[PATCH] mm,vmscan: compact memory from kswapd when lots of memory free already

[PATCH] mm,vmscan: compact memory from kswapd when lots of memory free already

[Rik van Riel]

If kswapd is woken up for a higher order allocation, for example
from alloc_skb, but the system already has lots of memory free,
kswapd_shrink_zone will rightfully decide kswapd should not free
any more memory.

However, at that point kswapd should proceed to compact memory, on
behalf of alloc_skb or others.

Currently kswapd will only compact memory if it first freed memory,
leading kswapd to never compact memory when there is already lots of
memory free.

On my home system, that lead to kswapd occasionally using up to 5%
CPU time, with many man wakeups from alloc_skb, and kswapd never
doing anything to relieve the situation that caused it to be woken
up.

Going ahead with compaction when kswapd did not attempt to reclaim
any memory, and as a consequence did not reclaim any memory, is the
right thing to do in this situation.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 71b1c29948db..9566a04b9759 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3343,7 +3343,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 		 * Compact if necessary and kswapd is reclaiming at least the
 		 * high watermark number of pages as requsted
 		 */
-		if (pgdat_needs_compaction && sc.nr_reclaimed > nr_attempted)
+		if (pgdat_needs_compaction && sc.nr_reclaimed >= nr_attempted)
 			compact_pgdat(pgdat, order);
 
 		/*
-- 
-- 
All rights reversed.

Re: [PATCH] mm,vmscan: compact memory from kswapd when lots of memory free already

[Vlastimil Babka]

On 02/23/2016 04:50 AM, Rik van Riel wrote:
> If kswapd is woken up for a higher order allocation, for example
> from alloc_skb, but the system already has lots of memory free,
> kswapd_shrink_zone will rightfully decide kswapd should not free
> any more memory.
>
> However, at that point kswapd should proceed to compact memory, on
> behalf of alloc_skb or others.
>
> Currently kswapd will only compact memory if it first freed memory,
> leading kswapd to never compact memory when there is already lots of
> memory free.
>
> On my home system, that lead to kswapd occasionally using up to 5%
> CPU time, with many man wakeups from alloc_skb, and kswapd never
> doing anything to relieve the situation that caused it to be woken
> up.

Hi,

I've proposed replacing kswapd compaction with kcompactd, so this hunk 
is gone completely in mmotm. This imperfect comparison was indeed one of 
the things I've noted, but it's not all:

http://marc.info/?l=linux-kernel&m=145493881908394&w=2

> Going ahead with compaction when kswapd did not attempt to reclaim
> any memory, and as a consequence did not reclaim any memory, is the
> right thing to do in this situation.
>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
>   mm/vmscan.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 71b1c29948db..9566a04b9759 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3343,7 +3343,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>   		 * Compact if necessary and kswapd is reclaiming at least the
>   		 * high watermark number of pages as requsted
>   		 */
> -		if (pgdat_needs_compaction && sc.nr_reclaimed > nr_attempted)
> +		if (pgdat_needs_compaction && sc.nr_reclaimed >= nr_attempted)
>   			compact_pgdat(pgdat, order);
>
>   		/*
>

Re: [PATCH] mm,vmscan: compact memory from kswapd when lots of memory free already

[Rik van Riel]

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On Tue, 2016-02-23 at 10:18 +0100, Vlastimil Babka wrote:
> On 02/23/2016 04:50 AM, Rik van Riel wrote:
> > If kswapd is woken up for a higher order allocation, for example
> > from alloc_skb, but the system already has lots of memory free,
> > kswapd_shrink_zone will rightfully decide kswapd should not free
> > any more memory.
> > 
> > However, at that point kswapd should proceed to compact memory, on
> > behalf of alloc_skb or others.
> > 
> > Currently kswapd will only compact memory if it first freed memory,
> > leading kswapd to never compact memory when there is already lots
> > of
> > memory free.
> > 
> > On my home system, that lead to kswapd occasionally using up to 5%
> > CPU time, with many man wakeups from alloc_skb, and kswapd never
> > doing anything to relieve the situation that caused it to be woken
> > up.
> 
> Hi,
> 
> I've proposed replacing kswapd compaction with kcompactd, so this
> hunk 
> is gone completely in mmotm. This imperfect comparison was indeed one
> of 
> the things I've noted, but it's not all:
> 
> http://marc.info/?l=linux-kernel&m=145493881908394&w=2

Never mind my patch, then. Your solution is nicer,
and already in -mm :)

-- 
All Rights Reversed.


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