In https://gerrit.wikimedia.org/r/#/c/238527/ I ran into the problem that our zend has an older version of phpredis that does not support some key features (like ecxlusive set with expiry). HHVM works fine.
If it will take a while before a full zend => hhvm migration (T86081), it would be nice to upgrade the phpredis extension as a stop gap.
See https://github.com/phpredis/phpredis/releases/tag/2.2.4 .