> One approach is to have just a "hint-map", which might indeed be good
> enough. The hint-map would contain entries only for private un-shared
> pages - which is often a large fraction of the pages. HOWEVER, I suspect
> that the UNIX fork()/exec() semantics would make even this be impractical:
> on average a lot of pages are not shared, but most pages are shared at
> least for a short while every once in a while. And even if they become
> unshared quickly after being shared, they will have lost the information
> about what single mapping they had.
On a system running Apache there is a small set of pages that is shared
between potentially hundreds of processes. I've seen httpds with SZ of
800k and SHARED of 720k on a particular busy server with 500 httpd
children. This is a static-only server, on servers with a more dynamic
mix the SZ will increase. On servers with mod_perl both SZ and SHARED can
increase (there's more stuff which is constant after config time). In
these cases the sharing stays in effect long after the fork().
Dean