Only --faiss-flat works in docker on Synology NAS???

[StephanSchuster]

On my Synology NAS (DS720+) I installed docker and tried to run imgdupes on a folder with two exact same images. It works only for --faiss-flat while I never get any output or result for --ngt or --hnsw, no matter what other options, values or images I provide.

admin@nas2:/volume1/docker$ ll
total 600
-rwxrwxrwx 1 admin users 304184 Apr  1  2013 test1.jpg
-rwxrwxrwx 1 admin users 304184 Apr  1  2013 test2.jpg
admin@nas2:/volume1/docker$ sudo docker run -it -v $PWD:/app knjcode/imgdupes . phash 0
admin@nas2:/volume1/docker$ sudo docker run -it -v $PWD:/app knjcode/imgdupes --ngt . phash 0
admin@nas2:/volume1/docker$ sudo docker run -it -v $PWD:/app knjcode/imgdupes --hnsw . phash 0
admin@nas2:/volume1/docker$ sudo docker run -it -v $PWD:/app knjcode/imgdupes --faiss-flat . phash 0
Building faiss index (dimension=64, num_proc=3)
Exact neighbor searching using faiss
100%|████████████████████████████████████████████████████| 2/2 [00:00<00:00, 2545.09it/s]
test1.jpg
test2.jpg

admin@nas2:/volume1/docker$

What could cause this issue? Am I doing something wrong? Is it a bug?

If I cannot fix it, what are the pros and cons of --faiss-flat in contrast to --ngt or --hnsw?