Re: copy on write memory

[Jacob Gorm Hansen <jacobg@diku.dk>]

Keir Fraser wrote:
> 
> Well, you also can over-commit on stuff that is read-only and fault in
> on demand, just as you can demand-CoW writable stuff e.g., no need to
> have all of kernel or glibc in memory all the time -- only hot parts
> of both will be in use by the system at any time.
> 
> i.e., 
>  1. There is fault in from no page -> shareable page on read accesses.
>  2. There is fault from shareable page -> shareable page + exclusive
>     page on write accesses. 
>  Both of these require extra allocation of memory.

But I will need some external service to give them to me right when I 
need them, or I may run into the 'paged-the-pager' problem and die?

>>Perhaps this should just be a one-way street, you give up pages to be 
>>nice to others (and get cheaper hosting or whatever kind of reward you 
>>can think of in return), and then you lose the right to write to them 
>>for good.  Should you need more writable pages, you will have to re-grow 
>>your reservation, and if that fails you will need to flush some slabs or 
>>buffer caches or or page stuff to disk or whatever you do in Linux when 
>>you have memory pressure.  Ultimately you may want to migrate to a less 
>>loaded machine.
> 
> 
> It's another way of looking at the problem (end-to-end style I
> suppose). Potetntially worth investigating. :-)

Perhaps I will have a go at some point. If going in this direction 
perhaps it will make sense to do this in Xen anyway.

Jacob



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