For HHVM, we have insight:
Under HHVM the maximum cache size / entry count was effectively unlimited (until OOM).
For PHP 7, this will require more careful monitoring.
See also T211488#5103171 which points a Grafana dashboard that plots some APCu-related metrics but I don't quite understand them (<https://grafana.wikimedia.org/d/000000550/mediawiki-application-servers>).
Looking at some early flame graphs from PHP 7 (Excimer), I suspect the successful use of local-server caching on PHP7-APCu may be dramatically worse than on HHVM (seeing a 3X increase in time spent on cache miss code paths e.g. for JS minfy for ResourceLoader).
* [ ] Have a Grafana dashboard (e.g. on a renamed "hhvm-apc-usage" dash, or on "mediawiki-application-servers") that shows:
** Current combined size in bytes of the APC cache values, over time.
** Current total APC entry key count, over time.
* [ ] Look at these and compare them to HHVM.