Lightsail killed my process due to out of memory, unattended-upgr invoked oom-killer

I have a process called server (developed in rust, but I don't think that is critical), it should run with about constant load 24/7. The CPU load for the process is in the sustainable zone. The system is running Ubuntu 20.04

Yesterday it was killed with an out of memory message. Looking at kern.log, I can see that there are a number of messages before the process was killed, the first was "unattended-upgr invoked oom-killer: gfp_mask=0x1100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO), order=0, oom_score_adj=0"

Feb 10 07:01:38 ip- kernel: [1133764.101038] unattended-upgr invoked oom-killer: gfp_mask=0x1100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO), order=0, oom_score_adj=0
Feb 10 07:01:38 ip- kernel: [1133764.101047] CPU: 0 PID: 640430 Comm: unattended-upgr Not tainted 5.15.0-1028-aws #32~20.04.1-Ubuntu
Feb 10 07:01:38 ip- kernel: [1133764.101050] Hardware name: Xen HVM domU, BIOS 4.11.amazon 08/24/2006

Looking at the memory usage you can see that my server app is using 155MB RAM. Surprisingly snapd is using 184MB, amazon-ssm-agen 307MB, and ssm-agent-worke 310MB

Feb 10 07:01:38 ip- kernel: [1133764.101272] Tasks state (memory values in pages):
Feb 10 07:01:38 ip- kernel: [1133764.101272] [  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
Feb 10 07:01:38 ip- kernel: [1133764.101275] [    266]     0   266    70052     4501    98304        0         -1000 multipathd
Feb 10 07:01:38 ip- kernel: [1133764.101279] [    441]     0   441    59353      451    90112        0             0 accounts-daemon
Feb 10 07:01:38 ip- kernel: [1133764.101281] [    442]     0   442      637      166    40960        0             0 acpid
Feb 10 07:01:38 ip- kernel: [1133764.101283] [    446]     0   446     2137      582    53248        0             0 cron
Feb 10 07:01:38 ip- kernel: [1133764.101286] [    449]   103   449     1982      753    61440        0          -900 dbus-daemon
Feb 10 07:01:38 ip- kernel: [1133764.101288] [    458]     0   458     7469     2827    94208        0             0 networkd-dispat
Feb 10 07:01:38 ip- kernel: [1133764.101290] [    459]     0   459    58181      224    81920        0             0 polkitd
Feb 10 07:01:38 ip- kernel: [1133764.101292] [    464]     0   464   307844     1343   180224        0             0 amazon-ssm-agen
Feb 10 07:01:38 ip- kernel: [1133764.101294] [    470]     0   470     4418      867    69632        0             0 systemd-logind
Feb 10 07:01:38 ip- kernel: [1133764.101297] [    473]     0   473    98274      773   122880        0             0 udisksd
Feb 10 07:01:38 ip- kernel: [1133764.101298] [    474]     0   474      951      455    49152        0             0 atd
Feb 10 07:01:38 ip- kernel: [1133764.101300] [    498]     0   498    60334      581   106496        0             0 ModemManager
Feb 10 07:01:38 ip- kernel: [1133764.101302] [    501]     0   501     1840      435    49152        0             0 agetty
Feb 10 07:01:38 ip- kernel: [1133764.101304] [    515]     0   515     1459      382    45056        0             0 agetty
Feb 10 07:01:38 ip- kernel: [1133764.101306] [    556]     0   556    27032     2749   114688        0             0 unattended-upgr
Feb 10 07:01:38 ip- kernel: [1133764.101308] [    639]     0   639     3046      704    61440        0         -1000 sshd
Feb 10 07:01:38 ip- kernel: [1133764.101310] [    788]     0   788   310204     1846   192512        0             0 ssm-agent-worke
Feb 10 07:01:38 ip- kernel: [1133764.101312] [   8869]     0  8869     2397      949    57344        0         -1000 systemd-udevd
Feb 10 07:01:38 ip- kernel: [1133764.101314] [  11347]   100 11347     6817      875    73728        0             0 systemd-network
Feb 10 07:01:38 ip- kernel: [1133764.101316] [  11355]   101 11355     6137     1403    81920        0             0 systemd-resolve
Feb 10 07:01:38 ip- kernel: [1133764.101318] [  11359]     0 11359    17149     1077   159744        0          -250 systemd-journal
Feb 10 07:01:38 ip- kernel: [1133764.101320] [  11463]   102 11463    22721      511    86016        0             0 systemd-timesyn
Feb 10 07:01:38 ip- kernel: [1133764.101323] [  11642]   104 11642    56125      343    81920        0             0 rsyslogd
Feb 10 07:01:38 ip- kernel: [1133764.101325] [ 190175]  1000 190175     4732      413    73728        0             0 systemd
Feb 10 07:01:38 ip- kernel: [1133764.101327] [ 190176]  1000 190176    42626     1132    94208        0             0 (sd-pam)
Feb 10 07:01:38 ip- kernel: [1133764.101329] [ 191381]     0 191381   184017     4350   249856        0          -900 snapd
Feb 10 07:01:38 ip- kernel: [1133764.101332] [ 355693]  1000 355693     2164      983    61440        0             0 tmux: server
Feb 10 07:01:38 ip- kernel: [1133764.101334] [ 355694]  1000 355694     2506      872    53248        0             0 bash
Feb 10 07:01:38 ip- kernel: [1133764.101336] [ 611154]     0 611154     3453      978    65536        0             0 sshd
Feb 10 07:01:38 ip- kernel: [1133764.101338] [ 611258]  1000 611258     3574      982    65536        0             0 sshd
Feb 10 07:01:38 ip- kernel: [1133764.101340] [ 611259]  1000 611259     2510      901    53248        0             0 bash
Feb 10 07:01:38 ip- kernel: [1133764.101342] [ 622945]     0 622945     3452      977    65536        0             0 sshd
Feb 10 07:01:38 ip- kernel: [1133764.101343] [ 623052]  1000 623052     3485      819    65536        0             0 sshd
Feb 10 07:01:38 ip- kernel: [1133764.101345] [ 623054]  1000 623054     3038     1483    57344        0             0 bash
Feb 10 07:01:38 ip- kernel: [1133764.101347] [ 629386]  1000 629386   155370    35002   393216        0             0 server
Feb 10 07:01:38 ip- kernel: [1133764.101349] [ 638483]     0 638483      654      434    45056        0             0 apt.systemd.dai
Feb 10 07:01:38 ip- kernel: [1133764.101351] [ 638487]     0 638487      654      434    45056        0             0 apt.systemd.dai
Feb 10 07:01:38 ip- kernel: [1133764.101353] [ 638521]     0 638521    81187    14207   425984        0             0 unattended-upgr
Feb 10 07:01:38 ip- kernel: [1133764.101355] [ 640399]     0 640399    70904      836   151552        0             0 packagekitd
Feb 10 07:01:38 ip- kernel: [1133764.101357] [ 640430]     0 640430    92707    24658   425984        0             0 unattended-upgr

Immediately after this, my process was killed:

Feb 10 07:01:38 ip- kernel: [1133764.101359] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/user.slice/user-1000.slice/session-296.scope,task=server,pid=629386,uid=1000
Feb 10 07:01:38 ip- kernel: [1133764.101385] Out of memory: Killed process 629386 (server) total-vm:621480kB, anon-rss:138304kB, file-rss:1704kB, shmem-rss:0kB, UID:1000 pgtables:384kB oom_score_adj:0

Why was my process killed? It was not the reason that the system ran out of memory. How much of the available memory can I use on a Lightsail server?