Re: A question about using a private anonymous mmap

[KAMEZAWA Hiroyuki]

On Fri, 5 Jun 2009 12:06:18 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Thu, 4 Jun 2009 17:48:29 +0100 (BST)
> Julian Phillips <julian@xxxxxxxxxxxxxxxxx> wrote:
> 
> > I have a program which creates a reasonably large private anonymous map. 
> > The program then writes into a few places in the map, but ends up reading 
> > from all of them.
> > 
> > When I run this program on a system running 2.6.20.7 the process only ever 
> > seems to use enough memory to hold the data that has actually been written 
> > (well - in units of PAGE_SIZE).  When I run the program on a system 
> > running 2.6.24.5 then as it reads the map the amount of memory used 
> > continues to increase until the complete map has actually been allocated 
> > (and since the total size is greater than the physically available RAM 
> > causes swapping).  Basically I seem to be seeing copy-on-read instead of 
> > copy-on-write type behaviour.
> > 
> > Is this an expected change, and is there any option I can tweak to get the 
> > old behaviour back?
> > 
> 
> It comes from removal of ZERO_PAGE.
> 
> IMHO, What you can do is mmap(/dev/zero, MAP_PRIVATE) instead of ANON.
> 
Ah, sorry, mmap(/dev/zero) trick is no help in current kernel, I misunderstood.

Thanks,
-Kame

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