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groupadd
Used to create a new workgroup
Additional information
groupaddThe command is used to create a new workgroup, and the new workgroup information will be added to the system file.
grammar
groupadd [选项] [参数]
Options
-g: Specify the id of the new workgroup;-r:Create a system workgroup, the group ID of the system workgroup is less than 500;-K: Overwrite configuration file/ect/login.defs;-o: Allows adding workgroups with non-unique group IDs.
parameter
- Group name: Specify the group name of the new workgroup.
Example
Create a new group and set the group ID to join the system:
groupadd -g 344 jsdigname
At this time, /etc/passwdan item with a group ID (GID) of 344 is generated in the file.
free
Show memory usage
Additional information
freeThe command can display the amount of unused and used memory in the current system, and can also display the memory buffer used by the kernel.
grammar
free [选项]
Options
-b: Display memory usage in Byte;-k: Display memory usage in KB;-m: Display memory usage in MB;-g: Display memory usage in GB;-o: Do not display the buffer adjustment column;-s<间隔秒数>: Continuously observe memory usage;-t: Display the memory sum column;-V: Display version information.
Example
Display memory usage information as a sum:
free -t
Periodically query memory usage information and execute the command every 10 seconds:
free -s 10
Show memory usage:
free -m
| total | used | free | shared | buffers | cached |
|---|---|---|---|---|---|
| 2016 | 1973 | 42 | 0 | 163 | 1497 |
| -/+ buffers/cache | 312 | 1703 | |||
| Swap | 4094 | 0 | 4094 |
Explanation of the first part of Memthe line:
total: Total memory;used: The amount of memory that has been used;free: The number of free memory;shared: Currently deprecated;buffers:Number of cache memory;cached: Number of cache memory.
relation:total = used + free
The second part (-/+ buffers/cache) explanation:
(-buffers/cache)Number of used memory:Memin the first part of the lineused – buffers – cached(+buffers/cache)Number of free memory:Memin the first part of the linefree + buffers + cached
It can be seen -buffers/cachethat what is reflected is the memory actually eaten by the program, and +buffers/cachewhat is reflected is the total amount of memory that can be appropriated.
The third part refers to the swap partition.
The fourth line of the output is for the swap partition SWAP, which is what we usually call virtual memory.
memDifference: The difference between used/free in the second line ( ) and used/free in the third line (-/+ buffers/cache). The difference between the two lies in the perspective of use. The first line is from the perspective of the OS. Because for the OS, it is buffers/cachedused, so its available memory is 2098428KB and the used memory is 30841684KB, including, The kernel (OS) uses +buffers+cached used by +Application (X, oracle, etc).
buffers/cachedThe third line refers to the fact that from the application point of view, it is available to the application , because buffer/cached is to improve the performance of file reading. When the application needs to use memory, buffer /cached will be recycled quickly.
So from the perspective of the application, available memory = system free memory + buffers + cached. For example, the available memory of this machine is:
18007156 = 2098428KB + 4545340KB + 11363424KB
Next, we explain when memory will be swapped, and in what way.
When the available memory is less than the rated value, a swap will occur. How to see the rating:
cat /proc/meminfo
Swapping will reduce the number of physical pages used in the system in three ways:
- Reduce buffer and page cache sizes,
- Swap out system V type memory pages,
- Swap out or discard pages (memory pages occupied by Application, that is, insufficient physical memory).
In fact, using swap sparingly does not affect system performance.
So buffersand cachedare both caches, what is the difference between the two?
In order to improve disk access efficiency, Linux has made some elaborate designs. In addition to dentrycaching (used in VFS to accelerate the conversion of file path names to inodes), Linux also adopts two main Cache methods:
- Buffer Cache: for reading and writing disk blocks;
- Page Cache: for reading and writing file inodes.
These Cache effectively shorten the time of I/O system calls (such as read, write, getdents). Disk operations are divided into logical level (file system) and physical level (disk block). These two caches cache logical and physical level data respectively.
Page cache is actually for the file system and is a cache of files. Data at the file level will be cached in the page cache. The logical layer of the file needs to be mapped to the actual physical disk, and this mapping relationship is completed by the file system. When the data in the page cache needs to be refreshed, the data in the page cache is handed over to the buffer cache, because the buffer cache caches disk blocks. However, this processing has become very simple after the 2.6 version of the kernel, and there is no real cache operation.
Buffer cache is a cache for disk blocks. That is, in the absence of a file system, data that directly operates on the disk will be cached in the buffer cache. For example, the metadata of the file system will be cached in the buffer cache.
Simply put, the page cache is used to cache file data, and the buffer cache is used to cache disk data. When there is a file system, when operating on a file, the data will be cached in the page cache; if you directly use ddother tools to read and write to the disk, the data will be cached in the buffer cache.
So when we look at Linux, as long as you don't use swap's swap space, you don't have to worry about having too little memory. If you often use a lot of swap, you may need to consider adding physical memory. This is also the standard used by Linux to determine whether the memory is sufficient.
If it is an application server, you generally only look at the second line, +buffers/cache, which means that the free memory is too little for the application, and it is time to consider optimizing the program or adding memory.
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