linux系统性能相关参数介绍

Linux系统cache的释放：
通过文件/proc/sys/vm/drop_caches来控制已使用内存的控制：
控制已经使用cache的释放方式：
drop_caches的详细文档如下：
Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.
To free pagecache:
* echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
* echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
* echo 3 > /proc/sys/vm/drop_caches
As this is a non-destructive operation, and dirty objects are notfreeable, the user should run “sync” first in order to make sure allcached objects are freed.
This tunable was added in 2.6.16.

 /proc/sys/vm/min_free_kbytes

该文件表示强制Linux VM最低保留多少空闲内存（Kbytes）。
缺省设置：724（512M物理内存）

 /proc/sys/vm/overcommit_memory

该文件指定了内核针对内存分配的策略，其值可以是0、1、2。
0， 表示内核将检查是否有足够的可用内存供应用进程使用；如果有足够的可用内存，内存申请允许；否则，内存申请失败，并把错误返回给应用进程。
1， 表示内核允许分配所有的物理内存，而不管当前的内存状态如何。
2， 表示内核允许分配超过所有物理内存和交换空间总和的内存（参照overcommit_ratio）。
缺省设置：0

/proc/sys/vm/overcommit_ratio

该文件表示，如果overcommit_memory=2，可以过载内存的百分比，通过以下公式来计算系统整体可用内存。
系统可分配内存=交换空间+物理内存*overcommit_ratio/100

/proc/sys/vm/swapiness

该文件表示系统进行交换行为的程度，数值（0-100）越高，越可能发生磁盘交换。

/proc/sys/vm/vfs_cache_pressure

该文件表示内核回收用于directory和inode cache内存的倾向

/proc/sys/vm/page-cluster

该文件表示在写一次到swap区的时候写入的页面数量，0表示1页，1表示2页，2表示4页。
缺省设置：3（2的3次方，8页）

关于page cache一些相关系统参数设置

• vm.dirty_background_ratio:
  is the percentage of system memory that can be filled with “dirty” pages — memory pages that still need to be written to disk — before the pdflush/flush/kdmflush background processes kick in to write it to disk. My example is 10%, so if my virtual server has 32 GB of memory that’s 3.2 GB of data that can be sitting in RAM before something is done.

• vm.dirty_ratio:

  is the absolute maximum amount of system memory that can be filled with dirty pages before everything must get committed to disk. When the system gets to this point all new I/O blocks until dirty pages have been written to disk. This is often the source of long I/O pauses, but is a safeguard against too much data being cached unsafely in memory.

• vm.dirty_background_bytes & vm.dirty_bytes:

are another way to specify these parameters. If you set the _bytes version the _ratio version will become 0, and vice-versa.

• vm.dirty_expire_centisecs：

is how long something can be in cache before it needs to be written. In this case it’s 30 seconds. When the pdflush/flush/kdmflush processes kick in they will check to see how old a dirty page is, and if it’s older than this value it’ll be written asynchronously to disk. Since holding a dirty page in memory is unsafe this is also a safeguard against data loss.

• vm.dirty_writeback_centisecs：

is how often the pdflush/flush/kdmflush processes wake up and check to see if work needs to be done.

动态修改系统配置参数，立刻生效，但不具有持久性（系统重启就会丢失）

$echo {parameters} /proc/sys/vm/{options}or$sysctl  -w variable=value

修改sysctl.conf系统配置文件，然后执行sysctl -p，重新加载生效，具有持久性：

$vim /etc/sysctl.conf$sysctl -p