threads getting stuck in vmem_bt_alloc() at "btalloc"?

[Rick Macklem]

Hi,

I've been doing some testing using pagesize clusters (4K) for NFS
instead of mclbytes (2K) on a single core i386.
Sometimes I get threads stuck sleeping on "btalloc", which seems
to happen in vmem_bt_alloc().

The comment in vmem_bt_alloc() basically says:
  out of address space or lost a fill race
Since this is persistent, I suspect it is the first case?

So, does anyone know what is going on here or what I should look
at to try and resolve this?

Btw, when I am testing, I don't see the pagesize cluster allocation
exceed 400, so it doesn't seem to be a leak or excessive allocation.

Thanks in advance for any help, rick
ps: Garrett, I cc'd you in part because I remember you mentioning
    something about mclbyte clusters being pre-allocated vs ??
    Maybe, you could explain what you were talking about, since
    I didn't follow it the last time.