Re: [PATCH 2.5.43-mm2] New shared page table patch

[ebiederm@xmission.com (Eric W. Biederman)]

"Martin J. Bligh" <mbligh@aracnet.com> writes:

> >> In many cases, this will stop the box from falling over flat on it's 
> >> face due to ZONE_NORMAL exhaustion (from pte-chains), or even total
> >> RAM exhaustion (from PTEs). Thus the performance gain is infinite ;-)
> > 
> > So why has no one written a pte_chain reaper?  It is perfectly sane
> > to allocate a swap entry and move an entire pte_chain to the swap
> > cache.  
> 
> I think the underlying subsystem does not easily allow for dynamic regeneration,
> so it's non-trivial. 

We swap pages out all of the time in 2.4.x, and that is all I was suggesting 
swap out some but not all of the pages, on a very long pte_chain.  And swapping
out a page is not terribly complex, unless something very drastic has changed.

> wli was looking at doing pagetable reclaim at some point,
> IIRC.
> 
> 
> IMHO, it's better not to fill memory with crap in the first place than
> to invent complex methods of managing and shrinking it afterwards. You
> only get into pathalogical conditions under sharing situation, else 
> it's limited to about 1% of RAM (bad, but manageable) ... thus providing
> this sort of sharing nixes the worst of it. Better cache warmth on 
> switches (for TLB misses), faster fork+exec, etc. are nice side-effects.

I will agree with that if everything works so the sharing happens,
this is a nice feature.

> The ultimate solution is per-object reverse mappings, rather than per
> page, but that's a 2.7 thingy now.
???

Last I checked we already had those in 2.4.x, and still in 2.5.x.  The
list of place the address space is mapped.

Eric