[JoshTriplett]

If you have enough CPU cache, the CPU will cache both kernel and user code/data for your process. If you run long enough and access enough user data that the kernel bits get evicted, then you'll take more of a performance hit, but the same thing applies to accessing enough user data that different parts of your user data get evicted.

[eloff]

Maybe this is more reflective on the kinds of software I work on, but I don't find that I have enough cache. So syscalls always come at a steep price. In fact I am tempted to break up one program into multiple processes and use the cache partitioning in Xeon v4 to prevent the different parts of my program from clobbering it's own cache.