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After some investigation it was found that there are memcached timeout errors that effectively make caching broken, and this is most visible for sessions. The fault seems to be with hhvm using the PHP memcached client. Issue does not happen with memcached-pecl client.
I see multiple possible actions:
- CACHE_MEMCACHED should default to native memcached client under HHVM and not the PHP implementation
- Investigate the root cause and fix it (possibly an upstream issue)
- Issue a warning (runtime, release notes...) for this combination
- Do nothing and drop official support for HHVM