The output of free has a total of four lines, the fourth behavior Swap area information, which is the total amount of the exchange, the amount of usage (used), and the number of idle swap areas, which are relatively clear, not too much.
The second and third rows in the free output are confusing. Both of these lines describe memory usage. The first column is total, the second column is the usage (used), and the third column is the available amount (free).
The first line of output is viewed from the operating system (OS). That is, from the OS point of view, the computer has a total of:
Here we get the first equation:
FO[2][4] indicates that the memory that is shared by several processes is now deprecated, and its value is always 0 (it may not be 0 on some systems, depending on how the free command is implemented).
FO[2][5] represents the memory that is trapped by the OS buffer. FO[2][6] Indicates the memory of the OS cache. In some cases, the two words buffer and cache are often mixed. But in some low-level software is to distinguish between the two words, look at the foreigner's Foreign Language:
That is, buffer is used to store the data to be output to disk (block device), and the cache is to store the data read from disk. Both are designed to improve IO performance and are managed by the OS.
Linux and other mature operating systems (such as Windows), in order to improve the performance of IO read, always have to cache some data, which is why Fo[2][6] (cached memory) is relatively large, and fo[2][3] relatively small reason.
The output from the second line looks at the system memory usage from an application perspective.
Because the memory consumed by the system cache and buffer can be quickly recycled, fo[3][3] is usually much larger than fo[2][3].
It also uses two equations:
Page cache and buffer cache have always been two more confusing concepts, and there are a lot of people on the internet arguing and guessing what the difference is between the two caches, and the discussion finally has not been a unified and correct conclusion, during my work this time, page The concept of cache and buffer cache has bothered me once, but the two concepts are actually very clear when analyzed carefully. If you can understand the nature of these two caches, then we may be more handy when analyzing IO issues.
The page cache is actually for the file system, the file cache, and the data at the file level is cached to page cache. The logical layer of the file needs to be mapped to the actual physical disk, and this mapping is done by the file system. When the page cache's data needs to be refreshed, the data in the page cache is given to buffer cache, but this processing is simple after the 2.6 kernel, with no real cache operation.
Buffer cache is a caching of disk blocks that, in the absence of a filesystem, caches data directly into the buffer cache, for example, the file system's metadata is cached in buffer cache.
In short, page cache is used to cache file data, and buffer cache is used to cache disk data. In the case of a file system, the data is cached to the page cache, and if the disk is read and written directly using tools such as DD, the data is cached to buffer cache.
To add, a Def_blk_ops file operation method is assigned to each device at the filesystem level, which is how the device operates, and there is a radix tree underneath each device's inode, which will place the page of the cached data under the radix tree. The page number will be displayed in the buffer column of the top program. If the device makes a file system, it generates an inode, which allocates operations such as Ext3_ops, which is the method of the file system, where there is also a radix tree under the inode, which caches the page of the file. The number of cached pages is counted in the cache column of the top program. As can be seen from the above analysis, the buffer cache and page cache in the 2.6 kernel are consistent in processing, but there is a conceptual difference between the page cache Cache,buffer for the file is the cache for the disk block data, that's all.
The difference between buffer and cache
A buffer is something that have yet to being "written" to disk. A cache is something that have been "read" from the disk and stored for later use.
For a more detailed explanation reference: difference between Buffer and Cache
For shared memory, which is primarily used to share data between different processes in a UNIX environment, is a method of interprocess communication, and the general application does not request the use of shared memory, nor does the author verify the effect of shared memory on the above equation. If you are interested, please refer to: What is Shared Memory?
The difference between cache and buffer:
Cache: Caching is a small but high-speed memory that sits between the CPU and the main memory. Because the CPU speed is much higher than the main memory, the CPU accesses the data directly from the memory to wait for a certain period of time, the cache holds the CPU just used or recycled part of the data, when the CPU re-use the part of the data can be directly called from the cache, which reduces the CPU waiting time, Improve the efficiency of the system. The cache is also divided into one-level cache (L1 cache) and level two cache (L2 cache), L1 cache is integrated within the CPU, L2 cache is usually soldered to the motherboard, and is now integrated into the CPU, with a common capacity of 256KB or 512KB L2 Cache.
Buffer: An area where data is transferred between devices that are not synchronized or that have different priority levels. Through buffers, you can reduce the number of waits between processes, so that when you read data from a slow device, the operating process of a fast device is uninterrupted.
Buffer and cache in free: (They are all memory-intensive):
Buffer: Memory as buffer cache, which is the read and write buffer of the block device
Cache: As the page cache memory, the file system cache
If the cache has a large value, it indicates that the cache has a high number of files. If frequently accessed files can be cache, then the disk read IO must be very small.
The output of free has a total of four lines, the fourth behavior Swap area information, which is the total amount of the exchange, the amount of usage (used), and the number of idle swap areas, which are relatively clear, not too much.
The second and third rows in the free output are confusing. Both of these lines describe memory usage. The first column is total, the second column is the usage (used), and the third column is the available amount (free).
The first line of output is viewed from the operating system (OS). That is, from the OS point of view, the computer has a total of:
Here we get the first equation:
FO[2][4] indicates that the memory that is shared by several processes is now deprecated, and its value is always 0 (it may not be 0 on some systems, depending on how the free command is implemented).
FO[2][5] represents the memory that is trapped by the OS buffer. FO[2][6] Indicates the memory of the OS cache. In some cases, the two words buffer and cache are often mixed. But in some low-level software is to distinguish between the two words, look at the foreigner's Foreign Language:
That is, buffer is used to store the data to be output to disk (block device), and the cache is to store the data read from disk. Both are designed to improve IO performance and are managed by the OS.
Linux and other mature operating systems (such as Windows), in order to improve the performance of IO read, always have to cache some data, which is why Fo[2][6] (cached memory) is relatively large, and fo[2][3] relatively small reason.
The output from the second line looks at the system memory usage from an application perspective.
Because the memory consumed by the system cache and buffer can be quickly recycled, fo[3][3] is usually much larger than fo[2][3].
It also uses two equations:
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