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Date: Fri, 16 Aug 2002 04:40:35 +0200
From: Andrea Arcangeli <andrea@suse.de>
To: Benjamin LaHaise <bcrl@redhat.com>
Cc: Linus Torvalds <torvalds@transmeta.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Chris Friesen <cfriesen@nortelnetworks.com>,
       Pavel Machek <pavel@elf.ucw.cz>, linux-kernel@vger.kernel.org,
       linux-aio@kvack.org
Subject: Re: aio-core why not using SuS? [Re: [rfc] aio-core for 2.5.29 (Re: async-io API registration for 2.5.29)]
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On Thu, Aug 15, 2002 at 10:16:47PM -0400, Benjamin LaHaise wrote:
> On Thu, Aug 15, 2002 at 07:08:30PM -0700, Linus Torvalds wrote:
> > 
> > On Thu, 15 Aug 2002, Benjamin LaHaise wrote:
> > > 
> > > A 4G/4G split flushes the TLB on every syscall.
> > 
> > This is just not going to happen. It will have to continue being a 3/1G 
> > split, and we'll just either find a way to move stuff to highmem and 
> > shrink the "struct page", or we'll just say "screw those 16GB+ machines on 
> > x86". 
> 
> I wish life were that simple.  Unfortunately, struct page isn't the only 
> problem with these abominations: the system can run out of kvm for 
> vm_area_struct, task_struct, files...  Personally, I *never* want to see 
> those data structures being kmap()'d as it would hurt kernel code quality 
> whereas a 4G/4G split is well confined, albeit sickening.

after the mem_map is gone, there's still the option of CONFIG_2G or even
CONFIG_1G if kernel metadata is the problem. Of course it wouldn't be
a generic kernel, but I guess a 4G/4G would probably be even less
generic.  In short we can do little at runtime to be generic. I guess a
16G with large softpagesize should be not too bad now that the
pagetables are in highmem, most problematic is >16G. Not that the
softpagesize is easy at all to implement (4G/4G is certainly simpler
because self contained in the include/arch) but at least it can payoff
for the lower mem setups too.

Andrea
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