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    SubjectRe: Why does the kernel need a gig of VM?
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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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