From: John Richard Moser <nigelenki@comcast.net>
To: Josh Boyer <jdub@us.ibm.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Why does the kernel need a gig of VM?
Date: Fri, 28 Jan 2005 15:48:30 -0500 [thread overview]
Message-ID: <41FAA51E.10000@comcast.net> (raw)
In-Reply-To: <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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next prev parent reply other threads:[~2005-01-28 21:09 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-01-28 20:06 Why does the kernel need a gig of VM? John Richard Moser
2005-01-28 20:42 ` Josh Boyer
2005-01-28 20:48 ` John Richard Moser [this message]
2005-02-01 15:15 ` Bill Davidsen
2005-01-28 21:42 ` Oliver Neukum
2005-01-28 20:44 ` Chris Friesen
2005-01-29 1:32 ` Andy Isaacson
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