linux-kernel - Re: why choose 896MB to the start point of ZONE

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Message-ID: <q2he6bf505b1004061847l31b2fe15v31d1556862d3a2f9@mail.gmail.com>
Date:	Tue, 6 Apr 2010 20:47:44 -0500
From:	Xianghua Xiao <xiaoxianghua@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Youngwhan Song <breadncup@...il.com>,
	Venkatram Tummala <venkatram867@...il.com>,
	Joel Fernandes <agnel.joel@...il.com>,
	Frank Hu <frank.hu.2001@...il.com>,
	hayfeng Lee <teklife.kernel@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-kernel@...kernel.org" <linux-kernel@...kernel.org>,
	"kernelnewbies@...linux.org" <kernelnewbies@...linux.org>
Subject: Re: why choose 896MB to the start point of ZONE_HIGHMEM

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@...or.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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