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Re: sparsemem support for mips with highmem

To: Randy Dunlap <>
Subject: Re: sparsemem support for mips with highmem
From: Christoph Lameter <>
Date: Mon, 18 Aug 2008 16:24:44 -0500
Cc: C Michael Sundius <>, Dave Hansen <>, Thomas Bogendoerfer <>,,,, Andy Whitcroft <>
In-reply-to: <>
Original-recipient: rfc822;
References: <> <1218753308.23641.56.camel@nimitz> <> <> <1218815299.23641.80.camel@nimitz> <> <> <> <1218821875.23641.103.camel@nimitz> <> <>
User-agent: Thunderbird (Windows/20080708)
Randy Dunlap wrote:

> +Sparsemem divides up physical memory in your system into N section of M
>                                                             sections
> +bytes. Page descriptors are created for only those sections that
> +actually exist (as far as the sparsemem code is concerned). This allows
> +for holes in the physical memory without having to waste space by
> +creating page discriptors for those pages that do not exist.
>                descriptors
> +When page_to_pfn() or pfn_to_page() are called there is a bit of overhead to
> +look up the proper memory section to get to the descriptors, but this
> +is small compared to the memory you are likely to save. So, it's not the
> +default, but should be used if you have big holes in physical memory.

This overhead can be avoided by configuring sparsemem to use a virtual vmemmap
(CONFIG_SPARSEMEM_VMEMMAP). In that case it can be used for non NUMA since the
overhead is less than even FLATMEM.

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