Messages in this thread |  | | Subject | Re: broken VM in 2.4.10-pre9 | From | (Eric W. Biederman) | Date | 19 Sep 2001 15:37:26 -0600 |
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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
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