Re: [PATCH] vmscan: prevent background aging of anon page in no swap system

[Ying Han <yinghan@google.com>]

On Sun, Aug 29, 2010 at 8:43 AM, Minchan Kim <minchan.kim@gmail.com> wrote:
> Ying Han reported that backing aging of anon pages in no swap system
> causes unnecessary TLB flush.
>
> When I sent a patch(69c8548175), I wanted this patch but Rik pointed out
> and allowed aging of anon pages to give a chance to promote from inactive
> to active LRU.
>
> It has a two problem.
>
> 1) non-swap system
>
> Never make sense to age anon pages.
>
> 2) swap configured but still doesn't swapon
>
> It doesn't make sense to age anon pages until swap-on time.
> But it's arguable. If we have aged anon pages by swapon, VM have moved
> anon pages from active to inactive. And in the time swapon by admin,
> the VM can't reclaim hot pages so we can protect hot pages swapout.
>
> But let's think about it. When does swap-on happen? It depends on admin.
> we can't expect it. Nonetheless, we have done aging of anon pages to
> protect hot pages swapout. It means we lost run time overhead when
> below high watermark but gain hot page swap-[in/out] overhead when VM
> decide swapout. Is it true? Let's think more detail.
> We don't promote anon pages in case of non-swap system. So even though
> VM does aging of anon pages, the pages would be in inactive LRU for a long
> time. It means many of pages in there would mark access bit again. So access
> bit hot/code separation would be pointless.
>
> This patch prevents unnecessary anon pages demotion in not-swapon and
> non-configured swap system. Of course, it could make side effect that
> hot anon pages could swap out when admin does swap on.
> But I think sooner or later it would be steady state.
> So it's not a big problem.
> We could lose someting but gain more thing(TLB flush and unnecessary
> function call to demote anon pages).
>
> I used total_swap_pages because we want to age anon pages
> even though swap full happens.
>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Reported-by: Ying Han <yinghan@google.com>
> Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
> ---
>  mm/vmscan.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 3109ff7..d8fd87d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2211,7 +2211,7 @@ loop_again:
>                         * Do some background aging of the anon list, to give
>                         * pages a chance to be referenced before reclaiming.
>                         */
> -                       if (inactive_anon_is_low(zone, &sc))
> +                       if (total_swap_pages > 0 && inactive_anon_is_low(zone, &sc))
>                                shrink_active_list(SWAP_CLUSTER_MAX, zone,
>                                                        &sc, priority, 0);
>
> --
> 1.7.0.5
>
>

There are few other places in vmscan where we check nr_swap_pages and
inactive_anon_is_low. Are we planning to change them to use
total_swap_pages
to be consistent ?

--Ying

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