[PATCH] ksm: change default values to better fit into mainline kernel

[PATCH] ksm: change default values to better fit into mainline kernel

[Izik Eidus]

Now that ksm is in mainline it is better to change the default values
to better fit to most of the users.

This patch change the ksm default values to be:
ksm_thread_pages_to_scan = 100 (instead of 200)
ksm_thread_sleep_millisecs = 20 (like before)
ksm_run = KSM_RUN_STOP (instead of KSM_RUN_MERGE - meaning ksm is
                        disabled by default)
ksm_max_kernel_pages = nr_free_buffer_pages / 4 (instead of 2046)

The important aspect of this patch is: it disable ksm by default, and set
the number of the kernel_pages that can be allocated to be a reasonable
number.

Signed-off-by: Izik Eidus <ieidus@redhat.com>
---
 mm/ksm.c |   14 +++++++++++---
 1 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 37cc373..f7edac3 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -30,6 +30,7 @@
 #include <linux/slab.h>
 #include <linux/rbtree.h>
 #include <linux/mmu_notifier.h>
+#include <linux/swap.h>
 #include <linux/ksm.h>
 
 #include <asm/tlbflush.h>
@@ -162,10 +163,10 @@ static unsigned long ksm_pages_unshared;
 static unsigned long ksm_rmap_items;
 
 /* Limit on the number of unswappable pages used */
-static unsigned long ksm_max_kernel_pages = 2000;
+static unsigned long ksm_max_kernel_pages;
 
 /* Number of pages ksmd should scan in one batch */
-static unsigned int ksm_thread_pages_to_scan = 200;
+static unsigned int ksm_thread_pages_to_scan = 100;
 
 /* Milliseconds ksmd should sleep between batches */
 static unsigned int ksm_thread_sleep_millisecs = 20;
@@ -173,7 +174,7 @@ static unsigned int ksm_thread_sleep_millisecs = 20;
 #define KSM_RUN_STOP	0
 #define KSM_RUN_MERGE	1
 #define KSM_RUN_UNMERGE	2
-static unsigned int ksm_run = KSM_RUN_MERGE;
+static unsigned int ksm_run = KSM_RUN_STOP;
 
 static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
 static DEFINE_MUTEX(ksm_thread_mutex);
@@ -183,6 +184,11 @@ 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);
@@ -1667,6 +1673,8 @@ static int __init ksm_init(void)
 	struct task_struct *ksm_thread;
 	int err;
 
+	ksm_init_max_kernel_pages();
+
 	err = ksm_slab_init();
 	if (err)
 		goto out;
-- 
1.5.6.5

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Re: [PATCH] ksm: change default values to better fit into mainline kernel

[Andrew Morton]

On Wed, 23 Sep 2009 23:05:47 +0300
Izik Eidus <ieidus@redhat.com> wrote:

> +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);
> @@ -1667,6 +1673,8 @@ static int __init ksm_init(void)
>  	struct task_struct *ksm_thread;
>  	int err;
>  
> +	ksm_init_max_kernel_pages();

Was it really worth creating a new function for this?

--- a/mm/ksm.c~ksm-change-default-values-to-better-fit-into-mainline-kernel-fix
+++ a/mm/ksm.c
@@ -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_init_max_kernel_pages();
+	ksm_max_kernel_pages = nr_free_buffer_pages() / 4;
 
 	err = ksm_slab_init();
 	if (err)
_

oh well, whatever.

