[PATCH 0/4] ksm - dynamic page sharing driver for linux v2

[Izik Eidus <ieidus@redhat.com>]

(From v1 to v2 the main change is much more documentation)

KSM is a linux driver that allows dynamicly sharing identical memory
pages between one or more processes.

Unlike tradtional page sharing that is made at the allocation of the
memory, ksm do it dynamicly after the memory was created.
Memory is periodically scanned; identical pages are identified and
merged.
The sharing is unnoticeable by the process that use this memory.
(the shared pages are marked as readonly, and in case of write
do_wp_page() take care to create new copy of the page)

This driver is very useful for KVM as in cases of runing multiple guests
operation system of the same type.
(For desktop work loads we have achived more than x2 memory overcommit
(more like x3))

This driver have found users other than KVM, for example CERN,
Fons Rademakers:
"on many-core machines we run one large detector simulation program per core.
These simulation programs are identical but run each in their own process and
need about 2 - 2.5 GB RAM.
We typically buy machines with 2GB RAM per core and so have a problem to run
one of these programs per core.
Of the 2 - 2.5 GB about 700MB is identical data in the form of magnetic field
maps, detector geometry, etc.
Currently people have been trying to start one program, initialize the geometry
and field maps and then fork it N times, to have the data shared.
With KSM this would be done automatically by the system so it sounded extremely
attractive when Andrea presented it."

(We have are already started to test KSM on their systems...)

KSM can run as kernel thread or as userspace application or both

example for how to control the kernel thread:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>
#include "ksm.h"

int main(int argc, char *argv[])
{
	int fd;
	int used = 0;
	int fd_start;
	struct ksm_kthread_info info;
	

	if (argc < 2) {
		fprintf(stderr,
			"usage: %s {start npages sleep | stop | info}\n",
			argv[0]);
		exit(1);
	}

	fd = open("/dev/ksm", O_RDWR | O_TRUNC, (mode_t)0600);
	if (fd == -1) {
		fprintf(stderr, "could not open /dev/ksm\n");
		exit(1);
	}

	if (!strncmp(argv[1], "start", strlen(argv[1]))) {
		used = 1;
		if (argc < 4) {
			fprintf(stderr,
		    "usage: %s start npages_to_scan max_pages_to_merge sleep\n",
		    argv[0]);
			exit(1);
		}
		info.pages_to_scan = atoi(argv[2]);
		info.max_pages_to_merge = atoi(argv[3]);
		info.sleep = atoi(argv[4]);
		info.flags = ksm_control_flags_run;

		fd_start = ioctl(fd, KSM_START_STOP_KTHREAD, &info);
		if (fd_start == -1) {
			fprintf(stderr, "KSM_START_KTHREAD failed\n");
			exit(1);
		}
		printf("created scanner\n");
	}

	if (!strncmp(argv[1], "stop", strlen(argv[1]))) {
		used = 1;
		info.flags = 0;
		fd_start = ioctl(fd, KSM_START_STOP_KTHREAD, &info);
		printf("stopped scanner\n");
	}

	if (!strncmp(argv[1], "info", strlen(argv[1]))) {
		used = 1;
		ioctl(fd, KSM_GET_INFO_KTHREAD, &info);
	 printf("flags %d, pages_to_scan %d npages_merge %d, sleep_time %d\n",
	 info.flags, info.pages_to_scan, info.max_pages_to_merge, info.sleep);
	}

	if (!used)
		fprintf(stderr, "unknown command %s\n", argv[1]);

	return 0;
}

example of how to register qemu to ksm (or any userspace application)

diff --git a/qemu/vl.c b/qemu/vl.c
index 4721fdd..7785bf9 100644
--- a/qemu/vl.c
+++ b/qemu/vl.c
@@ -21,6 +21,7 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  * DEALINGS IN
  * THE SOFTWARE.
  */
+#include "ksm.h"
 #include "hw/hw.h"
 #include "hw/boards.h"
 #include "hw/usb.h"
@@ -5799,6 +5800,37 @@ static void termsig_setup(void)
 
 #endif
 
+int ksm_register_memory(void)
+{
+    int fd;
+    int ksm_fd;
+    int r = 1;
+    struct ksm_memory_region ksm_region;
+
+    fd = open("/dev/ksm", O_RDWR | O_TRUNC, (mode_t)0600);
+    if (fd == -1)
+        goto out;
+
+    ksm_fd = ioctl(fd, KSM_CREATE_SHARED_MEMORY_AREA);
+    if (ksm_fd == -1)
+        goto out_free;
+
+    ksm_region.npages = phys_ram_size / TARGET_PAGE_SIZE;
+    ksm_region.addr = phys_ram_base;
+    r = ioctl(ksm_fd, KSM_REGISTER_MEMORY_REGION, &ksm_region);
+    if (r)
+        goto out_free1;
+
+    return r;
+
+out_free1:
+    close(ksm_fd);
+out_free:
+    close(fd);
+out:
+    return r;
+}
+
 int main(int argc, char **argv)
 {
 #ifdef CONFIG_GDBSTUB
@@ -6735,6 +6767,8 @@ int main(int argc, char **argv)
     /* init the dynamic translator */
     cpu_exec_init_all(tb_size * 1024 * 1024);
 
+    ksm_register_memory();
+
     bdrv_init();
 
     /* we always create the cdrom drive, even if no disk is there */

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