Causes of server buffer/cache and buffer/cache release

Catalog

• One 、 What is? buffer/cache？
• Two 、 What is? page cache？
• 3、 ... and 、buffer/cache Some features needing attention
• Four 、 How to manually buffer/cache Recycling ？
• 5、 ... and 、buffer/cache How to check high is caused by those processes
  • 5.1 hcache install
  • 5.2 hcache Common commands

One 、 What is? buffer/cache？

buffer/cache In fact, it is used as the file data cache of the server system , Especially for the process of file existence read/write During operation , So when your service process is reading and writing files ,Linux In order to improve the reading and writing speed of services , The file will be placed here buffer/cache For caching , because Linux The characteristic of service is that everything will exist in the form of documents , So you will find that whether or not you have done large-scale reading and writing to the file , Mechanical buffer/cache It always exists , And keep increasing , This is because the network connection generated by the server is good , User agreement （UDP） Socket, too , The data system in this part will create corresponding file descriptors for the application , And the use of these file descriptors , Will re-enter buffer/cache For reading and writing , So this is also your machine will always exist higher buffer/cache Why ！（ Because all documents will be used for reading and writing buffer/cache, When the memory is reasonable ）

Two 、 What is? page cache？

Page cache It is mainly used as the cache of file data on the file system , Especially when the process has read／write During operation . If you think about it , As a system call that can map files to memory ：mmap Is it natural to use page cache？ In the current system implementation ,page cache It is also used as a cache device of other file types , So in fact page cache Also responsible for most of the block device file cache work .

3、 ... and 、buffer/cache Some features needing attention

When the service memory is sufficient ,Linux In order to speed up the efficiency of reading and writing files, the kernel will put files in buffer/cache in To ensure reading and writing efficiency , But in fact , Although when your application finishes reading and writing files ,buffer/cache This part of memory will not be released automatically , It is reserved as a buffer , Wait until your service process reads and writes the same file the next time , All operations of re initializing the memory are omitted ; So this will lead to , When your application process frequently reads and writes to different files , You will find that the service can be used directly free An important reason why memory will be less and less ; Don't buffer/cache In this endless cache, it will not be automatically released ？ Of course not. , When the server is under high memory pressure , The memory will be recycled automatically , As free Space is allocated to other processes , One of the main memory reclaimed is buffer/cache Buffer memory block for ;

Four 、 How to manually buffer/cache Recycling ？

In addition to reclaiming memory when the system process memory is under great pressure , We can also do it manually buffer/cache Recycling , But because of buffer/cache It is mainly used for reading and writing files , So when you recycle files , Usually associated with a system IO Puma is high , Because the system kernel is also compared cache Whether the data in is consistent with the data in the hard disk , If inconsistent, write , Then the memory can be recycled ;

$ sync
#  Force the data in memory to be flushed to disk first 

 clear Buffer Buffer area 
$ echo 1 > /proc/sys/vm/drop_caches  #  Said clear pagecache.
$ echo 2 > /proc/sys/vm/drop_caches  #  Represents clean recovery slab The object in the allocator （ Includes directory item cache and inode cache ）.slab An allocator is a mechanism for managing memory in the kernel , Many of these cache data implementations are used pagecache.
$ echo 3 > /proc/sys/vm/drop_caches  #  Said clear pagecache and slab The cache object in the allocator 

If you have to clear the disk cache , The first command is the safest in the enterprise and production environment ,"...echo 1> ..." It will only clear the page cache . The third option above is not recommended in production environments "...echo 3 > ..." , Unless you know what you're doing , Because it will clear the cache page , Catalog entries and inodes.

stay Linux Release buffers that may be used by the kernel （Buffer） And caching （Cache） Is it a good idea ？

When you set many settings and want to check the effect , If it is actually specific to I/O Scope benchmarking , Then you may need to clear the buffer and cache . You can delete the cache as shown above , There is no need to restart the system （ That is, no shutdown is required ）.

Linux It is designed to look in the disk cache before looking for the disk . If it finds the resource in the cache , Then the request will not be sent to disk . If we clean up the cache , Disk caching doesn't work , The system will look for resources on the disk .

Besides , When the cache is cleared, it will also slow down the system , The system will load each requested resource into the disk cache again .

Now? , We're going to create one shell Script , Through one cron Scheduling tasks every afternoon 2 Automatic clearing of points RAM cache . Create a shell Script clearcache.sh And add the following line ：

 #!/bin/bash
 #  Be careful , We use  "echo 3", However, it is not recommended to use in the product environment , You should use  echo 1
 echo 3 > /proc/sys/vm/drop_caches"

to clearcache.sh File set execution permissions

 # chmod 755 clearcache.sh

Now? , When you need to clear the memory cache, just call the script .

Now set a daily afternoon 2 Click the scheduled task to clear RAM cache , open crontab Editing .

 # crontab -e

Add the following lines , Save and exit .

 0 3 * * * /path/to/clearcache.sh

5、 ... and 、buffer/cache How to check high is caused by those processes

5.1 hcache install

$ wget https://silenceshell-1255345740.cos.ap-shanghai.myqcloud.com/hcache -O /usr/local/bin/hcache ;\
chmod +x /usr/local/bin/hcache

5.2 hcache Common commands

$ hcache --top 10           #  Global display 10 The largest cached file 
$ lsof /usr/lib/vmware-tools/lib64/libxerces-c-3.1.so/libxerces-c-3.1.so
#  Find the corresponding process according to the file 、pid
$ hcache -pid 1070   #  Get all the file information opened by the current process number 
$ lsof -p 1070     #  Get all the file information opened by the current process number ( A choice )

lsof Command use reference URL ：https://man.linuxde.net/lsof