Thanks for the replies - so the rule of thumb would be:
"For binaries other than driver modules it's usually not all that critical but in general it would be best to compile against 2.4.xx if you want the binary to be forwards- compatible with newer kernels."
(Obviously not talking about optimising performance here).
I take it then that binaries in both woody and sarge repositories are probably compiled against 2.4.xx.
not going to see any difference most likely from application A compiled on a 2.6.x series kernel compared to application A also compiled on a 2.4.x series kernel.
This is something of a relief - I've been trying to custom compile a 2.4.26 kernel (from backports.org) on my new 3.1 installation in order to compile binaries that I want to run on a Knoppix-based 2.4.26 kernel system (problems so far but I'll get there?) - but if all else fails I'll just rebuild the 2.4.27 kernel that came with 3.1 and compile against that. I was hoping to get optimum performance for 2.4.26 (because it's a resource-intensive application) but if there's not going to be much or any performance difference on 2.4.26 to a binary compiled against 2.4.27 then I'm wondering if I should bother .... I'm wondering how much difference there could be between 2.4.26 and 2.4.27 ....