On Linux memory management mechanism

Often encounter some new Linux novice will ask how much memory consumption?
In Linux often found that the free memory is very small, it seems that all the memory is occupied by the system, the surface of the memory is not enough to use, it is not. This is an excellent feature of Linux memory management, which differs from Windows memory management in this regard. The main feature is that no matter how large the physical memory is, Linux is fully utilized to read some program-called hard disk data into memory, using the high-speed features of memory read and write to improve the data access performance of Linux system. Instead, Windows allocates memory for the application only when it needs memory, and does not take full advantage of the large capacity of the memory space. In other words, with each additional physical memory, Linux will be able to take full advantage of the benefits of hardware investment, and Windows will only do it as a device, even if the increase of 8GB or even larger.
This feature of Linux, mainly uses the free physical memory, divides the part space, as the cache, buffers, improves the data access performance.
The page cache (cache) is a primary disk cache implemented by the Linux kernel. It is primarily used to reduce I/O operations on disks. Specifically, by caching the data in the disk into physical memory, access to the disk becomes access to physical memory.
The value of the disk cache is two: first, accessing the disk is much slower than accessing the memory, so accessing the data from memory is faster than accessing it from disk. Second, once the data is accessed, it is likely to be accessed again in the short term.
Here's a look at the Linux memory management mechanism:
a physical memory and virtual memory
We know that reading and writing data directly from physical memory is much faster than reading and writing data from a hard disk, so we want all of the data read and written to be done in memory, and memory is limited, which leads to the concept of physical and virtual memory.
Physical memory is the amount of memory provided by the system hardware, is the real memory, relative to the physical memory, under Linux there is a virtual memory concept, virtual memory is to meet the lack of physical memory of the proposed strategy, it is the use of disk space virtual out of a piece of logical memory, The disk space used as virtual memory is called swap space.
As an extension of physical memory, Linux will use the virtual memory of the swap partition when physical memory is low, in more detail, the kernel will write the memory block information that is temporarily unused to the swap space, so that the physical memory has been released, this memory can be used for other purposes, when the original content needs to be used, This information is re-read into the physical memory from the swap space.
Linux memory management is a paging access mechanism, in order to ensure that the physical memory can be fully utilized, the kernel will be in the physical memory of infrequently used data blocks automatically swapped into virtual memory, and the information often used to retain the physical memory.
To learn more about Linux memory operating mechanisms, you need to know the following:
The Linux system will occasionally make a paging operation to keep as much free physical memory as possible, even if there is nothing to do with memory, and Linux will swap out the memory pages that are temporarily unused. This avoids the time required to wait for the interchange.
Linux for the page exchange is conditional, not all the pages are swapped to virtual memory when not in use, the Linux kernel based on the "most Frequently used" algorithm, only some infrequently used paging files to virtual memory, sometimes we will see a phenomenon: Linux physical memory is still many, But the swap space is also used a lot. In fact, this is not surprising, for example, a very large memory of the process to run, the need to spend a lot of memory resources, there will be some infrequently used paging file is swapped into virtual memory, but later this memory-intensive process ended and freed a lot of memory, the page just swapped out The file is not automatically swapped into physical memory, unless it is necessary, then the system physical memory will be much more idle, while the swap space is also being used, there is a phenomenon just said. Don't worry about that, just know what's going on.
The swap space pages are swapped to physical memory first, and if there is not enough physical memory to accommodate them, they will be swapped out immediately, so there may not be enough space in the virtual memory to store the swap pages, which can eventually lead to a fake crash, service exception, etc. While Linux can recover itself over time, the restored system is largely unusable.
Therefore, it is very important to plan and design the use of Linux memory rationally.
two monitoring of memory
As a Linux system administrator, monitoring memory usage status is very important, through monitoring to help understand the use of memory state, such as memory consumption is normal, memory shortage, and so on, the most commonly used to monitor memory commands have free, top, etc., the following is the output of a system free:
[[email protected] ~]# free
