Re: [PATCH/RFC 11/14] Reclaim Scalability: swap backed pages are nonreclaimable when no swap space available

[Lee Schermerhorn <Lee.Schermerhorn@hp.com>]

On Tue, 2007-09-18 at 16:01 -0400, Rik van Riel wrote:
> Lee Schermerhorn wrote:
> > On Sun, 2007-09-16 at 22:53 -0400, Rik van Riel wrote:
> >> Lee Schermerhorn wrote:
> >>> PATCH/RFC  11/14 Reclaim Scalability: treat swap backed pages as
> >>> 	non-reclaimable when no swap space is available.
> >>>
> >>> Against:  2.6.23-rc4-mm1
> >>>
> >>> Move swap backed pages [anon, shmem/tmpfs] to noreclaim list when
> >>> nr_swap_pages goes to zero.   Use Rik van Riel's page_anon() 
> >>> function in page_reclaimable() to detect swap backed pages.
> >>>
> >>> Depends on NORECLAIM_NO_SWAP Kconfig sub-option of NORECLAIM
> >>>
> >>> TODO:   Splice zones' noreclaim list when "sufficient" swap becomes
> >>> available--either by being freed by other pages or by additional 
> >>> swap being added.  How much is "sufficient" swap?  Don't want to
> >>> splice huge noreclaim lists every time a swap page gets freed.
> >> Yet another reason for my LRU list split between filesystem
> >> backed and swap backed pages: we can simply stop scanning the
> >> anon lists when swap space is full and resume scanning when
> >> swap space becomes available.
> > 
> > 
> > Hi, Rik:
> > 
> > It occurs to me that we probably don't want to stop scanning the anon
> > lists [active/inactive] when swap space is full.  We might have LOTS of
> > anon pages that already have swap space allocated to them that can be
> > reclaimed.  It's just those that don't already have swap space that
> > aren't reclaimable until more swap space becomes available.
> 
> Well, "lots" is a relative thing.

Agreed.  See below.

> 
> If we run into those pages in our normal course of scanning,
> we should free the swap space.
> 
> If swap space finally ran out, I suspect we should just give
> up.
> 
> If you have a system with 128GB RAM and 2GB swap, it really
> does not make a lot of sense to scan all the way through 90GB
> of anonymous pages to free maybe 1GB of swap...

I agree.  However:

1) that's the reason I'm putting swap-backed pages that are in excess of
available swap space on a noreclaim list.  So that only reclaimable
pages end up on the [anon] lru list.

2) consider the case of 128GB RAM and 64GB swap:  that's plenty of swap
space to make scanning of anon pages worthwhile.  But, if we can avoid
scanning the other 26GB [your "90GB" of anon less the 64GB of swappable
anon] in the process, scanning will be more efficient, I think.  Theory
needs testing, of course.

> 
> If swap is large, we can free swap space during the normal
> LRU scanning, before we completely run out.

If this works--we keep sufficient swap free during scanning--we'll never
declare anon/shmem/tmpfs pages non-reclaimable due to lack of swap
space.  If it doesn't we can still move the non-reclaimable ones
aside--if that's a performance win overall.  This depends on how
efficiently we can bring "unswappable" pages back from noreclaim-land.

Later,
Lee

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