RAM detected but not used in Ubuntu

System:

cat /etc/os-releasePRETTY_NAME="Ubuntu 22.04.4 LTS"NAME="Ubuntu"VERSION_ID="22.04"VERSION="22.04.4 LTS (Jammy Jellyfish)"VERSION_CODENAME=jammyID=ubuntuID_LIKE=debianHOME_URL="https://www.ubuntu.com/"SUPPORT_URL="https://help.ubuntu.com/"BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"UBUNTU_CODENAME=jammy

Problem: I bought my computer with 8gb RAM. I installed another 8gb RAM in the empty slot. Ubuntu shows 16gb ram installed.

• htopenter image description here

lsmemRANGE                                 SIZE  STATE REMOVABLE  BLOCK0x0000000000000000-0x000000007fffffff   2G online       yes   0-150x0000000100000000-0x000000047fffffff  14G online       yes 32-143Memory block size:       128MTotal online memory:      16GTotal offline memory:      0B

sudo lshw -class memory*-memory       description: System Memory       physical id: 3a       slot: System board or motherboard       size: 16GiB     *-bank:0          description: SODIMM DDR4 Synchronous 2667 MHz (0,4 ns)          product: CT8G4SFS8266.M8FD          vendor: 859B          physical id: 0          serial: 25B3207D          slot: ChannelA-DIMM0          size: 8GiB          width: 64 bits          clock: 2667MHz (0.4ns)     *-bank:1          description: [empty]          physical id: 1          slot: ChannelA-DIMM1     *-bank:2          description: SODIMM DDR4 Synchronous 2667 MHz (0,4 ns)          vendor: Fujitsu          physical id: 2          serial: 8B5A0100          slot: ChannelB-DIMM0          size: 8GiB          width: 64 bits          clock: 2667MHz (0.4ns)     *-bank:3          description: [empty]          physical id: 3          slot: ChannelB-DIMM1

In the image above and from the code outputs I see that the RAM is 16GB, but when the computer is under pressure the RAM fills up to 7.9GB and then uses it from the swap.

When you look at the htop image, it shows around 50% full even though it uses 4GB. I was suspicious at this point and ran the stress command and the result was exactly as I expected. The CPU filled every core to 100%, and the RAM only filled 7.9 GB. It does not exceed 7.9GB. Why does it not use the second connected RAM even though it sees it?

stress --cpu 8 --io 4 --vm 4 --vm-bytes 1024M --timeout 8s

enter image description here

free -h               total        used        free      shared  buff/cache   availableMem:            15Gi       3,8Gi       3,0Gi       709Mi       8,7Gi        10GiSwap:          1,0Gi          0B       1,0Gi