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Date: Thu, 12 Jan 2012 15:46:17 +0000
From: Mel Gorman <mel@....ul.ie>
To: Rik van Riel <riel@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
akpm@...ux-foundation.org,
KOSAKI Motohiro <kosaki.motohiro@...il.com>,
Johannes Weiner <hannes@...xchg.org>, hughd@...gle.com,
aarcange@...hat.com
Subject: Re: [PATCH -mm 1/2] mm: kswapd test order 0 watermarks when
compaction is enabled
On Mon, Jan 09, 2012 at 09:33:13PM -0500, Rik van Riel wrote:
> When built with CONFIG_COMPACTION, kswapd does not try to free
> contiguous pages. Because it is not trying, it should also not
> test whether it succeeded, because that can result in continuous
> page reclaim, until a large fraction of memory is free and large
> fractions of the working set have been evicted.
>
hmm, I'm missing something about your explanation.
1. wakeup_kswapd passes requested order to kswapd_max_order. Bear in
mind that this does *not* happen for THP.
2. kswapd reads this and passes it to balance_pgdat
3. balance_pgdat puts that in scan_control
4. shrink_zone gets that scan_control and so on
kswapd does try to free contiguous pages.
What is the source of the contiguous allocations of concern? The
mm_vmscan_wakeup_kswapd tracepoint should be able to get you a stack
trace to identify the source of high-order allocations.
To confirm I was not on crazy pills I fired up a systemtap script
that did a burst of order-8 allocations (ok, some crazy pills) and
observed this with tracepoints
<idle>-0 [002] 236009.284803: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=8
kswapd0-53 [002] 236009.285028: mm_vmscan_kswapd_wake: nid=0 order=8
kswapd0-53 [002] 236009.285034: mm_vmscan_lru_isolate: isolate_mode=1 order=8 nr_requested=9 nr_scanned=0 nr_taken=0 contig_taken=0 contig_dirty=0 contig_failed=0
kswapd0-53 [002] 236009.285035: mm_vmscan_lru_isolate: isolate_mode=1 order=8 nr_requested=22 nr_scanned=0 nr_taken=0 contig_taken=0 contig_dirty=0 contig_failed=0
kswapd0-53 [002] 236009.285038: mm_vmscan_lru_isolate: isolate_mode=1 order=8 nr_requested=1 nr_scanned=1 nr_taken=1 contig_taken=0 contig_dirty=0 contig_failed=1
kswapd0-53 [002] 236009.285049: mm_vmscan_lru_isolate: isolate_mode=1 order=8 nr_requested=32 nr_scanned=32 nr_taken=32 contig_taken=12 contig_dirty=0 contig_failed=20
kswapd0-53 [002] 236009.285080: mm_vmscan_lru_isolate: isolate_mode=2 order=8 nr_requested=32 nr_scanned=38 nr_taken=38 contig_taken=24 contig_dirty=0 contig_failed=14
kswapd0-53 [002] 236009.285090: mm_vmscan_lru_isolate: isolate_mode=1 order=8 nr_requested=23 nr_scanned=24 nr_taken=24 contig_taken=4 contig_dirty=0 contig_failed=20
This is with CONFIG_COMPACTION.
You're still in the right area though. kswapd does contiguous-aware
reclaim it does not do any compaction and so potentially it is doing
excessive reclaim while depending on another process to do the
compaction for it. That is a problem.
> Also remove a line of code that increments balanced right before
> exiting the function.
>
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
> mm/vmscan.c | 22 +++++++++++++++++-----
> 1 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f54a05b..c3eec6b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2608,7 +2608,7 @@ loop_again:
> */
> for (i = 0; i <= end_zone; i++) {
> struct zone *zone = pgdat->node_zones + i;
> - int nr_slab;
> + int nr_slab, testorder;
> unsigned long balance_gap;
>
> if (!populated_zone(zone))
> @@ -2637,11 +2637,25 @@ loop_again:
> * gap is either the low watermark or 1%
> * of the zone, whichever is smaller.
> */
> + testorder = order;
> balance_gap = min(low_wmark_pages(zone),
> (zone->present_pages +
> KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
> KSWAPD_ZONE_BALANCE_GAP_RATIO);
> - if (!zone_watermark_ok_safe(zone, order,
> + /*
> + * Kswapd reclaims only single pages when
> + * COMPACTION_BUILD. Trying too hard to get
> + * contiguous free pages can result in excessive
> + * amounts of free memory, and useful things
> + * getting kicked out of memory.
> + * Limit the amount of reclaim to something sane,
> + * plus space for compaction to do its thing.
> + */
> + if (COMPACTION_BUILD) {
> + testorder = 0;
> + balance_gap += 2<<order;
> + }
> + if (!zone_watermark_ok_safe(zone, testorder,
> high_wmark_pages(zone) + balance_gap,
> end_zone, 0)) {
kswapd does reclaim high-order pages so this comment is misleading.
However I see the type of problem you are talking about.
Direct reclaim in shrink_zones() does a check for compaction_suitable()
when deciding whether to abort reclaim or not. How about doing the same
for kswapd and if compaction can go ahead, goto out?
> shrink_zone(priority, zone, &sc);
> @@ -2670,7 +2684,7 @@ loop_again:
> continue;
> }
>
> - if (!zone_watermark_ok_safe(zone, order,
> + if (!zone_watermark_ok_safe(zone, testorder,
> high_wmark_pages(zone), end_zone, 0)) {
> all_zones_ok = 0;
> /*
> @@ -2776,8 +2790,6 @@ out:
>
> /* If balanced, clear the congested flag */
> zone_clear_flag(zone, ZONE_CONGESTED);
> - if (i <= *classzone_idx)
> - balanced += zone->present_pages;
> }
> }
>
>
--
Mel Gorman
SUSE Labs
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