Re: [Alsa-user] new source of MIDI playback slow-down identified - 5a03b051ed87e72b959f32a86054e1142ac4cf55 thp: use compaction in kswapd for GFP_ATOMIC order > 0

[Arthur Marsh <arthur.marsh@internode.on.net>]

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Clemens Ladisch wrote, on 22/02/11 18:07:
> Arthur Marsh wrote:
>> I'm experiencing MIDI playback slow-downs when I'm observing kswapd0
>> active (a few percent of cpu in top output) in recent kernels.
>>
>> I git-bisected the problem down to:
>>
>> commit 5a03b051ed87e72b959f32a86054e1142ac4cf55
>> Author: Andrea Arcangeli<aarcange@redhat.com>
>> Date:   Thu Jan 13 15:47:11 2011 -0800
>>
>>       thp: use compaction in kswapd for GFP_ATOMIC order>  0
>>
>>       This takes advantage of memory compaction to properly generate pages of
>>       order>  0 if regular page reclaim fails and priority level becomes more
>>       severe and we don't reach the proper watermarks.
>>
>>       Signed-off-by: Andrea Arcangeli<aarcange@redhat.com>
>>       Signed-off-by: Andrew Morton<akpm@linux-foundation.org>
>>       Signed-off-by: Linus Torvalds<torvalds@linux-foundation.org>
>>
>> I ran git-bisect over the weekend, building and installing ALSA 1.0.24
>> with each kernel. After identifying the above commit, I rebuilt the 2.6
>> head with that commit reverted and verified that the problem was no
>> longer present.
>
> Apparently, huge page compaction disables interrupts for much too long.
>
>> MIDI playback was via aplaymidi -p 17:0 to a Soundblaster Audigy 2 ZS
>> (SB0350) wavetable.
>
> The ALSA sequencer uses either the system timer or an HR timer at 1 kHz
> to deliver MIDI commands (notes); the wavetable driver requires its own
> interrupts in regular 5.3 ms intervals.
>
>
> Regards,
> Clemens
>

Hi, Andrea Arcangeli's "z1" patch (attached) solved the problem for me, 
even with significant swap activity.

Regards,

Arthur.

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diff --git a/mm/vmscan.c b/mm/vmscan.c
index 17497d0..5718bca 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2385,7 +2385,6 @@ loop_again:
 		 * cause too much scanning of the lower zones.
 		 */
 		for (i = 0; i <= end_zone; i++) {
-			int compaction;
 			struct zone *zone = pgdat->node_zones + i;
 			int nr_slab;
 
@@ -2408,7 +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,
@@ -2416,24 +2415,23 @@ loop_again:
 			sc.nr_reclaimed += reclaim_state->reclaimed_slab;
 			total_scanned += sc.nr_scanned;
 
-			compaction = 0;
+#if 0
 			if (order &&
 			    zone_watermark_ok(zone, 0,
 					       high_wmark_pages(zone),
 					      end_zone, 0) &&
 			    !zone_watermark_ok(zone, order,
 					       high_wmark_pages(zone),
-					       end_zone, 0)) {
+					       end_zone, 0))
 				compact_zone_order(zone,
 						   order,
 						   sc.gfp_mask, false,
 						   COMPACT_MODE_KSWAPD);
-				compaction = 1;
-			}
+#endif
 
 			if (zone->all_unreclaimable)
 				continue;
-			if (!compaction && nr_slab == 0 &&
+			if (nr_slab == 0 &&
 			    !zone_reclaimable(zone))
 				zone->all_unreclaimable = 1;
 			/*