In past versions, I used to delete all files in
/data/dalvik-cache to clear the dalvik cache, which is the only directory that shows up in TWRP log:
In Lollipop and Marshmallow where Android uses ART VM, the dalvik cache is often a gigabyte or so big, and more if you installed a lot of apps.
In Nougat, however, with 200+ total apps (system + user), the dalvik cache folder is less than 800 MB as large, so I doubt it contains all dalvik cache: some must be residing at somewhere else.
How do I clear all dalvik cache? They're useless when backed up in a recovery.