Re: Why is get_user_pages so slow?
From: Kevin Easton
Date: Wed Aug 11 2010 - 23:50:40 EST
Quoting Micha Nelissen <micha@xxxxxxxxxxxxxx>:
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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