Here are two possible approaches.
Using initialization code
Firstly, one notes that the process priority is set (in
SetSystemOptions["ProcessPriority" -> -1];
where the numeric value corresponds to a priority as follows:
-1: below normal
1: above normal
Process priorities on Windows are rather complex and it doesn't seem like there's a way to set idle priority directly, but low will probably be good enough. To execute this code in each subkernel after launch, you just need to evaluate:
Needs["Parallel`Developer`"]; (* Load required package *)
$InitCode = Hold[SetSystemOptions["ProcessPriority" -> -2];];
and now each kernel's process priority will change from normal to low shortly after it starts.
Starting local kernels as if they were remote
Another, Windows-specific, approach is as follows. I will use this essentially as an excuse to post the command needed to start remote kernels on Windows where we usually do not have
ssh available; I found the widely cited Remote Kernel Strategies presentation less than helpful here and devised this alternative, which I believe works better than other approaches.
We use the
rcmdsvc executables from the Windows 2000 Resource Kit, which must be installed first (although the installation is utterly trivial). Mathematica's installation directory must also be in the system's
PATH environment variable for the
math command to execute properly on the remote computer (even if in reality this is the same as the local computer).
We now change
SubKernels`RemoteKernels`$RemoteCommand as follows:
"runas /savedcreds /user:\"`3`\" /noprofile \"rcmd \\\\`1` start /b " <>
If[SubKernels`LocalKernels`Private`lowerprio, "/LOW", "/NORMAL"] <>
" math -noinit -subkernel -mathlink -linkmode Connect `4` -linkname `2`\"";
The username (slot 3) defaults to the username of the logged-on user but can be changed using e.g.
SubKernels`RemoteKernels`$RemoteUserName = "mathematica";
Each remote kernel configured with this option will start with reduced priority, depending on whether or not the option is set for local kernels. While it isn't ideal for the process priority for remote kernels to be set using the tickbox on the Local Kernels tab, this option has to be set somewhere and short of adding our own checkbox this seemed like the most reasonable choice. Remote hosts can also be used, of course, if you have them available; if you want to save this command line for each of the hosts set up (say, you want to set up both Windows and Linux hosts, and need to use a different command line for each) then tick the "Use custom launch command" box when configuring the host.