On Tue, Jan 17, 2012 at 05:39:32PM +0900, KAMEZAWA Hiroyuki wrote:

On Tue, 17 Jan 2012 17:13:57 +0900
Minchan Kim [off-list ref] wrote:

quoted

This patch insert memory pressure notify point into vmscan.c
Most problem in system slowness is swap-in. swap-in is a synchronous
opeartion so that it affects heavily system response.

This patch alert it when reclaimer start to reclaim inactive anon list.
It seems rather earlier but not bad than too late.

Other alert point is when there is few cache pages
In this implementation, if it is (cache < free pages),
memory pressure notify happens. It has to need more testing and tuning
or other hueristic. Any suggesion are welcome.

Signed-off-by: Minchan Kim <minchan@kernel.org>

In my 1st impression, isn't this too simple ?

I agree It's too simple. It would be good start point rather than
unnecessary complicated things.

quoted

---
 mm/vmscan.c |   28 ++++++++++++++++++++++++++++
 1 files changed, 28 insertions(+), 0 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2880396..cfa2e2d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -43,6 +43,7 @@
 #include <linux/sysctl.h>
 #include <linux/oom.h>
 #include <linux/prefetch.h>
+#include <linux/low_mem_notify.h>
 
 #include <asm/tlbflush.h>
 #include <asm/div64.h>

@@ -2082,16 +2083,43 @@ static void shrink_mem_cgroup_zone(int priority, struct mem_cgroup_zone *mz,
 {
 	unsigned long nr[NR_LRU_LISTS];
 	unsigned long nr_to_scan;
+
 	enum lru_list lru;
 	unsigned long nr_reclaimed, nr_scanned;
 	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
 	struct blk_plug plug;
+#ifdef CONFIG_LOW_MEM_NOTIFY
+	bool low_mem = false;
+	unsigned long free, file;
+#endif
 
 restart:
 	nr_reclaimed = 0;
 	nr_scanned = sc->nr_scanned;
 	get_scan_count(mz, sc, nr, priority);
+#ifdef CONFIG_LOW_MEM_NOTIFY
+	/* We want to avoid swapout */
+	if (nr[LRU_INACTIVE_ANON])
+		low_mem = true;

IIUC, nr[LRU_INACTIVE_ANON] can be easily > 0.

Yes. But I thought it would be better than late notification.
Late notification ends up swap out which is a big concern about this patch.
More proper timing suggestion helps me a lot.

And get_scan_count() now check per-memcg-lru. So, this only works when
memcg is not used.

Hmm, I didn't look at recent memcg/global reclaim unify patch of Johannes.
I need time to look at it.
Thanks.

quoted

+	/*
+	 * We want to avoid dropping page cache excessively
+	 * in no swap system
+	 */
+	if (nr_swap_pages <= 0) {
+		free = zone_page_state(mz->zone, NR_FREE_PAGES);
+		file = zone_page_state(mz->zone, NR_ACTIVE_FILE) +
+			zone_page_state(mz->zone, NR_INACTIVE_FILE);
+		/*
+		 * If we have very few page cache pages,
+		 * notify to user
+		 */
+		if (file < free)
+			low_mem = true;
+	}

I can't understand why you think you can check lowmem condition by "file < free".

The reason I thought so is I want to maintain some page cache to some degree.
But I admit It's very naive heuristic and should be improved.

And I don't think using per-zone data is good.
(I'm not sure how many zones embeded guys using..)

Agree. In case of swapless system, we need another heuristic.

Another idea:
1. can't we use some technique like cleancache to detect the condition ?

I totally forgot cleancache approach. Could you remind that?

2. can't we measure page-in/page-out distance by recording something ?

I can't understand your point. What's relation does it with swapout prevent?

3. NR_ANON + NR_FILE_MAPPED can't mean the amount of core memory if we can
   ignore the data file cache ?

It's good but how do we define some amount?
It's very vague but I guess we can get a good idea from that.
Perhaps, you already has it.

4. how about checking kswapd's busy status ?

Could you elaborate on your idea?

Kame, Thanks for reply, 

Thanks,
-Kame

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