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

[Rik van Riel <riel@redhat.com>]

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.

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...

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

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