linux-kernel - Re: [PATCH] vmscan: fix missing place to check nr_swap

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Date:	Thu, 26 Aug 2010 20:31:00 -0700
From:	Ying Han <yinghan@...gle.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH] vmscan: fix missing place to check nr_swap_pages.

On Thu, Aug 26, 2010 at 6:03 PM, Minchan Kim <minchan.kim@...il.com> wrote:
>
> Hello.
>
> On Fri, Aug 27, 2010 at 9:11 AM, Ying Han <yinghan@...gle.com> wrote:
> > Fix a missed place where checks nr_swap_pages to do shrink_active_list. Make the
> > change that moves the check to common function inactive_anon_is_low.
> >
>
> Hmm.. AFAIR, we discussed it at that time but we concluded it's not good.
> That's because nr_swap_pages < 0 means both "NO SWAP" and "NOT enough
> swap space now". If we have a swap device or file but not enough space
> now, we need to aging anon pages to make inactive list enough size.
> Otherwise, working set pages would be swapped out more fast before
> promotion.

We found the problem on one of our workloads where more TLB flush
happens without the change. Kswapd seems to be calling
shrink_active_list() which eventually clears access bit of those ptes
and does TLB flush
with ptep_clear_flush_young(). This system does not have swap
configured, and why aging the anon lru in that
case?

> That aging is done by kswapd so I think it's not big harmful in the system.
> But if you want to remove aging completely in non-swap system, we need
> to identify non swap system and not enough swap space. I thought we
> need it for embedded system.

Lots of TLB flush hurts the performance especially on large smp system. So does
it make sense if change it to:

+       if (nr_swap_pages == 0)
+               return 0;

--Ying


> Thanks.
>
>
> --
> Kind regards,
> Minchan Kim
>
> --
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