kernel memory

[Peter Wemm]

On Thursday 21 July 2005 11:58 am, Petri Helenius wrote:
> Peter Wemm wrote:
> >2GB for paged kernel memory.  But in addition we access memory via
> > the direct map area to avoid the need for temporary mappings in
> > many cases. uma (malloc, mbufs) etc use this, as does the sfbuf
> > temporary mapping system.
>
> So there is no limitation for malloced memory? Say if my driver would
> like to have 4 or 8 gig lookup cache that would work?

AARGH.  I've just found a bug/feature in the memory allocator.

There are two code paths, one for small (<PAGE_SIZE) allocations, which 
uses the direct map allocations instead of kvm allocations, and the 
other large chunk allocator that simply allocates pages at a time from 
kvm. :-(

I suspect this is because malloc's semantics depend on objects being 
contiguous. The direct map method would allocate physically 
discontiguous pages.

So, if you allocated your lookup cache in <4K chunks, you could have as 
much as you like. :-/
-- 
Peter Wemm - peter at wemm.org; peter at FreeBSD.org; peter at yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5