I have recently tried out to run our huge, about 10 years old application using HHVM. Interestingly enough, aside of few 5.6 features it didn't support yet, the process was surprisingly painless.
However, HHVM was generally between 2 and 5 times slower than vanilla PHP at rendering pages. In micro benchmarks it was way ahead, but running the whole application was a very different matter.
I decided to wait until the thing matures a bit more before trying again and then hopefully with more time to investigate the causes of this huge difference in performance.
That's really interesting, as I brought one of our services at my old job over onto HHVM and gained 4 times more performance, I'm really curious what workload you were throwing at it, and I know that the HHVM people would love to take a look at it; performance regressions from PHP to HHVM have been basically nil for me over the past 12 months.
also, if this is an open source workload, any chance you could submit it to https://github.com/hhvm/oss-performance ? We generally optimize based on those benchmarks, and Facebook itself.
However, HHVM was generally between 2 and 5 times slower than vanilla PHP at rendering pages. In micro benchmarks it was way ahead, but running the whole application was a very different matter.
I decided to wait until the thing matures a bit more before trying again and then hopefully with more time to investigate the causes of this huge difference in performance.