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highmem questions

Subject: highmem questions
From: Srinivas Kommu <>
Date: Wed, 02 Aug 2006 15:11:35 -0700
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1. If I have 1 gig physical memory and CONFIG_HIGHMEM disabled, would the user processes be able to see the high memory? Only 256 meg (this is on BCM1250; so 256 meg is expected) shows up in /proc/meminfo and I couldn't malloc more than that much from user processes.

2. With highmem enabled, is there a penalty to the user processes? From what I understood, highmem mappings are needed for the kernel to access that memory. For the pages belonging to user processes, does it still use pkmaps?

3. How do I measure the penalty of highmem on kernel modules? Since the code and data for modules resides in highmem, the kernel has to constantly map and unmap while running inside the modules? Is there a way to quantify the overhead of running with highmem enabled versus not?

thanks in advance!


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