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From: Peter Wemm <peter@wemm.org>
To: Petri Helenius <pete@he.iki.fi>
Date: Sat, 23 Jul 2005 12:12:24 -0700
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Cc: freebsd-amd64@freebsd.org
Subject: Re: kernel memory
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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@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5