mjhjmtu

I have Ubuntu 18.04 dual booted with Windows 10. Windows 10 shows full 8 GB RAM.

But in Ubuntu it shows only 6.8 GB.

Is there any way to make it near 8 GB or is this natural?

~$ free
 total used free shared buff/cache available
Mem: 7097484 1775180 3633172 67964 1689132 4994508
Swap: 1000444 0 1000444

After turning the swap area off,

~$ free
 total used free shared buff/cache available
Mem: 7097484 1776700 4761256 111464 559528 4949096
Swap: 0 0 0

Output

~# dmesg | grep -i memory:
[ 0.000000] PM: Registered nosave memory: [mem 0x00000000-0x00000fff]
[ 0.000000] PM: Registered nosave memory: [mem 0x00087000-0x00087fff]
[ 0.000000] PM: Registered nosave memory: [mem 0x000a0000-0x000bffff]
[ 0.000000] PM: Registered nosave memory: [mem 0x000c0000-0x000fffff]
[ 0.000000] PM: Registered nosave memory: [mem 0x09b00000-0x09dfffff]
[ 0.000000] PM: Registered nosave memory: [mem 0x09f00000-0x09f09fff]
[ 0.000000] PM: Registered nosave memory: [mem 0x6427a000-0x64b79fff]
[ 0.000000] PM: Registered nosave memory: [mem 0x6b58f000-0x6b78efff]
[ 0.000000] PM: Registered nosave memory: [mem 0x6b78f000-0x6d78efff]
[ 0.000000] PM: Registered nosave memory: [mem 0x6d78f000-0x6f78efff]
[ 0.000000] PM: Registered nosave memory: [mem 0x6f78f000-0x6f7fefff]
[ 0.000000] PM: Registered nosave memory: [mem 0x6f800000-0xafffffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xb0000000-0xf7ffffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xf8000000-0xfbffffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfc000000-0xfebfffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfec00000-0xfec00fff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfec01000-0xfec0ffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfec10000-0xfec10fff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfec11000-0xfed7ffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfed80000-0xfed80fff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfed81000-0xfedfffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfee00000-0xfee00fff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfee01000-0xff7fffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xff800000-0xfff4ffff]
[ 0.000000] PM: Registered nosave memory: [mem 0xfff50000-0xffffffff]
[ 0.000000] Memory: 6929972K/7238032K available (12300K kernel code, 2472K rwdata, 4248K rodata, 2408K init, 2416K bss, 308060K reserved, 0K cma-reserved)
[ 0.039763] Freeing SMP alternatives memory: 36K
[ 1.147121] Freeing initrd memory: 54212K
[ 141.519341] Freeing unused kernel memory: 2408K
[ 153.310684] Freeing unused kernel memory: 2008K
[ 153.317976] Freeing unused kernel memory: 1896K
[ 153.869546] [TTM] Zone kernel: Available graphics memory: 3548742 kiB
[ 153.869547] [TTM] Zone dma32: Available graphics memory: 2097152 kiB

This line:

[ 0.000000] Memory: 6929972K/7238032K available (12300K kernel code, 2472K rwdata, 4248K rodata, 2408K init, 2416K bss, 308060K reserved, 0K cma-reserved)

indicates that BIOS is giving the kernel 7238032K of memory. The rest of the memory is being used by the BIOS:

8,388,608 K Total - 7,238,032 K Kernel = 1,150,576 K reserved by BIOS.

That is an unusually large amount for BIOS to reserve. You would have to look at your BIOS settings to perhaps try to figure out why.

From: Why my acer nitro 5 (AN515-42, Radeon RX 560X) 8gb ram but its just 6.9gb ram usable?

This can only be "fixed" in the bios! But not all bios allow you to
change the pre-allocated memory of the video card, because there is a
risk of the notebook turning a brick! Can you take a picture of this
setting?

Ubuntu is using your CPU's integrated graphics, which consumes ~1.1GB of system memory, whereas Windows is using the discrete graphics, which consumes no system memory.

I'm not sure why Ubuntu defaults to the iGPU, but you can possibly get it to use the dGPU by ensuring that your AMD GPU driver is up to date. If that does not work, you can try using the hybrid graphics feature to switch between GPUs, but do note that this feature is very experimental and very low-level and could easily prevent your Ubuntu install from booting if your hardware setup isn't correctly supported.