Linux cached and buffers memory
我们都知道Linux 的free memory是有两种算法的，一种是系统实际剩余的物理内存，一种是加上系统的cached住的临时缓存后的总剩余内存数，如下面标示的两个free值。
total used free shared buffers cached Mem: 1986376 891920 1094456 0 44352 462224 -/+ buffers/cache: 385344 1601032 Swap: 3695604 0 3695604
内核通过一组内核线程pdflush去管理缓存，有三种方式去释放缓存区。
1. 当脏缓存页保留一段时间后，内核就会显式地开始把脏页写到磁盘。
2. 缓存区达到一定阀值时，pdflush会去主动回收。
3. 使用sync、fsync和fdatasync系统调用将缓存区刷新到磁盘。
所以那些buffers、cached中的缓存空间，在系统需要的时候，它会主动让出空间来做它用。
buffers和cached的区别主要是， buffers是块设备的读写缓存区，是物理级的，如你对裸盘的读写。cached是作为page caches，是针对文件系统的页缓存，文件的读写都是缓存在这里面的。当page cache的数据需要刷新时，page cache中的数据交给buffer cache（这种操作在2.6内核后变的简单了，具体怎么简单？）。
手动回收cache的方法：
方法一：
# And free up caches
#
echo Freeing the page cache:
echo 1 > /proc/sys/vm/drop_caches
echo Free dentries and inodes:
echo 2 > /proc/sys/vm/drop_caches
echo Free the page cache, dentries and the inodes:
echo 3 > /proc/sys/vm/drop_caches
当然这种方法也适用与sysctl。
方法二：
fcntl(fd, F_SETFL, O_DIRECT)
对文件描述符设置O_DRECT，会避免cache该文件的内容，直接读写磁盘。在大量数据传输的时候，会避免占用大量缓存，而且传输效率也不会下降什么。
方法三：
降低如下阀值的数值，有利于回收缓存，但会影响性能，这些设置要根据具体环境来调优。
/proc/sys/vm/dirty_ratio
/proc/sys/vm/dirty_background_ratio
/proc/sys/vm/dirty_expire_centisecs
方法四：
sysctl -w vm.vfs_cache_pressure=n (n > 100)
这会使Kernel更勤于回收cache。
2019年9月14日 15:56
Pertaining to larger jobs like an apartment sophisticated, a full price complex, an workplace, a lodge, or a huge leasing area, a fulltime cleaning service is liable in get. Janitorial services might be much more cost efficient than obtaining an in-house better. Plus, you can find a promise from nearly all cleaning companies and also proof involving insurance.
2019年11月14日 00:41
I . t consultants accomodate business clients to extend the i . t department during an organization. Consultants help out companies with the implementation in new technology with the organization.
2020年3月19日 20:05
Reading through the document online as well as watching 24-hour information sites is becoming a lot more popular. The reason being it is actually cheaper and also you get much more news. You can observe what is going on on the planet, as this happens.
2020年3月19日 20:06
Technology within the long-run is actually irrelevant. That's what a person of mine explained when We made the presentation in order to him in regards to a new item. I have been talking concerning the product's functions and advantages and detailed "state-of-the-art technology" or even something to that particular effect, as one of these.
2020年3月19日 20:06
Lots of people mistakenly believe that it is technology that drives development. Yet in the definitions over, that is actually clearly false. It is actually opportunity that defines development and technologies which allows innovation. Think about the traditional "Build a much better mousetrap" instance taught in many business colleges.
2020年3月19日 20:06
Within the sports industry, peak overall performance in sports happens to be a much desired state through players as well as coaches of levels. If the athletes tend to be school kids soccer gamers or Olympians striving for his or her Gold medals, maximum performance within sports offers always drawn athletes as well as coaches as well.
2020年3月19日 20:07
That is literally true in terms of buying a property. To turn into a first-time residence buyer, you must know where and the way to begin your home buying method. The pursuing questions and also answers are already carefully selected to offer a base of basic familiarity with home acquiring.
2020年3月19日 20:08
That is literally true in terms of buying a property. To turn into a first-time residence buyer, you must know where and the way to begin your home buying method. The pursuing questions and also answers are already carefully selected to offer a base of basic familiarity with home acquiring.
2020年4月28日 19:14
For anyone who is recruiting servant maid with the agency, look at its consistency, referrals connected with current buyers, about the maid's insurance coverage, how long there're doing ebay etc. Ask whether or not they guarantee the servants and whether or not they performed any criminal history checks on the maids.
2020年4月28日 19:15
In contrast, if you choosed to go additional route in addition to hire household painters; you're not required to help lift 1 finger. Professionals will work the meet your needs exactly, of course providing you pay these individuals considerably. Painters will be sure that everything is finished according for your requirements and needs.