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Re: Ascertaining dirty pages

On Feb 18, 2008, at 7:39 AM, Vincent Coetzee <email@hidden> wrote: 
Hi Steven,

I am aware of vmmap, but perhaps I did not make the nature of my problem very clear. My application needs to know about these pages, and needs to be able to access information about the pages. Additionally vmmap does not tell one which pages are dirty, ,it just tells you about virtual memory allocated to a process. I need to know on a page by page basis (i.e. currently 4096 bytes per page) which pages have been written to and the exact address of the changes within the page.



You can't know which specific addresses in a page have changed, without keeping shadow copies, raising an exception (by intentionally setting the page permissions incorrectly), and then doing a byte compare.

So for example, if you did a strcpy() of a 60 byte string including the NUL, you would take 60 exceptions, have to remember all of them, and figure out some way to aggregate them to account them to a single "event", the strcpy() call.

Unless you are writing an "electric fence" type system, and this is solely for debugging and you do not care if your application runs 1000's of time slower, this is the wrong approach to the problem.

For BSS data (global initialized data), the data is mapped in COW. The vmmap utility will tell you if this data is written by having remapped the region PRV, if the in core copy differs from the on disk copy, since writing it triggers the COW.

For your own allocations, there are some tricks you can play, but they mostly come down to using mallocdebug properly.

If you are using Objective C and have enabled garbage collection, without explicit collection calls to create synchronization points (and only looking after you hit one), the issue becomes significantly more complex.

So tell us if you need the information at runtime vs. debug, and tell us why you think you need the information at all, i.e. what problem you are trying to solve that leads you to believe the data is necessary.

-- Terry
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