Hello,
My system consistently freezes when about 45% of my memory is used, the CPU seems to become consumed by IOWait. This is what I notice on the system monitor, if I close some applications before IOWait consumes everything then it doesn't crash but as soon as I get to 45% of memory it freezes. I understand that if all the memory is used up it would crash but I'm not even getting to the point where swap memory is used. When it's frozen I use the SysRq + k to get back to the graphical. You probably need some more output info but I'm not sure how to get this when the system is frozen. Any advice on how to post that would be much appreciated! I'm running KDE on Debian 10
here is my system info:

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H/W path         Device      Class       Description
====================================================
                             system      0880 (Lenovo)
/0                           bus         NITU1
/0/0                         memory      106KiB BIOS
/0/4                         processor   Intel(R) Core(TM)2 Duo CPU     T6570  @ 2.10GHz
/0/4/5                       memory      64KiB L1 cache
/0/4/6                       memory      2MiB L2 cache
/0/d                         memory      4GiB System Memory
/0/d/0                       memory      2GiB SODIMM Synchronous 800 MHz (1,2 ns)
/0/d/1                       memory      2GiB SODIMM Synchronous 800 MHz (1,2 ns)
/0/100                       bridge      Mobile 4 Series Chipset Memory Controller Hub
/0/100/2                     display     Mobile 4 Series Chipset Integrated Graphics Controller
/0/100/2.1                   display     Mobile 4 Series Chipset Integrated Graphics Controller
/0/100/1a                    bus         82801I (ICH9 Family) USB UHCI Controller #4
/0/100/1a/1      usb2        bus         UHCI Host Controller
/0/100/1a.1                  bus         82801I (ICH9 Family) USB UHCI Controller #5
/0/100/1a.1/1    usb4        bus         UHCI Host Controller
/0/100/1a.2                  bus         82801I (ICH9 Family) USB UHCI Controller #6
/0/100/1a.2/1    usb5        bus         UHCI Host Controller
/0/100/1a.7                  bus         82801I (ICH9 Family) USB2 EHCI Controller #2
/0/100/1a.7/1    usb1        bus         EHCI Host Controller
/0/100/1b                    multimedia  82801I (ICH9 Family) HD Audio Controller
/0/100/1c                    bridge      82801I (ICH9 Family) PCI Express Port 1
/0/100/1c.1                  bridge      82801I (ICH9 Family) PCI Express Port 2
/0/100/1c.2                  bridge      82801I (ICH9 Family) PCI Express Port 3
/0/100/1c.2/0    wlan0       network     BCM4312 802.11b/g LP-PHY
/0/100/1c.3                  bridge      82801I (ICH9 Family) PCI Express Port 4
/0/100/1c.5                  bridge      82801I (ICH9 Family) PCI Express Port 6
/0/100/1c.5/0    enp7s0      network     NetLink BCM5906M Fast Ethernet PCI Express
/0/100/1d                    bus         82801I (ICH9 Family) USB UHCI Controller #1
/0/100/1d/1      usb6        bus         UHCI Host Controller
/0/100/1d/1/2                input       AND
/0/100/1d.1                  bus         82801I (ICH9 Family) USB UHCI Controller #2
/0/100/1d.1/1    usb7        bus         UHCI Host Controller
/0/100/1d.2                  bus         82801I (ICH9 Family) USB UHCI Controller #3
/0/100/1d.2/1    usb8        bus         UHCI Host Controller
/0/100/1d.7                  bus         82801I (ICH9 Family) USB2 EHCI Controller #1
/0/100/1d.7/1    usb3        bus         EHCI Host Controller
/0/100/1d.7/1/3              multimedia  Lenovo EasyCamera
/0/100/1e                    bridge      82801 Mobile PCI Bridge
/0/100/1f                    bridge      ICH9M LPC Interface Controller
/0/100/1f.2      scsi0       storage     82801IBM/IEM (ICH9M/ICH9M-E) 4 port SATA Controller [AHCI mode]
/0/100/1f.2/0    /dev/sda    disk        320GB WDC WD3200BEVT-2
/0/100/1f.2/0/1  /dev/sda1   volume      579MiB Windows NTFS volume
/0/100/1f.2/0/2  /dev/sda2   volume      58GiB Windows NTFS volume
/0/100/1f.2/0/3  /dev/sda3   volume      238GiB EXT4 volume
/0/100/1f.2/1    /dev/cdrom  disk        DVDRAM GT30N
/0/100/1f.3                  bus         82801I (ICH9 Family) SMBus Controller
/0/100/1f.6                  generic     82801I (ICH9 Family) Thermal Subsystem
/0/1                         system      PnP device PNP0103
/0/2                         system      PnP device PNP0c02
/0/3                         system      PnP device PNP0b00
/0/5                         input       PnP device PNP0303
/0/6                         input       PnP device PNP0f13
/0/7                         system      PnP device PNP0c02


Thanks!

Which exact Plasma version and distribution is this about?

I'm running Debian 10.3 'Buster' plasma-desktop_5.14.5.1 Kernel 4.19.0-8-amd64

I am probably a bit tired already, but I don't see how much SWAP you actually have, it should ideally be at least double the physical RAM, especially since you only have 4 GB. On sda there doesn't seem to be any SWAP partition, or is this an oversight?

Also, how many applications do you open when this happens, and how many browser tabs? Browsers tend to use up a lot of memory, so limiting the number of tabs is always a good idea with little RAM.

you are right, it appears I have no Swap

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$ free
              total        used        free      shared  buff/cache   available
Mem:        3972972     2600168      659036      288620      713768      857708
Swap:             0           0           0


could that explain why the system freezes when I get close to using about 3GB of my 4GB of memory it tries to put some in swap but there is no swap so it freezes?
I have followed the steps I found here http://www.linuxandubuntu.com/home/how-to-create-or-increase-swap-space-in-linux to add 8GB of swap,

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$ free
              total        used        free      shared  buff/cache   available
Mem:        3972972     2681632      670880      294704      620460      767948
Swap:       8191996           0     8191996


will restart and see if this solves the problem
*Edit So after a restart, problem solved, thanks Mamarok for noticing this.