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

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Apr 21, 2009 at 06:23:31PM +0900, KAMEZAWA Hiroyuki wrote:
> 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 ;)

Hmpf, not good.

> > > 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.

shrink_zone() and shrink_inactive_list() use whatever is set in
sc->swap_cluster_max and for __zone_reclaim() this is:

	.swap_cluster_max = max_t(unsigned long, nr_pages, SWAP_CLUSTER_MAX)

SWAP_CLUSTER_MAX is 32 (2^5), so if you have an order 6 allocation
doing reclaim, you end up with sc->swap_cluster_max == 64 already.
Not common, but it happens.

> > 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 ?

This goes in the direction of extent-based allocations.  I tried that
once by providing every reclaimer with a cookie that is passed in for
swap allocations and used to find per-reclaimer offsets.

Something went wrong, I can not quite remember.  Will have another
look at this.