Commit c73602ad authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

ksm: more on default values

Adjust the max_kernel_pages default to a quarter of totalram_pages,
instead of nr_free_buffer_pages() / 4: the KSM pages themselves come from
highmem, and even on a 16GB PAE machine, 4GB of KSM pages would only be
pinning 32MB of lowmem with their rmap_items, so no need for the more
obscure calculation (nor for its own special init function).

There is no way for the user to switch KSM on if CONFIG_SYSFS is not
enabled, so in that case default run to KSM_RUN_MERGE.

Update KSM Documentation and Kconfig to reflect the new defaults.

Signed-off-by: default avatarHugh Dickins <>
Cc: Izik Eidus <>
Cc: Andrea Arcangeli <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 0eca52a9
......@@ -52,15 +52,15 @@ The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
readable by all but writable only by root:
max_kernel_pages - set to maximum number of kernel pages that KSM may use
e.g. "echo 2000 > /sys/kernel/mm/ksm/max_kernel_pages"
e.g. "echo 100000 > /sys/kernel/mm/ksm/max_kernel_pages"
Value 0 imposes no limit on the kernel pages KSM may use;
but note that any process using MADV_MERGEABLE can cause
KSM to allocate these pages, unswappable until it exits.
Default: 2000 (chosen for demonstration purposes)
Default: quarter of memory (chosen to not pin too much)
pages_to_scan - how many present pages to scan before ksmd goes to sleep
e.g. "echo 200 > /sys/kernel/mm/ksm/pages_to_scan"
Default: 200 (chosen for demonstration purposes)
e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan"
Default: 100 (chosen for demonstration purposes)
sleep_millisecs - how many milliseconds ksmd should sleep before next scan
e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
......@@ -70,7 +70,8 @@ run - set 0 to stop ksmd from running but keep merged pages,
set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
set 2 to stop ksmd and unmerge all pages currently merged,
but leave mergeable areas registered for next run
Default: 1 (for immediate use by apps which register)
Default: 0 (must be changed to 1 to activate KSM,
except if CONFIG_SYSFS is disabled)
The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
......@@ -86,4 +87,4 @@ pages_volatile embraces several different kinds of activity, but a high
proportion there would also indicate poor use of madvise MADV_MERGEABLE.
Izik Eidus,
Hugh Dickins, 30 July 2009
Hugh Dickins, 24 Sept 2009
......@@ -224,7 +224,9 @@ config KSM
the many instances by a single resident page with that content, so
saving memory until one or another app needs to modify the content.
Recommended for use with KVM, or with other duplicative applications.
See Documentation/vm/ksm.txt for more information.
See Documentation/vm/ksm.txt for more information: KSM is inactive
until a program has madvised that an area is MADV_MERGEABLE, and
root has set /sys/kernel/mm/ksm/run to 1 (if CONFIG_SYSFS is set).
int "Low address space to protect from user allocation"
......@@ -184,11 +184,6 @@ static DEFINE_SPINLOCK(ksm_mmlist_lock);
sizeof(struct __struct), __alignof__(struct __struct),\
(__flags), NULL)
static void __init ksm_init_max_kernel_pages(void)
ksm_max_kernel_pages = nr_free_buffer_pages() / 4;
static int __init ksm_slab_init(void)
rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
......@@ -1673,7 +1668,7 @@ static int __init ksm_init(void)
struct task_struct *ksm_thread;
int err;
ksm_max_kernel_pages = totalram_pages / 4;
err = ksm_slab_init();
if (err)
......@@ -1697,6 +1692,9 @@ static int __init ksm_init(void)
goto out_free2;
ksm_run = KSM_RUN_MERGE; /* no way for user to start it */
#endif /* CONFIG_SYSFS */
return 0;
Supports Markdown
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment