Removing the Lock for Small GC Allocations: Clarification of GC Design?

[dsimcha]

I have a plan to avoid the GC lock for most small (<1 page) GC 
allocations.  I hope to have a pull request within a week or two, 
in time for the next release.  There's one detail I need 
clarified by Sean, Walter or someone who designed the D GC.

Currently small allocations are handled by popping a block off a 
free list, if a block is available.  I plan to make each page 
owned by a single thread, and make the free lists thread-local.  
The array of free lists (one for each power of two size) is 
stored in the Gcx struct.  The easiest way to make this array 
thread-local is to move it out of the Gcx struct and make it 
global.  Is there any reason why >1 instance of Gcx would exist 
(maybe as an implementation detail of shared libraries, etc.)?  
If not, what's to point of having the Gcx struct instead of just 
making its variables global?