linux-kernel - Re: [PATCH 00/18] Make common x86 arch area for i386 and x86_64

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Date:	Fri, 16 Mar 2007 13:52:18 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	David Miller <davem@...emloft.net>
cc:	andi@...stfloor.org, rostedt@...dmis.org,
	torvalds@...ux-foundation.org, mbligh@...igh.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	chrisw@...s-sol.org, rusty@...tcorp.com.au, glommer@...il.com
Subject: Re: [PATCH 00/18] Make common x86 arch area for i386 and x86_64 -
 Take 2

On Fri, 16 Mar 2007, David Miller wrote:

> > It is primarily a performance improvement since the sparsemem table 
> > lookups would no longer be necessary and it also streamlines other 
> > frequent cacheline uses. These page -> page_struct and vice versa 
> > operations are key to the performance of various subsystem among them 
> > the slab allocator.
> 
> If you set the bit range small enough and don't use sparsemem-extreme,
> the cost is extremely low considering the flexibility you obtain.

What flexibility? 
 
> There are always going to be holes on large systems, there isn't
> really a way to avoid this given how addressing is done on those
> machines.

Virtual mmap allows holes in the same way as page tables do. These 
structures were designed for sparseness. I really do not think we should 
rediscuss the material here that was discussed on linux-mm. The outcome of 
that was that we will introduce virtual memmap to reduce sparsemem 
overhead and to increase flexibility.



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