SUMMARY
On memory-restrained systems, rebuilding all thumbnails can lead to the OOM killer being triggered.

Description copied from bug #497163 where this was initially reported:
> However, several times the process was terminated by the OOM killer due to excessive RAM taken by the spawned ffmpeg processes (I don't know for sure, unfortunately, I haven't analysed this issue), I'm running this on a VM with 8 GB of RAM


ADDITIONAL INFORMATION

OOM Killer dmesg log

Dec 13 17:17:23 kubuntu-virtualbox kernel: Tasks state (memory values in pages):
Dec 13 17:17:23 kubuntu-virtualbox kernel: [  pid  ]   uid  tgid total_vm      rss rss_anon rss_file rss_shmem pgtables_bytes swapents oom_score_adj name
[...other lines omitted]
Dec 13 17:17:23 kubuntu-virtualbox kernel: [  32893]  1000 32893   191297    67431    67302      129         0   929792        0           200 gdb
Dec 13 17:17:23 kubuntu-virtualbox kernel: [  32916]  1000 32916   875180   504573   499724      783      4066  4845568        0           200 kphotoalbum
Dec 13 17:17:23 kubuntu-virtualbox kernel: [  33194]     0 33194   119161     1592     1306      286         0   266240        0             0 fwupd
Dec 13 17:17:23 kubuntu-virtualbox kernel: [  59488]  1000 59488   787532   433595   433103      492         0  4382720        0           200 ffmpeg
Dec 13 17:17:23 kubuntu-virtualbox kernel: [  59531]  1000 59531   652165   378273   377419      854         0  3870720        0           200 ffmpeg
Dec 13 17:17:23 kubuntu-virtualbox kernel: oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/user.slice/user-1000.slice/user@1000.service/app.slice/app-code@269c1bf1a3e843ee914e3bab80c1c>
Dec 13 17:17:23 kubuntu-virtualbox kernel: Out of memory: Killed process 32916 (kphotoalbum) total-vm:3500720kB, anon-rss:1998896kB, file-rss:3132kB, shmem-rss:16264kB, UID:1000 pgtables:4732kB oom_score_adj:200