Total used free shared buffers Cached
mem:3894036 3473544 420492 0 72972 1332348
-/+ buffers/cache:2068224 1825812
swap:4095992 906036 3189956
Meaning of each option:
First line:
Total: Overall size of physical memory
Used: The amount of physical memory already in use
Free: idle physical memory size
Shared: The amount of memory that multiple processes share
buffers/cached: Size of the disk cache
Second line of Mem: representing physical memory usage
Third row (-/+ buffers/cached): Represents the disk cache usage status
Line four: Swap indicates swap space memory usage state
The memory state of the free command output can be viewed in two angles: one from the kernel point of view, from the perspective of the application layer
view the state of memory from the perspective of the kernel
Is that the kernel can now be directly assigned to, do not need additional operations, that is, the output of the above free command of the second mem value, it can be seen that this system physical memory 3894036K, free memory only 420492K, that is, 40M a little more, we do a calculation such as:
3894036-3473544 = 420492
In fact, the total physical memory minus the physical memory that has been used to get the free physical memory size, note that the available memory value of 420492 does not contain the memory size in the buffers and cached states.
If you think that the system is too small, you are wrong, in fact, the kernel completely control the use of memory, Linux will need to memory, or when the system is running progressively, the buffers and cached state memory into the Free State of memory for the system to use.
use state of system memory from the perspective of the application layer
That is, the amount of memory that an application running on Linux can use, that is, the output of the third line of the free command-/+ buffers/cached, and you can see that the memory used by this system is 2068224K, and the idle memory reaches 1825812K, and continues to do such a calculation:
420492+ (72972+1332348) =1825812
This equation shows that the physical memory value available to an application is the sum of the free value of the MEM item plus the buffers and cached values, that is, the value of this value is buffers and cached item size, and for applications, buffers/ Cached occupied memory is available because buffers/cached is designed to improve the performance of file reads, and when applications need to use memory, buffers/cached is quickly recycled for application use.
bufferssimilarities and differences with the cached
In the Linux operating system, when the application needs to read the data in the file, the operating system allocates some memory, reads the data from the disk into the memory, and then distributes the data to the application, and when the data needs to be written to the file, the operating system allocates the memory to receive the user data first. The data is then written from memory to disk. However, if there is a large amount of data that needs to be read from disk to memory or written to disk by memory, the system's read and write performance becomes very low, since it is a time-consuming and resource-intensive process to read data from disk or write data to disk, in which case Linux introduces buffers and Cached mechanism.
Buffers and cached are memory operations that are used to save files and file attribute information that have been opened by the system, so that when the operating system needs to read some files, it will first look in the buffers and cached memory areas, and if found, read them directly to the application. If you do not find the data needed to read from disk, this is the operating system caching mechanism, through the cache, greatly improve the performance of the operating system. But the content of buffers and cached buffer is different.
Buffers is used to buffer the block device, it only records the file system metadata (metadata) and tracking in-flight pages, and cached is used to buffer the file. More commonly said: buffers is mainly used to store content in the directory, file attributes and permissions and so on. and cached is used directly to memorize the files and programs we have opened.
In order to verify our conclusion is correct, you can open a very large file by VI, look at the change of cached, and then again VI this file, feel how the speed of two times to open the similarities and differences, is not the second opening speed significantly faster than the first time?
Then execute the following command: see if the value of buffers changes, and then repeat the Find command to see how the two-time display is different.
Linux operating system memory operation Principle, is largely based on the needs of the server design, such as the system's buffering mechanism will be used to cache files and data in cached, Linux is always trying to cache more data and information, so that the need for this data can be directly from the memory Without the need for a lengthy disk operation, this design approach improves the overall performance of the system.

Design ideas improve the overall performance of the system.
In-depth understanding of Linux memory Management mechanisms: reference: http://ixdba.blog.51cto.com/2895551/541355

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On Linux memory management mechanism