Re: [patch] mm: use-once mapped file pages

[Johannes Weiner <hannes@cmpxchg.org>]

Hello Minchan,

On Wed, Sep 16, 2009 at 12:26:27AM +0900, Minchan Kim wrote:
> Hi, Hannes.

[snapped stuff Rik answered already]

> > When dropping into reclaim, the VM has a hard time making progress
> > with these pages dominating. A And since all mapped pages are treated
> > equally (i.e. anon pages as well), a major part of the anon working
> > set is swapped out before the hashing completes as well.
> >
> > Failing reclaim and swapping show up pretty quickly in decreasing
> > overall system interactivity, but also in the throughput of the
> > hashing process itself.
> >
> > This patch implements a use-once strategy for mapped file pages.
> >
> > For this purpose, mapped file pages with page table references are not
> > directly activated at the end of the inactive list anymore but marked
> > with PG_referenced and sent on another roundtrip on the inactive list.
> > If such a page comes in again, another page table reference activates
> > it while the lack thereof leads to its eviction.
> >
> > The deactivation path does not clear this mark so that a subsequent
> > page table reference for a page coming from the active list means
> > reactivation as well.
> 
> It seems to be good idea. but I have a concern about embedded.
> AFAIK, some CPUs don't have accessed bit by hardware.
> maybe ARM series.
> (Nowadays, Some kinds of CPU series just supports access bit.
> but there are still CPUs that doesn't support it)
> 
> I am not sure there are others architecture.
> Your idea makes mapped page reclaim depend on access bit more tightly.
>  :(

ARM seems to emulate the accessed bit by ensuring a subsequent access
will fault when the young bit is cleared, so we should get one extra
minor fault per finally activated mapped file page as a trade-off.

I am not too concerned about that because it should be a rather rare
event.  Only fresh pages go through that.  PG_referenced is remembered
over activation/deactivation and once it's set, a page is treated just
like it is now: activated if referenced, reclaimed if not.

So yeah, there is a bit more overhead for ARM to approximate the
working set initially.  But the results are more trustworthy and we
get rid of a badly performing corner case in the VM.

> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 28aafe2..0c88813 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -508,9 +508,6 @@ int page_referenced(struct page *page,
> > A {
> > A  A  A  A int referenced = 0;
> >
> > - A  A  A  if (TestClearPageReferenced(page))
> > - A  A  A  A  A  A  A  referenced++;
> > -

This hunk should also get removed from the !CONFIG_MMU dummy function.
I'll wait a bit for more feedback and send a fixed revision.

Thanks,
	Hannes

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