[Devel] [PATCH RHEL7 COMMIT] mm, fs: make vfs_cache_min_ratio=0 by default

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-693.1.1.vz7.37.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-693.1.1.vz7.37.12
------>
commit 592e27fa3be43f9a54b1d24751da764dc332591e
Author: Andrey Ryabinin <aryabinin at virtuozzo.com>
Date:   Thu Oct 12 12:57:45 2017 +0300

    mm,fs: make vfs_cache_min_ratio=0 by default
    
    vfs_cache_min_ratio is a sysctl ported from PCS6. It is supposed
    to increase performance, but it was noticed to do the opposite
    https://jira.sw.ru/browse/PSBM-68644.
    
    It supposed to protect slabs from reclaiming if their percentage is lower
    than the sysctl value (2% by default).
    However, it's not quite clear in what scenario it supposed to improve
    performance. Moreover, I don't quite understand how is this 2% protection
    supposed to change anything in reclaim logic. If we have relatively
    small number of reclaimable slab objects  they won't be reclaimed anyway
    because of priority protection. The following simple test:
    
    	#!/bin/bash
    	mkdir -p /vz/tst/
    	mkdir -p /sys/fs/cgroup/memory/tst
    
    	for i in {1..4} ; do
    		mkdir -p /sys/fs/cgroup/memory/tst/test$i
    		# 1G
    		echo $((1024*1024*1024)) > /sys/fs/cgroup/memory/tst/test$i/memory.limit_in_bytes
    		echo $$ > /sys/fs/cgroup/memory/tst/test$i/tasks
    		mkdir -p /vz/tst/test$i
    
    		#fill slab_reclaimable
    		stat /vz/tst/test$i > /dev/null
    		for j in {1..10065} ; do
    			mkdir -p /vz/tst/test$i/test$j
    			stat /vz/tst/test$i/test$j > /dev/null
    		done
    
    		#read 1G file to fill page cache in memcg.
    		cat /vz/test$i > /dev/null
    	done
    
    	cat /sys/fs/cgroup/memory/tst/memory.stat
    	echo $$ > /sys/fs/cgroup/memory/tasks
    
    	#create memory pressure, sizeof /vz/fill_ram
    	#should be bigger than total ram.
    	cat /vz/fill_ram > /dev/null
    
    show that there is no much difference between 0 and 2% settings.
    In both cases we prefer to reclaim page cache.
    
    So let's set vfs_cache_min_ratio to 0 and see how it goes.
    
    https://jira.sw.ru/browse/PSBM-69672
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 fs/dcache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 2cb018a..ce8e24f 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -86,7 +86,7 @@
 int sysctl_vfs_cache_pressure __read_mostly = 100;
 EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);
 
-int sysctl_vfs_cache_min_ratio __read_mostly = 2;
+int sysctl_vfs_cache_min_ratio __read_mostly = 0;
 
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);