By realtime I mean something that can help with fast network I/O; Low-latency and high-throughput.
closed as not constructive by mgorven, EEAA, Zoredache, Shane Madden♦, Tim Brigham Jun 7 '12 at 0:44
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I'd say CentOS or Scientific Linux with a realtime kernel compiled from Red Hat MRG source RPMs or using the CERN pre-built binaries.
The caveat is that simply installing a realtime distro won't automagically make everything faster. These kernels and related tools give you more "knobs" to tweak and ways to measure performance. This can be done for free (I've done it), but you will have to invest the time into working with your application and environment. Using the option I outlined, you can at least start with the RHEL MRG Tuning Guide.
On the tuning side, there's a lot more to work with in this realm. E.g.