What is the difference between linux and windows memory?

The difference between Linux memory and Windows memory: 1. Linux uses physical memory first and puts it on the swap partition when the memory is not enough, while Windows uses memory and virtual memory together; 2. Windows always gives memory It is faster to start new programs by leaving a certain amount of free space. However, Linux memory is often fully used. It is necessary to clear out a piece of memory before allocating it to new programs. It is slower to start new programs.

The operating environment of this tutorial: windows10&&linux7.3 system, Dell G3 computer.

What is the difference between Linux and Windows memory?

Linux uses physical memory first. When the physical memory is still free, Linux will not release the memory. The program occupying the memory has been closed. (This part of the memory is used for caching). In other words, even if you have a large amount of memory, it will be full after using it for a period of time. The advantage of this is that it will be faster to start programs that have just been opened or read data that has just been accessed, which is very beneficial to the server.

Difference

windows always leaves a certain amount of free space in the memory. Even if the memory is free, the program will use some virtual memory. , the advantage of this is that it is faster to start a new program, just allocate some free memory to it,

And what about Linux? Since the memory is often fully used, a piece of memory must be cleared first and then allocated to a new program. Therefore, the startup of the new program will be slower.

The Linux kernel basically puts all the data in the memory first. If the memory is not enough, it will put it in the swap partition (virtual memory). The detail is that only frequently used data will be placed in the memory. Infrequently used data will be placed in the memory. The operated data will be gradually placed on the swap partition and written back to the hard disk when appropriate.

The processing method of Windows is to use memory and virtual memory together, rather than focusing on memory operations. The result is that the IO burden is relatively large, which sometimes slows down the processing speed. The philosophy of Linux is to use memory as much as possible, because the speed of memory is more than 100 times faster than the speed of the hard disk.

Expand knowledge

Linux divides physical memory into three levels for management

The physical memory of the system is divided into several nodes (nodes). One node corresponds to a memory cluster bank, that is, each memory cluster is considered a node. (You can use NODE_DATA(node_id) to find the node numbered node_id in the system)

The memory is divided into nodes, and each node is associated with a processor of the system. pg_data_t is used in the kernel to instantiate each node in the system. Nodes are linked to a NULL-terminated pgdat_list linked list, where each node is linked to the next node using the pg_data_tnode_next field. For the UMA structure, only the static pg_data structure of contig_page_data is used. At this time, NODE_DATA directly points to the global contig_page_data.

Nodes are divided into memory management areas. A memory management area is described using struct zone_struct, zone_t, to represent a certain range of memory. The 16MB of the low-end range is described as ZONE_DMA, then the ordinary memory domain ZONE_NORMAL that can be directly mapped to the kernel, and finally the physical area beyond the kernel segment. Address field ZONE_HIGHMEM (0xF8000000~0xFFFFFFFF), high-end memory, is the available memory space reserved in the system and cannot be directly mapped by the kernel. (In order to be compatible with hot-plugging and memory fragmentation processing, the kernel introduces some logical memory areas:

1. The kernel defines a pseudo memory area ZONE_MOVEABLE, which needs to be used in mmeory mirgation, a mechanism to prevent physical memory fragmentation. This memory area is for the ultimate use of memory fragmentation

2. ZONE_DEVICE: Non Volatile Memory allocated to support hot-swappable devices. Non-volatile memory).

Page frame (page frame): represents the smallest unit of memory. Each page in the heap memory will create an instance of struct page. Traditionally, memory is regarded as continuous bytes, that is, memory is a byte array, and the number (address) of the memory unit can be used as the index of the byte array. During paging management, several bytes are converted into one page, such as 4K bytes. At this time, the memory becomes a continuous page, that is, the memory is a page array, and each page of physical memory is a page frame. The memory is numbered in units of pages. This number serves as an index into the page array and is called the page frame number. (The data structure objects of the page are stored in the mem_map global array. This array is usually stored at the head of ZONE_NORMAL, or in the area reserved for loading the kernel image in a small memory system. After loading the kernel's low address to The memory area behind the memory area, that is, the data structure objects of the memory page where ZONE_NORMAL starts, are all stored in this global array).

The paging unit can convert linear addresses into physical addresses. Linear addresses are divided into fixed-length groups, called pages. The linear addresses within the page are mapped to continuous physical addresses. This allows the kernel to specify the physical address of a page and its storage permissions without specifying the storage permissions for the entire linear address of the page.

The paging unit divides all RAM into fixed-length page frames (also called page frames). Each page frame contains a page, which means that the length of the page frame and the page are the same. Page frames are part of memory and therefore a storage area. ----mm_types.h The mapping in the struct page structure not only saves a pointer, but also saves some additional information, which is used to determine whether the page belongs to an anonymous memory area in an unassociated address space. Method to restore anon_vma through mapping: anon_vma=(struct anon_vma *)(mapping-PAGE_MAPPING_ANON).

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