linux-kernel - Why is get_user

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Message-ID: <4C61AD7A.7040400@neli.hopto.org>
Date:	Tue, 10 Aug 2010 21:50:18 +0200
From:	Micha Nelissen <micha@...i.hopto.org>
To:	linux-kernel@...r.kernel.org
Subject: Why is get_user_pages so slow?

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.

Thanks,

Micha

View attachment "memspeed.c" of type "text/x-csrc" (1585 bytes)