We are currently seeing a continuing and pretty severe memory leak on the API appserver cluster only. I am unsure when this leak started, but it surely got more serious since we've removed a few servers from the cluster on April 1st:
if a HHVM server doesn't crash, its occupied memory grows approximately by 2 Gbs/day. Moreover, when memory usage grows, the older appservers crash and need a soft reboot, the newer ones spike in terms of cpu usage.
IIRC, we memory profiling is problematic on HHVM 3.12, but I am going to try to gather some data anyways today.