Re: [RFC][PATCH] split file and anonymous page queues #2

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

On Mon, 2007-03-19 at 20:52 -0400, Rik van Riel wrote:
> Split the anonymous and file backed pages out onto their own pageout
> queues.  This we do not unnecessarily churn through lots of anonymous
> pages when we do not want to swap them out anyway.
> 
> This should (with additional tuning) be a great step forward in
> scalability, allowing Linux to run well on very large systems where
> scanning through the anonymous memory (on our way to the page cache
> memory we do want to evict) is slowing systems down significantly.
> 
> This patch has been stress tested and seems to work, but has not
> been fine tuned or benchmarked yet.  For now the swappiness parameter
> can be used to tweak swap aggressiveness up and down as desired, but
> in the long run we may want to simply measure IO cost of page cache
> and anonymous memory and auto-adjust.
> 
> We apply pressure to each of sets of the pageout queues based on:
> - the size of each queue
> - the fraction of recently referenced pages in each queue,
>     not counting used-once file pages
> - swappiness (file IO is more efficient than swap IO)
> 
> Please take this patch for a spin and let me know what goes well
> and what goes wrong.

Rick:  Which tree is the patch against.  Diffs say 2.6.20.x86_64, but
doesn't apply to 2.6.20 which doesn't use __inc_zone_state() for things
like nr_active, nr_inactive, ...

Also, in the snippet:

>--- linux-2.6.20.x86_64/mm/swap_state.c.vmsplit 2007-02-04
>13:44:54.000000000 -0500
>+++ linux-2.6.20.x86_64/mm/swap_state.c 2007-03-19 12:00:23.000000000
-0400
>@@ -354,7 +354,7 @@ struct page *read_swap_cache_async(swp_e
>                        /*
>                         * Initiate read into locked page and return.
>                         */
>-                       lru_cache_add_active(new_page);
>+                       lru_cache_add_anon(new_page);
>                        swap_readpage(NULL, new_page);
>                        return new_page;
>                }

Should that be lru_cache_add_active_anon()? Or did you intend to add it
to the inactive anon list?

Finally, could you [should you?] skip scanning the anon lists--or at
least the inactive anon list--when nr_swap_pages == 0?  The anon pages
aren't going anywhere, right?  I think this would obviate Christoph L's
patch to exclude anon pages from the LRU when there is no swap.  

Lee