linux-kernel - Re: Why is get_user

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Message-ID: <4C63C807.102@neli.hopto.org>
Date:	Thu, 12 Aug 2010 12:08:07 +0200
From:	Micha Nelissen <micha@...i.hopto.org>
To:	Kevin Easton <kevin@...rana.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Why is get_user_pages so slow?

Hi Kevin,

Kevin Easton wrote:
> Yep, you are.  Because your pointer 'p' is a pointer to int, when you 
> increment it by 0x1000 in your loops you are actually incrementing it by 
> 0x1000 * sizeof(int) - so you're only actually touching one page in four.

Oops sorry, thanks for catching my mistake.

I also discovered the following: if I read from all pages, then call 
get_user_pages, it is still quite slow (did I get a read-only page?). 
However, if I touch all pages by writing to them, then get_user_pages 
becomes a factor 40 times faster or so.

All is clear now, I think. Thanks.

Micha
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