vm.swappiness=0在最新内核中可能会OOM，MySQL被意外kill_MySQL
在之前的《LINUX上MYSQL优化三板斧》中，我们建议大家把 vm.swappiness = 0 设置好。来尽量避免MySQL的服务器内存被交换出去。这样Linux在把内存交换出去时更偏向于将cache页交换出去，而不是将inactive页交换出去。详细描述请参考：http://www.woqutech.com/?p=1200。
经常有人会问， vm.swappiness = 0会不会导致Linux在有swap空间的时候也不交换出去，从而导致内存溢出（OOM）。参照《LINUX上MYSQL优化三板斧》介绍，我们知道，这个值只是一个Linux在判断是否交换内存(swap)的一个“倾向”参考值，而并不是说，设置为0以后，Linux就完全不会使用内存交换空间。
但是，在较新的内核中(2.6.32-303.el6及以后)，vm.swappiness = 0 的默认行为修改掉了，这个说法不再成立。设置该参数vm.swappiess=0，有可能导致MySQL数据库所在的系统出现内存溢出。一般来说，MySQL数据库占用的内存是整个服务器中最大的，根据Linux的策略，它会会首先把MySQL给Kill掉（调整 /proc/(pidof -s mysqld)/oom_adj可以改变OOM时kill的优先级），从而导致应用故障等。
这个修改是在内核3.5-rc1中提交的，并且合并到了2.6.32-303.el6及之后的各个版本。先让我们来看看这个patch：
http://gitorious.ti.com/ti-linux-kernel/ti-linux-kernel/commit/fe35004fbf9eaf67482b074a2e032abb9c89b1dd?format=patch
From fe35004fbf9eaf67482b074a2e032abb9c89b1dd Mon Sep 17 00:00:00 2001
From: Satoru Moriya
Date: Tue, 29 May 2012 15:06:47 -0700
Subject: [PATCH] mm: avoid swapping out with swappiness==0
Sometimes we'd like to avoid swapping out anonymous memory. In
particular, avoid swapping out pages of important process or process
groups while there is a reasonable amount of pagecache on RAM so that we
can satisfy our customers' requirements.
OTOH, we can control how aggressive the kernel will swap memory pages with
/proc/sys/vm/swappiness for global and
/sys/fs/cgroup/memory/memory.swappiness for each memcg.
But with current reclaim implementation, the kernel may swap out even if
we set swappiness=0 and there is pagecache in RAM.
This patch changes the behavior with swappiness==0. If we set
swappiness==0, the kernel does not swap out completely (for global reclaim
until the amount of free pages and filebacked pages in a zone has been
reduced to something very very small (nr_free + nr_filebacked
watermark)).
Signed-off-by: Satoru Moriya
Acked-by: Minchan Kim
Reviewed-by: Rik van Riel
Acked-by: Jerome Marchand
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds
---
mm/vmscan.c | 6 +++---
1 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 67a4fd4..ee97530 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1761,10 +1761,10 @@ static void get_scan_count(struct mem_cgroup_zone *mz, struct scan_control *sc,
* proportional to the fraction of recently scanned pages on
* each list that were recently referenced and in active use.
*/
- ap = (anon_prio + 1) * (reclaim_stat->recent_scanned[0] + 1);
+ ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
ap /= reclaim_stat->recent_rotated[0] + 1;
- fp = (file_prio + 1) * (reclaim_stat->recent_scanned[1] + 1);
+ fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
fp /= reclaim_stat->recent_rotated[1] + 1;
spin_unlock_irq(&mz->zone->lru_lock);
@@ -1777,7 +1777,7 @@ out:
unsigned long scan;
scan = zone_nr_lru_pages(mz, lru);
- if (priority || noswap) {
+ if (priority || noswap || !vmscan_swappiness(mz, sc)) {
scan >>= priority;
if (!scan && force_scan)
scan = SWAP_CLUSTER_MAX;
--
1.7.5
就像Satoru Moriya所说的那样，在之前的版本中，就算我们设置了swappiness=0并且RAM中还有pagecache，内核也可能会交换出部分匿名内存页。而为了“满足用户的需求”，这个patch修改了swappiness=0的行为，如果你设置swappiness=0，那么只有在(nr_free + nr_filebacked
在2.6.32-303.el6 RHEL/CentOS及更新版本的内核中，该patch就已经被合并进来：
* Mon Aug 27 2012 Jarod Wilson
...
- [mm] avoid swapping out with swappiness==0 (Satoru Moriya) [787885]
其他分发版本的Linux(比如Debian，Ubuntu)的版本中，请各位自己查阅一下，看看时候已经合并该patch。
RHEL/CentOS 6.3的内核版本是2.6.32-279，而RHEL/CentOS 6.4的内核为2.6.32-358，从这个版本开始，swappiness的行为就已经修改了，使用这个版本及之后版本的同志们需要特别注意一下。
解决的办法其实也很简单，
1、尽量保证Linux操作系统还有足够的内存
2、最新的内核，建议把vm.swappiness设置1
3、考虑设置 /proc/(pidof -s mysqld)/oom_adj为较小的值来尽量避免MySQL由于内存不足而被关闭。
参考：
https://github.com/torvalds/linux/blob/master/mm/vmscan.c
http://gitorious.ti.com/ti-linux-kernel/ti-linux-kernel/commit/fe35004fbf9eaf67482b074a2e032abb9c89b1dd?format=patch
http://www.mysqlperformanceblog.com/2014/04/28/oom-relation-vm-swappiness0-new-kernel/
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