Re: [patch 3/3][rfc] vmscan: batched swap slot allocation

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 21 Apr 2009 10:52:31 +0200
Johannes Weiner <hannes@cmpxchg.org> wrote:

> > Keeping multiple pages locked while they stay on private list ? 
> 
> Yeah, it's a bit suboptimal but I don't see a way around it.
> 
Hmm, seems to increase stale swap cache dramatically under memcg ;)

> > BTW, isn't it better to add "allocate multiple swap space at once" function
> > like
> >  - void get_swap_pages(nr, swp_entry_array[])
> > ? "nr" will not be bigger than SWAP_CLUSTER_MAX.
> 
> It will sometimes be, see __zone_reclaim().
> 
Hm ? If I read the code correctly, __zone_reclaim() just call shrink_zone() and
"nr" to shrink_page_list() is SWAP_CLUSTER_MAX, at most.

> I had such a function once.  The interesting part is: how and when do
> you call it?  If you drop the page lock in between, you need to redo
> the checks for unevictability and whether the page has become mapped
> etc.
> 
> You also need to have the pages in swap cache as soon as possible or
> optimistic swap-in will 'steal' your swap slots.  See add_to_swap()
> when the cache radix tree says -EEXIST.
> 

If I was you, modify "offset" calculation of
  get_swap_pages()
     -> scan_swap_map()
to allow that a cpu  tends to find countinous swap page cluster.
Too difficult ?

Regards,
-Kame

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