Linux memory being eaten

This is the difference between Windows and Linux in memory management. At first glance, the Linux system eats our memory (Linux ate my ram), but in fact this is also the characteristic of its memory management.

free command introduction

The following is the result of using the free command to view the memory of our laboratory file server. The -m option indicates using MB as the unit:

The second line of output indicates the system memory usage:

Mem: total (total) = 3920MB,

used (used) = 1938MB,

free (free) = 1982MB,

shared (shared memory) = 0MB,

buffers = 497MB,

cached = 1235MB

Note: The first four items are relatively easy to understand. There are no suitable words to translate buffer and cache. The difference between them is:

• A buffer is something that has yet to be “written” to disk.
• A cache is something that has been “read” from the disk and stored for later use.

That is, the buffer is used to store data to be output to the disk, and the cache is the data that is read from the disk and stored in the memory for future use. They are introduced to provide IO performance.

The third line of output indicates what is obtained based on the second line -/buffers/cache:

– buffers/cache used = Mem used – buffers – cached = 1938MB – 497MB – 1235MB = 205MB

buffers/cache free = Mem free buffers cached = 1982MB 497MB 1235MB = 3714MB

The third line of output indicates the use of swap partition:

Swap：total = 4095MB

used(used)= 0MB

free(free)= 4095MB

Since the system currently has sufficient memory, the swap partition is not used.

The third line of the output results above may be more difficult to understand. Why should this line of data be shown to the user? What does the memory usage minus the system buffer/cached memory mean? The system free memory plus buffer/ What does cached memory mean?

Classification of memory

We divide memory into three categories, with different names for its usage from the perspective of users and operating systems:

Something in the above table represents the memory of "buffers/cached" in the free command. This memory is indeed used from the perspective of the operating system, but if the user wants to use it, this memory can be recycled quickly. It is used by user programs, so this memory should be classified as idle from the user's perspective.

Go back to the results output by the free command again. The results output in the third line should be understandable. The numbers in this line represent the system memory usage from the user's perspective. Therefore, if you use the top or free command to check how much memory is left in the system, you should actually add the free memory to the buffer/cached memory. That is the actual free memory of the system.

buffers/cached benefits

Linux memory management has made a lot of careful designs. In addition to caching dentry (used in VFS to accelerate the conversion of file path names to inodes), it also adopts two main Cache methods: Buffer Cache and Page Cache. The purpose is In order to improve disk IO performance. Data read from a slow block device is temporarily held in memory. Even if the data is no longer needed at that time, the next time the application accesses the data, it can be read directly from memory, thus bypassing the slow speed. block devices, thus improving the overall performance of the system.

Linux will make full use of these free memories. The design idea is that if the memory is free, it is better to use it to cache more data. The next time the program accesses the data again, the speed will be faster. If the program wants to use memory and the memory in the system is When it is insufficient, instead of using the swap partition, part of the cache is quickly recycled and left for user programs to use.

Therefore, it can be seen that buffers/cached is really beneficial and harmless. The real disadvantage may give users the illusion that Linux consumes memory!

In fact, it is not the case. Linux has not eaten up your memory. As long as the swap partition has not been used and your memory is running low, you should feel lucky because Linux caches a large amount of data. Maybe you can use it next time. Benefit.

Experimental proof

The following experiments are used to verify the above conclusion:

We read a large file one after another and compare the practices of reading twice:

1. First generate a 1G large file

2. Clear cache

3. Read this file and test the time it takes

4. Read the file again and test the time it takes

As can be seen from the above, the first time reading this 1G file took about 18s, and the second time it was read again, it only took 0.3s, a full 60 times improvement!

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