Re: 2.4 vm, program load, page faulting, ...

[Andrew Morton]

Alex Tomas <[e-mail]> wrote:
>
>  w> You should ask Andrew about his patch to do exactly that: he
>  w> forced all PROC_EXEC mmaps to be nonlinear-mapped and this forced
>  w> all programs to suck entire binaries into memory...  I recall he
>  w> saw at least 25% improvement at launching gnome.
> 
> they talked about pages _already present_ in pagecache.

2.5.64-mm8 does that too.  At mmap-time it will, for a PROT_EXEC mapping,
pull every affected page off disk and it will instantiate pte's against
them all via install_page().

So there should be zero major and minor faults against that mmap region
during application startup.

The improved IO layout appears to halve startup time for big things.  I
haven't attempted to instrument the effects of the reduced minor fault rate. 
If indeed the rate _has_ decreased.  If it hasn't, it's a bug...

This is all a bit dubious for several reasons.  Most particularly, the
up-front instantiation of the pages in pagetables makes unneeded pages harder
to reclaim.  It would be really neat if someone could try putting the
madvise(MADV_WILLNEED) into glibc and test that.  Maybe on a 2.4 kernel.