Re: too big min_free_kbytes

[alex shi <lkml.alex@gmail.com>]

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I am tried the patch, but seems it has no effect for our regression.

Regards
Alex

On Thu, Jan 27, 2011 at 11:27 PM, Andrea Arcangeli <aarcange@redhat.com>wrote:

> On Thu, Jan 27, 2011 at 01:40:58PM +0000, Mel Gorman wrote:
> > On Wed, Jan 26, 2011 at 05:42:37PM +0000, Mel Gorman wrote:
> > > On Wed, Jan 26, 2011 at 04:36:55PM +0000, Mel Gorman wrote:
> > > > > But the wmarks don't
> > > > > seem the real offender, maybe it's something related to the tiny
> pci32
> > > > > zone that materialize on 4g systems that relocate some little
> memory
> > > > > over 4g to make space for the pci32 mmio. I didn't yet finish to
> debug
> > > > > it.
> > > > >
> > > >
> > > > This has to be it. What I think is happening is that we're in
> balance_pgdat(),
> > > > the "Normal" zone is never hitting the watermark and we constantly
> call
> > > > "goto loop_again" trying to "rebalance" all zones.
> > > >
> > >
> > > Confirmed.
> > > <SNIP>
> >
> > How about the following? Functionally it would work but I am concerned
> > that the logic in balance_pgdat() and kswapd() is getting out of hand
> > having being adjusted to work with a number of corner cases already. In
> > the next cycle, it could do with a "do-over" attempt to make it easier
> > to follow.
>
> That number 8 is the problem, I don't think anybody was ever supposed
> to free 8*highwmark pages. kswapd must work in the hysteresis range
> low->high area and then sleep wait low to hit again before it gets
> wakenup. Not sure how that number 8 ever come up... but to be it looks
> like the real offender and I wouldn't work around it.
>
> totally untested... I will test....
>
> ====
> Subject: vmscan: kswapd must not free more than high_wmark pages
>
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> When the min_free_kbytes is set with `hugeadm
> --set-recommended-min_free_kbytes" or with THP enabled (which runs the
> equivalent of "hugeadm --set-recommended-min_free_kbytes" to activate
> anti-frag at full effectiveness automatically at boot) the high wmark
> of some zone is as high as ~88M. 88M free on a 4G system isn't
> horrible, but 88M*8 = 704M free on a 4G system is definitely
> unbearable. This only tends to be visible on 4G systems with tiny
> over-4g zone where kswapd insists to reach the high wmark on the
> over-4g zone but doing so it shrunk up to 704M from the normal zone by
> mistake.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f5d90de..9e3c78e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2407,7 +2407,7 @@ loop_again:
>                         * zone has way too many pages free already.
>                         */
>                         if (!zone_watermark_ok_safe(zone, order,
> -                                       8*high_wmark_pages(zone), end_zone,
> 0))
> +                                       high_wmark_pages(zone), end_zone,
> 0))
>                                 shrink_zone(priority, zone, &sc);
>                        reclaim_state->reclaimed_slab = 0;
>                        nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,
>
>
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