Re: [PATCH] Rmap speedup

[Andrew Morton]

Rik van Riel wrote:
> 
> On Fri, 2 Aug 2002, Andrew Morton wrote:
> > Daniel Phillips wrote:
> > >
> > > This patch eliminates about 35% of the raw rmap setup/teardown overhead by
> > > adopting a new locking interface that allows the add_rmaps to be batched in
> > > copy_page_range.
> >
> > Well that's fairly straightforward, thanks.  Butt-ugly though ;)
> 
> It'd be nice if the code would be a bit more beautiful and the
> reverse mapping scheme more modular.

I changed it to, essentially:

foo()
{
	spinlock_t *rmap_lock = NULL;
	unsigned rmap_lockno = -1;
	...
	for (stuff) {
		cached_rmap_lock(page, &rmap_lock, &rmap_lockno);
		__page_add_rmap(page, ptep);
		..
	}
	drop_rmap_lock(&rmap_lock, &rmap_lockno);
}

See http://www.zip.com.au/~akpm/linux/patches/2.5/2.5.30/daniel-rmap-speedup.patch

Fixing zap_pte_range pretty much requires the pagemap_lru_lock
rework; otherwise we couldn't hold the rmap lock across
tlb_remove_page().

> Remember that we're planning to go to an object-based scheme
> later on, turning the code into a big monolithic mesh really
> makes long-term maintenance a pain...

We have short-term rmap problems:

1) Unexplained pte chain state with ntpd
2) 10-20% increased CPU load in fork/exec/exit loads
3) system lock under heavy mmap load
4) ZONE_NORMAL pte_chain consumption

Daniel and I are on 2), Bill is on 4) (I think).