Re: Garbage collection

Nick Roberts <nick.roberts@acm.org> kirjoitti 13.08.2004:
> On 11 Aug 2004 12:58:25 -0400, glen herrmannsfeldt <gah@ugcs.caltech.edu>
> wrote:
>> As I understand it, it is common on many systems for the memory
>> allocation system to fill page tables with pointers to one zero
>> filled page, and then allocate a real page with the first write
>> operation.
>
> I don't understand why it is necessary for the PTEs to be set to
> point to any frame. Why not just set the P bit to 0?


Presumably because when the P bit is 0, even reading the page triggers a
page fault; if all zero-filled allocated pages are physically shared,
read operations do not trigger a page fault, and only when a write
operation is attempted is the operating system called.


I'm not sure how useful that is; presumably most pages are allocated in
order to write to them, not in order to read zeros from them.


I am also unsure how this relates to compilers any more :)
--
Antti-Juhani Kaijanaho http://antti-juhani.kaijanaho.info/
[There still seems to be a fair amount of C code that likes to dereference
random pointers and expects to find zeros, but I agree that this has
wandered away from compilers. -John]

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