Re: too big min_free_kbytes

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Jan 27, 2011 at 04:27:55PM +0100, Andrea Arcangeli wrote:
> totally untested... I will test....

The below patch is fixing my problem and working fine for me... as
expected it can't possibly lead to any D state, it's pretty much like
setting min_free_kbytes lower, and it's not going to alter anything
other than the levels of free memory kept by kswapd.

$ while :; do ps xa|grep [k]swapd; sleep 1; done
  452 ?        R      1:20 [kswapd0]
  452 ?        S      1:20 [kswapd0]
  452 ?        S      1:20 [kswapd0]
  452 ?        S      1:20 [kswapd0]
  452 ?        S      1:20 [kswapd0]
  452 ?        R      1:20 [kswapd0]
  452 ?        R      1:20 [kswapd0]
  452 ?        R      1:20 [kswapd0]
  452 ?        R      1:20 [kswapd0]
  452 ?        S      1:20 [kswapd0]
  452 ?        R      1:20 [kswapd0]
$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system--
  ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us
  sy id wa
 2  1   1784 111040 2393336 807924    0    0    63   992   56   70  1   1 96  2
 0  1   1784 108928 2402556 801864    0    0 122624     0 1619 2150  0   5 80 16
 0  1   1784 110664 2401244 801140    0    0 122496     0 1602 2081  0   3 81 16
 0  1   1784 109796 2410184 792984    0    0 122752     0 1685 2149  0   4 80 16
 0  1   1784 110416 2411856 791208    0    0 120448     4 1599 2075  0   4 81 16
 1  0   1784 113516 2415344 785336    0    0 122496     0 1636 2125  0   4 81 15

I doubt we'll get any regression because of the below (see also my
prev email in this thread), and I would only expect more cache and
maybe better lru. Previously the free memory levels were stuck at
~700M now they're stuck at the right level for a 4G system with THP on
(I'd still like to try to reduce the requirements only 1 hugepage for
each migratetype in the set_min_free_kbytes to reduce the requirements
to the minium, but only if possible..). But this saves 600M over 4G so
it's the highest prio to address.

Comments welcome,
Thanks!
Andrea

> ====
> 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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