Ubuntu Wiki

-lowlatency == -generic + more aggressive low latency kernel configuration It can offers all things that Ubuntu offer with -generic so -backport modules, video closed drivers and so on. It is very solid and oriented to "soft" users. Trade-off between low latency and power consumption.

-rt == -generic + plus PREEMPT_RT patchset (this is the realtime Linux) It would want offer the same but require a lot of works. it would want be aligned with -generic version but it can't be able. It isn't very solid and it is oriented to "hard" users only. Power consumption is a secondary concern.

-realtime == vanilla (kernel.org) + plus PREEMPT_RT patchset (this is the realtime Linux) It don't be interested neither to be aligned with -generic kernel nor be compatible with all closed video rivers shipped (and worked) with Ubuntu. It don't provide -backport drivers. it is very stable and it is oriented to "hard" users only. Power consumption is a secondary concern.

From a technical point of view -rt and -realtime are the same kernel. A minor difference is that the -rt kernel offer the "really stable" and upstream official release 2.6.31 whereas -realtime offers the last official upstream release that is 2.6.33. But there are the same kernel (that is PREEMPT_RT). The main difference is the external support. In -rt I have tried to offer an usable system as like Ubuntu do (so I have worked on compatibility with closed video drivers for example like nvidia or fglrx) whereas with -realtime I don't enforce it at all.

In less words: if you need of closed video drivers, external DKMS kernel modules, linux-backports-* you should probably start to use -lowlatency (when it will be available through Ubuntu repos). Instead if you really need of an real-time system you should avoid all above or trying to make those working alone.

To put it simply: -lowlatency all the way! It delivers impressive results for maintenance requirements way lower than -realtime (or -rt even more), meaning less work for maintainers and new kernel candy for users.

Life is a trade-off: someone want a very hard real-time system without adopt other hardware architectures than pc (MCU, FPGA/ASIC, and so on), others want use normal and cheap pc, others want use also accelerated drivers also, others want a real-system which take care of they laptop's batteries too. And at the end we have limited resources(*) for give an answer on these needs (we are all volunteers).