[Devel] [PATCH RHEL7 COMMIT] Revert "mm: Change formula of calculation of default min_free_kbytes"

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-862.11.6.vz7.71.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-862.11.6.vz7.71.14
------>
commit ef5829e402f632dcf0b68e9bee5cf7f4f0a26ed2
Author: Kirill Tkhai <ktkhai at virtuozzo.com>
Date:   Wed Sep 26 18:18:29 2018 +0300

    Revert "mm: Change formula of calculation of default min_free_kbytes"
    
    This reverts commit 923c3bfcf26b7666257231689334504f73b3497e.
    
    We do not need this crutch after shrink_slab() improving
    patchset is implemented. It was used to protect the system
    against agressive memory eaters like dd on original jira BUG.
    
    But now we are safe, and since the test shows that defaults
    have better performance, we restore them.
    
    http://perf.sw.ru/2/cmp/1701?html=1#chart1
    https://jira.sw.ru/browse/PSBM-69296
    
    Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 mm/page_alloc.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3d22c4301dc9..bb4d34f923ba 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6586,6 +6586,27 @@ void setup_per_zone_wmarks(void)
 
 /*
  * Initialise min_free_kbytes.
+ *
+ * For small machines we want it small (128k min).  For large machines
+ * we want it large (64MB max).  But it is not linear, because network
+ * bandwidth does not increase linearly with machine size.  We use
+ *
+ * 	min_free_kbytes = 4 * sqrt(lowmem_kbytes), for better accuracy:
+ *	min_free_kbytes = sqrt(lowmem_kbytes * 16)
+ *
+ * which yields
+ *
+ * 16MB:	512k
+ * 32MB:	724k
+ * 64MB:	1024k
+ * 128MB:	1448k
+ * 256MB:	2048k
+ * 512MB:	2896k
+ * 1024MB:	4096k
+ * 2048MB:	5792k
+ * 4096MB:	8192k
+ * 8192MB:	11584k
+ * 16384MB:	16384k
  */
 int __meminit init_per_zone_wmark_min(void)
 {
@@ -6593,14 +6614,14 @@ int __meminit init_per_zone_wmark_min(void)
 	int new_min_free_kbytes;
 
 	lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
-	new_min_free_kbytes = lowmem_kbytes * 2 / 100; /* 2% */
+	new_min_free_kbytes = int_sqrt(lowmem_kbytes * 16);
 
 	if (new_min_free_kbytes > user_min_free_kbytes) {
 		min_free_kbytes = new_min_free_kbytes;
 		if (min_free_kbytes < 128)
 			min_free_kbytes = 128;
-		if (min_free_kbytes > 4194304)
-			min_free_kbytes = 4194304;
+		if (min_free_kbytes > 65536)
+			min_free_kbytes = 65536;
 	} else {
 		pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\n",
 				new_min_free_kbytes, user_min_free_kbytes);