need for 3G/1G split

Hi,

Why is kernel restricted to use virtual addresses of only 1 GB? Or
what is the need to split up the virtual address space into user
addresses and kernel addresses?

TIA,

Rick
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Re: need for 3G/1G split

> Why is kernel restricted to use virtual addresses of only 1 GB? Or
> what is the need to split up the virtual address space into user
> addresses and kernel addresses?

We can have 4GB for kernel and 4GB for userspace but this has a overhead
of flushing of tlb for every syscall.
You can also make the split 2/2GB but this has a problem of having less
user address space.
3/1 is a better choice.

--
Vivek Kutal


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Re: need for 3G/1G split

Also, go through the following article, they have explained things in
a very nice way.

http://kerneltrap.org/node/2450



On 3/21/07, Vivek wrote:
> > Why is kernel restricted to use virtual addresses of only 1 GB? Or
> > what is the need to split up the virtual address space into user
> > addresses and kernel addresses?
>
> We can have 4GB for kernel and 4GB for userspace but this has a overhead
> of flushing of tlb for every syscall.
> You can also make the split 2/2GB but this has a problem of having less
> user address space.
> 3/1 is a better choice.
>
> --
> Vivek Kutal
>
>
>
> --
> To unsubscribe from this list: send an email with
> "unsubscribe kernelnewbies" to ecartis@nl.linux.org
> Please read the FAQ at http://kernelnewbies.org/FAQ
>
>


--
Regards,
Sandeep






If the facts don't fit the theory, change the facts.
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Re: need for 3G/1G split

3/21/07, Vivek wrote:
> > Why is kernel restricted to use virtual addresses of only 1 GB? Or
> > what is the need to split up the virtual address space into user
> > addresses and kernel addresses?
>
> We can have 4GB for kernel and 4GB for userspace but this has a overhead
> of flushing of tlb for every syscall.
> You can also make the split 2/2GB but this has a problem of having less
> user address space.
> 3/1 is a better choice.

Does TLB size change 4, 3, or 2 GB virtual addresses are used? I
though it would be same irrespective of whether virtual addresses are
4 or 1 GB.
If the TLB size is at all same for any number of virtual addresses
then what difference it make while flushing the TLB?

>
> --
> Vivek Kutal
>
>
>
> --
> To unsubscribe from this list: send an email with
> "unsubscribe kernelnewbies" to ecartis@nl.linux.org
> Please read the FAQ at http://kernelnewbies.org/FAQ
>
>
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Re: need for 3G/1G split

On 3/22/07, Prasad Joshi wrote:
> 3/21/07, Vivek wrote:
> > > Why is kernel restricted to use virtual addresses of only 1 GB? Or
> > > what is the need to split up the virtual address space into user
> > > addresses and kernel addresses?
> >
> > We can have 4GB for kernel and 4GB for userspace but this has a overhead
> > of flushing of tlb for every syscall.
> > You can also make the split 2/2GB but this has a problem of having less
> > user address space.
> > 3/1 is a better choice.
>
> Does TLB size change 4, 3, or 2 GB virtual addresses are used? I
> though it would be same irrespective of whether virtual addresses are
> 4 or 1 GB.
> If the TLB size is at all same for any number of virtual addresses
> then what difference it make while flushing the TLB?
>

The difference is that the TLB needs not be flushed for the kernel
virtual addresses because it remains the same for each processes. The
bigger the kernel virtual address space the better it is (but lesser
virtual memory for user space).
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