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Re: [PATCH] ksm: change default values to better fit into mainline kernel

[Hugh Dickins]

On Wed, 23 Sep 2009, Izik Eidus wrote:
> Now that ksm is in mainline it is better to change the default values
> to better fit to most of the users.
> 
> This patch change the ksm default values to be:
> ksm_thread_pages_to_scan = 100 (instead of 200)
> ksm_thread_sleep_millisecs = 20 (like before)
> ksm_run = KSM_RUN_STOP (instead of KSM_RUN_MERGE - meaning ksm is
>                         disabled by default)
> ksm_max_kernel_pages = nr_free_buffer_pages / 4 (instead of 2046)
> 
> The important aspect of this patch is: it disable ksm by default, and set
> the number of the kernel_pages that can be allocated to be a reasonable
> number.
> 
> Signed-off-by: Izik Eidus <ieidus@redhat.com>

You rather caught me by surprise with this one, Izik: I was thinking
more rc7 than rc1 for switching it off; but no problem, you're probably
right to get people into the habit of understanding it's off even when
they choose CONFIG_KSM=y.

There's several reasons why I couldn't Ack your patch, but I see Andrew
is already sending it on to Linus, so here's another to go on top of it:


[PATCH] 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: Hugh Dickins <hugh.dickins@tiscali.co.uk>
---

 Documentation/vm/ksm.txt |   13 +++++++------
 mm/Kconfig               |    4 +++-
 mm/ksm.c                 |   10 ++++------
 3 files changed, 14 insertions(+), 13 deletions(-)

--- 2.6.31-git+izik/Documentation/vm/ksm.txt	2009-09-23 16:05:43.000000000 +0100
+++ linux/Documentation/vm/ksm.txt	2009-09-24 16:07:16.000000000 +0100
@@ -52,15 +52,15 @@ The KSM daemon is controlled by sysfs fi
 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 fr
                    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 differen
 proportion there would also indicate poor use of madvise MADV_MERGEABLE.
 
 Izik Eidus,
-Hugh Dickins, 30 July 2009
+Hugh Dickins, 24 Sept 2009
--- 2.6.31-git+izik/mm/Kconfig	2009-09-23 16:05:56.000000000 +0100
+++ linux/mm/Kconfig	2009-09-24 16:07:16.000000000 +0100
@@ -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).
 
 config DEFAULT_MMAP_MIN_ADDR
         int "Low address space to protect from user allocation"
--- 2.6.31-git+izik/mm/ksm.c	2009-09-24 16:15:26.000000000 +0100
+++ linux/mm/ksm.c	2009-09-24 16:07:16.000000000 +0100
@@ -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_init_max_kernel_pages();
+	ksm_max_kernel_pages = totalram_pages / 4;
 
 	err = ksm_slab_init();
 	if (err)
@@ -1697,6 +1692,9 @@ static int __init ksm_init(void)
 		kthread_stop(ksm_thread);
 		goto out_free2;
 	}
+#else
+	ksm_run = KSM_RUN_MERGE;	/* no way for user to start it */
+
 #endif /* CONFIG_SYSFS */
 
 	return 0;

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Re: [PATCH] ksm: change default values to better fit into mainline kernel

[Izik Eidus]

On 09/24/2009 06:52 PM, Hugh Dickins wrote:
> You rather caught me by surprise with this one, Izik: I was thinking
> more rc7 than rc1 for switching it off;

I thought that after the merge window -> only fixes can get in, but I 
guess I was wrong...

> +#else
> +	ksm_run = KSM_RUN_MERGE;	/* no way for user to start it */
> +
>    

That is a good point, didnt notice that I am blocking the usage of it 
when sysfs is not build.

>   #endif /* CONFIG_SYSFS */
>
>   	return 0;
>    

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Re: [PATCH] ksm: change default values to better fit into mainline kernel

[Hugh Dickins]

On Thu, 24 Sep 2009, Izik Eidus wrote:
> On 09/24/2009 06:52 PM, Hugh Dickins wrote:
> > You rather caught me by surprise with this one, Izik: I was thinking
> > more rc7 than rc1 for switching it off;
> 
> I thought that after the merge window -> only fixes can get in, but I guess I
> was wrong...

Linus much prefers new features in rc1, though sometimes allowed in rc2;
from then on it should be mainly bugfixing, yes, but lots of work does
go in then, and tweaks to defaults, additions to documentation, etc,
are perfectly acceptable.

Hugh

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