Re: why choose 896MB to the start point of ZONE_HIGHMEM

[Xianghua Xiao <xiaoxianghua@gmail.com>]

If the last 128MB out of the kernel 1GB space is used to for highmen,
meanwhile it's also used for IO/vmalloc, how does this work?

Xianghua

On Tue, Apr 6, 2010 at 6:32 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 04/06/2010 04:27 PM, Youngwhan Song wrote:
>> Nice explanation, Venkatram,
>>
>> Just one question pop up mind.
>>
>> What if actual physical memory is only 256MB? How does kernel divide
>> virtual memory? Do we need to specify the region to kernel? Or will
>> kernel itself decide it automatically?
>>
>
> If there is less than 896 MB of physical memory, the vmalloc region is
> automatically extended (in your case, it will be 768 MB in size.)  There
> will be no HIGHMEM in such a case, and if you are compiling your own
> kernel you will gain considerable speed by disabling HIGHMEM support
> completely.
>
> This, of course, was the norm back when Linux was first created, and a
> typical amount of memory was 8 MB or so.  That we'd have gigabytes of
> memory seemed very distant at the time.
>
>        -hpa
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