntl.h>
227
228#define FILE_NAME "/mnt/hugepagefile"
229#define LENGTH (256UL*1024*1024)
230#define PROTECTION (PROT_READ | PROT_WRITE)
231
232/* Only ia64 requires this */
233#ifdef __ia64__
234#define ADDR (void *)(0x8000000000000000UL)
235#define FLAGS (MAP_SHARED | MAP_FIXED)
236#else
237#define ADDR (void *)(0x0UL)
238#define FLAGS (MAP_SHARED)
239#endif
240
241void check_bytes(char *addr)
242{
243 printf("First hex is %x\n", *((unsigned int *)addr));
244}
245
246void write_bytes(char *addr)
247{
248 unsigned long i;
249
250 for (i = 0; i < LENGTH; i++)
251 *(addr + i) = (char)i;
252}
253
254void read_bytes(char *addr)
255{
256 unsigned long i;
257
258 check_bytes(addr);
259 for (i = 0; i < LENGTH; i++)
260 if (*(addr + i) != (char)i) {
261 printf("Mismatch at %lu\n", i);
262 break;
263 }
264}
265
266int main(void)
267{
268 void *addr;
269 int fd;
270
271 fd = open(FILE_NAME, O_CREAT | O_RDWR, 0755);
272 if (fd < 0) {
273 perror("Open failed");
274 exit(1);
275 }
276
277 addr = mmap(ADDR, LENGTH, PROTECTION, FLAGS, fd, 0);
278 if (addr == MAP_FAILED) {
279 perror("mmap");
280 unlink(FILE_NAME);
281 exit(1);
282 }
283
284 printf("Returned address is %p\n", addr);
285 check_bytes(addr);
286 write_bytes(addr);
287 read_bytes(addr);
288
289 munmap(addr, LENGTH);
290 close(fd);
291 unlink(FILE_NAME);
292
293 return 0;
294}