We need to limit the number of random configuration parameters - just because something could possible have some minor performance gain if it's tweaked does not mean it should be exposed.

This just needs to be set to equal a small number of page sizes. Since memory maps are aligned on page boundaries, then on average you expect to have 1/2 of a page of extra memory use. If the block size is 2 pages or less, then this overhead is really significant. Sure, maybe if we made it 3 or 5 instead of 2, but the range of useful values here is small and I don't think any user would ever want to tune this.