Linux memory management-physical memory and virtual memory

We know that reading and writing data directly from the physical memory is much faster than reading and writing data from the hard disk. Therefore, we hope that all data can be read and written in the memory, while the memory is limited, this introduces the concept of physical memory and virtual memory.

Physical memory is the memory size provided by the system hardware and is the real memory. Compared with physical memory, there is also a concept of virtual memory in Linux, virtual Memory is a strategy proposed to meet the shortage of physical memory. It is a logical memory virtualized by the disk Space. The disk Space used as the virtual memory is called the Swap Space ).

As an extension of the physical memory, Linux uses the virtual memory of swap partitions when the physical memory is insufficient. In more details, the kernel will write the memory block information that is not used for the moment to the swap space, in this way, the physical memory has been released, and this memory can be used for other purposes. When you need to use the original content, this information will be re-read from the swap space into the physical memory.

Linux memory management adopts a paging access mechanism. To ensure that the physical memory can be fully utilized, when appropriate, the kernel automatically exchanges infrequently used data blocks in the physical memory to the virtual memory, and retains frequently used information to the physical memory.

To learn more about the Linux memory running mechanism, you need to know the following aspects:

First, the Linux system will perform page switching from time to maintain as much free physical memory as possible. Even if there is no need for memory, Linux will swap out memory pages that are temporarily unavailable. This avoids waiting for the switching time.

Second, Linux provides Page Swap conditions. Not all pages are switched to the virtual memory when not in use. Linux kernel uses the "most recently used" algorithm, swap some infrequently used page files to the virtual memory. Sometimes we can see that there is still a lot of Linux physical memory, but the swap space is also used. In fact, this is not surprising. For example, a process that occupies a large amount of memory needs to consume a lot of memory resources, and some infrequently used page files will be exchanged to the virtual memory, however, when the process that occupies a lot of memory resources ends and releases a lot of memory, the page files that have just been exchanged will not be automatically exchanged into the physical memory unless necessary, now the physical memory of the system will be much idle and the swap space will be used. You don't have to worry about this. You just need to know what it is.

Finally, the pages in the swap space will be first exchanged to the physical memory during use. If there is not enough physical memory to accommodate these pages, they will be immediately exchanged, there may not be enough space in the virtual memory to store these swap pages, which will eventually lead to issues such as false crashes and service exceptions in Linux. Although Linux can be restored by itself within a period of time, however, the recovered system is basically unavailable.

Therefore, it is very important to reasonably plan and design the Linux memory usage.

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