[PATCH] mm/vmpressure: scale window size based on machine memory and CPU count

[PATCH] mm/vmpressure: scale window size based on machine memory and CPU count

[Benjamin Lee McQueen]

the vmpressure window size was recently fixed at 512 pages regardless
of machine size, this patch makes it scale based on the machine memory
and CPU count.

Signed-off-by: Benjamin Lee McQueen <mcq@disroot.org>
---
 mm/vmpressure.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 3fbb86996c4d..b2989c70dd39 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -32,10 +32,20 @@
  * As the vmscan reclaimer logic works with chunks which are multiple of
  * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
  *
- * TODO: Make the window size depend on machine size, as we do for vmstat
- * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
+ * Window size is now scaled based on RAM and CPU size, similarly to how
+ * vmstat checks them.
  */
-static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
+static unsigned long vmpressure_win;
+
+static int __init vmpressure_win_init(void)
+{
+        unsigned long mem = totalram_pages() >> (27 - PAGE_SHIFT);
+
+	vmpressure_win = SWAP_CLUSTER_MAX * max(16UL,
+		2UL * fls(num_online_cpus()) * (1 + fls(mem)));
+        return 0;
+}
+core_initcall(vmpressure_win_init);
 
 /*
  * These thresholds are used when we account memory pressure through
-- 
2.53.0

Re: [PATCH] mm/vmpressure: scale window size based on machine memory and CPU count

[Andrew Morton]

On Fri, 27 Feb 2026 10:33:33 -0600 Benjamin Lee McQueen <mcq@disroot.org> wrote:

> the vmpressure window size was recently fixed at 512 pages regardless
> of machine size, this patch makes it scale based on the machine memory
> and CPU count.

Why?  Presumably the current code is causing some problem - please
fully describe that.


> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -32,10 +32,20 @@
>   * As the vmscan reclaimer logic works with chunks which are multiple of
>   * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
>   *
> - * TODO: Make the window size depend on machine size, as we do for vmstat
> - * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
> + * Window size is now scaled based on RAM and CPU size, similarly to how
> + * vmstat checks them.
>   */
> -static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
> +static unsigned long vmpressure_win;
> +
> +static int __init vmpressure_win_init(void)
> +{
> +        unsigned long mem = totalram_pages() >> (27 - PAGE_SHIFT);
> +
> +	vmpressure_win = SWAP_CLUSTER_MAX * max(16UL,
> +		2UL * fls(num_online_cpus()) * (1 + fls(mem)));
> +        return 0;
> +}
> +core_initcall(vmpressure_win_init);

Whitespace is odd.

[PATCH] mm/vmpressure: scale window size based on machine memory and CPU count

[Benjamin Lee McQueen]

on systems of different sizes, the fixed 512 page window may not
be suitable and cause excessive false positive memory pressure
notifications.

or should window size be capped to avoid excessive notification
delays on very large systems?

v2: better commit msg, also tried to fix the whitespace.

Signed-off-by: Benjamin Lee McQueen <mcq@disroot.org>
---
 mm/vmpressure.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 3fbb86996c4d..925659f28dcb 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -32,10 +32,20 @@
  * As the vmscan reclaimer logic works with chunks which are multiple of
  * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
  *
- * TODO: Make the window size depend on machine size, as we do for vmstat
- * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
+ * Window size is now scaled based on RAM and CPU size, similarly to how
+ * vmstat checks them.
  */
-static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
+static unsigned long vmpressure_win;
+
+static int __init vmpressure_win_init(void)
+{
+	unsigned long mem = totalram_pages() >> (27 - PAGE_SHIFT);
+
+	vmpressure_win = SWAP_CLUSTER_MAX * max(16UL,
+		2UL * fls(num_online_cpus()) * (1 + fls(mem)));
+	return 0;
+}
+core_initcall(vmpressure_win_init);
 
 /*
  * These thresholds are used when we account memory pressure through
-- 
2.51.0

[PATCH v2] mm/vmpressure: scale window size based on machine memory and CPU count

[Benjamin Lee McQueen]

on systems of different sizes, the fixed 512 page window may not
be suitable and cause excessive false positive memory pressure
notifications.

or should window size be capped to avoid excessive notification
delays on very large systems?

v2: better commit msg, also tried to fix the whitespace.

also my bad for the multiple sends, i couldn't get the format
right..

Signed-off-by: Benjamin Lee McQueen <mcq@disroot.org>
---
 mm/vmpressure.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 3fbb86996c4d..925659f28dcb 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -32,10 +32,20 @@
  * As the vmscan reclaimer logic works with chunks which are multiple of
  * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
  *
- * TODO: Make the window size depend on machine size, as we do for vmstat
- * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
+ * Window size is now scaled based on RAM and CPU size, similarly to how
+ * vmstat checks them.
  */
-static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
+static unsigned long vmpressure_win;
+
+static int __init vmpressure_win_init(void)
+{
+	unsigned long mem = totalram_pages() >> (27 - PAGE_SHIFT);
+
+	vmpressure_win = SWAP_CLUSTER_MAX * max(16UL,
+		2UL * fls(num_online_cpus()) * (1 + fls(mem)));
+	return 0;
+}
+core_initcall(vmpressure_win_init);
 
 /*
  * These thresholds are used when we account memory pressure through
-- 
2.53.0