Max Kernel Memory?

Setting vm.kmem_size to what seem to be usable values causes all kinds of memory allocations on boot. For example setting it to 1GB with 2GB installed results in a failure. 

What is the maximum amount of memory that can be allocated to the kernel, and why can't 1/2 of the system's memory be allocated safely? We have a module with a potentially very large dynamic lookup table, with little going on in userland, so we want to allocate as much as possible to the kernel.

Barney