Why Is the Memory of an ECS Obtained by Running the free Command Inconsistent with the Actual Memory?

Symptom

After you create an ECS, you run the free -m command to view the ECS memory. The ECS memory is less than the memory configured during ECS creation.

For example:

When you are creating an ECS, the configured memory size is 4,194,304 KB (4,096 MB). After the ECS is created, you run the free -m command to view its memory. The command output is as follows:

[root@localhost ~]# free -m
total used free shared buff/cache available
Mem: 3790 167 3474 8 147 3414
Swap: 1022 0 1022

The memory in the command output is 3,790 MB, which is less than the configured 4,096 MB.

Run the dmidecode -t memory command to check the actual memory configured for the ECS. The command output is as follows:

[root@localhost ~]# dmidecode -t memory
# dmidecode 3.0
Getting SMBIOS data from sysfs.
SMBIOS 2.8 present.

Handle 0x1000, DMI type 16, 23 bytes
Physical Memory Array
Location: Other
Use: System Memory
Error Correction Type: Multi-bit ECC
Maximum Capacity: 4 GB
Error Information Handle: Not Provided
Number Of Devices: 1

Handle 0x1100, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x1000
Error Information Handle: Not Provided
Total Width: Unknown
Data Width: Unknown
Size: 4,096 MB
Form Factor: DIMM
Set: None
Locator: DIMM 0
Bank Locator: Not Specified
Type: RAM
Type Detail: Other
Speed: Unknown
Manufacturer: QEMU
Serial Number: Not Specified
Asset Tag: Not Specified
Part Number: Not Specified
Rank: Unknown
Configured Clock Speed: Unknown
Minimum Voltage: Unknown
Maximum Voltage: Unknown
Configured Voltage: Unknown

The memory in the command output is the same as that configured during ECS creation.

Possible Causes

When the OS is started, related devices are initialized, which occupies memory. In addition, when the kernel is started, it also occupies memory. The memory occupied by kdump can be set. Unless otherwise specified, do not change the memory size occupied by kdump.

The command output of free -m shows the available memory of the ECS, and that of dmidecode -t memory shows the hardware memory.

The memory obtained by running the free -m command is less than the memory configured for the ECS. This is a normal phenomenon.

This is a normal phenomenon even for physical servers.