Problems with actual memory not equal to physical memory in Ubuntu Server

Recording

Source V2ex, mentioned a usually not very noticeable problem, Ubuntu Server system by default will occupy 128M of memory, for CVM internal Kdump service.

Science

1. View the physical memory owned by the CVM
   View the underlying hardware information with the Dmidecode command
2. Why does free see less memory?
   Free sees the memory that is managed by the partner system, which does not include the consumption of some basic data structures in the OS kernel and some of the kernel's reservations. Kernel reserved memory, can view DMESG by command after CVM boot | Grep-i Memory
3. Ubuntu reserved 128M memory for CVM internal Kdump service
4. What is Kdump service? Why Open Kdump?
   When the OS kernel fails (such as a kernel bug), the Kdump service assists in producing a dump file, similar to the application's coredump. The purpose of the kdump is to provide enough information to enable the client to locate the kernel bug problem in case of a bug in the kernel.
5. How to turn off Kdump
   1. Modify the Grub startup parameters to close the Kdump reservation. In the/etc/default/grub.d/kexec-tools.cfg and/etc/default/grub configuration files, remove the configuration parameters such as "crashkernel=xxx", and then perform UPDATE-GRUB2 update grub Boot file
   2. Close Kdump Service systemctl disable Kdump-tools
   3. Restart CVM, you can see more memory
6. Why does CentOS look more like 1G of memory?
   In 1G memory, the CentOS system does not turn on Kdump by default, does not reserve to memory, so there will be more, and Ubuntu default on the Kdump

Problems with actual memory not equal to physical memory in Ubuntu Server