During the PHP5-HHVM migration, memcached connection failures were observed. More information about that in this puppet change: https://gerrit.wikimedia.org/r/#/c/operations/puppet/+/183436/.
I'm unable to find now whether this was found due to us merely paying closer attention to the migration (and thus not HHVM-specific), or whether it was due some kind of problem with HHVM (which would likely not apply to PHP7), or whether it had to do with new kinds of uses we started doing on the "newer" code only (which would still apply to PHP7 today).
The above puppet change created a UNIX domain socket with Nutcracker listening on it directly. This seems like a good performance optimisation. This task is to see whether that makes sense to keep with PHP7 or not.
As it stands, due to the code being conditional on HHVM (from when we were still partly on PHP5), we have lost this optimisation effectively.
- Use localhost:11212 instead of 127.0.0.1:11212. – https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/183774/
- Use /var/run/nutcracker/nutcracker.sock:0 instead of 127.0.0.1:11212. – https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/183526/
- https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/542183/. This removes the above code as it is now unused due to having been conditional on HHVM and we're on PHP7 now (T235142).