Message-ID: <41FAA51E.10000@comcast.net>
Date: Fri, 28 Jan 2005 15:48:30 -0500
From: John Richard Moser <nigelenki@comcast.net>
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To: Josh Boyer <jdub@us.ibm.com>
CC: linux-kernel@vger.kernel.org
Subject: Re: Why does the kernel need a gig of VM?
References: <41FA9B37.1020100@comcast.net> <1106944969.7542.13.camel@windu.rchland.ibm.com>
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Wow.

I'd heard that there was a way to set 3.5/0.5 GiB split, and that there
was a patch that removed the split and isolated the kernel (but that was
slow), so I was just curious about all this stuff with people screaming
about how tight 4G of VM is vs a half gig or a gig that can be freed up.

Josh Boyer wrote:
> On Fri, 2005-01-28 at 15:06 -0500, John Richard Moser wrote:
> 
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>>Can someone give me a layout of what exactly is up there?  I got the
>>basic idea
>>
>>K 4G
>>A 3G
>>A 2G
>>A 1G
>>
>>App has 3G, kernel has 1G at the top of VM on x86 (dunno about x86_64).
>>
>>So what's the layout of that top 1G?  What's it all used for?  Is there
>>some obscene restriction of 1G of shared memory or something that gets
>>mapped up there?
>>
>>How much does it need, and why?  What, if anything, is variable and
>>likely to do more than 10 or 15 megs of variation?
> 
> 
> Because of various reasons.  Normal kernel space virtual addresses
> usually start at 0xc0000000, which is where the 3GiB userspace
> restriction comes from.  
> 
> Then there is the vmalloc virtual address space, which usually starts at
> a higher address than a normal kernel address.  Along the same lines are
> ioremap addresses, etc.
> 
> Poke around in the header files.  I bet you'll find lots of reasons.
> 
> josh
> 
> 

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