linux-kernel - Re: Why is get_user

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Message-ID: <20100812134338.14856uchty1ra39c@guarana.org>
Date:	Thu, 12 Aug 2010 13:43:38 +1000
From:	Kevin Easton <kevin@...rana.org>
To:	Micha Nelissen <micha@...i.hopto.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Why is get_user_pages so slow?

Quoting Micha Nelissen <micha@...i.hopto.org>:

> Hi all,
>
> Why is get_user_pages much slower than taking the faults? (I would  
> expect it to be faster).
>
> Attached example program first mallocs a piece of memory (64MB in  
> this case) then reads it "to take the faults". Afterwards, it uses  
> mmap with MAP_POPULATE to "speed up" and not to have to take the  
> faults, but have everything mapped in one go. I think mmap is using  
> get_user_pages in this case.
>
> $ ./memspeed
> malloc took 0 msecs
> read took 14 msecs
> write took 0 msecs
> free took 1 msecs
> mmap took 45 msecs
> munmap took 5 msecs
>
> Using MAP_POPULATE is 3 times as slow as the 'stupid'  
> implementation! I'm running a Core 2 duo e6300 system with linux  
> 2.6.28.4.
>
> Am I doing something wrong? MAP_POPULATE seems a bit of a joke to me.

Hi Micha,

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.

If you change the types of 'buf', 'p' and 'e' to 'char *' then it  
touches every page - and (and least on my test box) the MAP_POPULATE  
case pulls ahead.

     - Kevin

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