Re: broken VM in 2.4.10-pre9

[ebiederm@xmission.com (Eric W. Biederman)]

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

> > On September 17, 2001 06:03 pm, Eric W. Biederman wrote:
> > > In linux we have avoided reverse maps (unlike the BSD's) which tends
> > > to make the common case fast at the expense of making it more
> > > difficult to handle times when the VM system is under extreme load and
> > > we are swapping etc.
> > 
> > What do you suppose is the cost of the reverse map?  I get the impression you
> 
> > think it's more expensive than it is.
> 
> We can keep the typical page table cost lower than now (including reverse
> maps) just by doing some common sense small cleanups to get the page struct
> down to 48 bytes on x86

While there is a size cost I suspect you will notice reverse maps
a lot more in operations like fork where having them tripples the amount
of memory that you need to copy.  So you should see a double or more
in the time it takes to do a fork.

That I think is a significant cost.

Eric