re: AMAP_SHARED (was Re: XIP)

[matthew green <mrg%eterna.com.au@localhost>]

> > > - For highly tuned, XIP'ed systems, programs should be designed to
> > >   avoid .data, because they're COW'ed to page cache sooner or later.
> > 
> > why is this a problem?
> > 
> > if the data is needed, and it will be written to, then these pages
> > will be allocated (COW'd) eventually, and the same space will be used.
> 
> Not a problem, as in it works.
> 
> As already explained, we allocate PV for XIP segments, only for
> vnode-backed AMAP_SHARED == shared .data.  Careful users may design
> the whole system to not allocate PV at all, by giving up that
> feature.  To help user's design decision, I stated the obvious -
> .data is XIP-unfriendly.

but why is it unfriendly?  i don't see why.  there's going to
be the same number of pages allocated for writeable data in
both cases, so the same amount of resources will be consumed.


.mrg.