Re: Free memory never fully used, swapping

[Simon Kirby]

On Thu, Nov 25, 2010 at 04:12:38PM +0000, Mel Gorman wrote:

> On Thu, Nov 25, 2010 at 01:03:28AM -0800, Simon Kirby wrote:
> > > > <SNIP>
> > > >
> > > > This x86_64 box has 4 GB of RAM; zones are set up as follows:
> > > > 
> > > > [    0.000000] Zone PFN ranges:
> > > > [    0.000000]   DMA      0x00000001 -> 0x00001000
> > > > [    0.000000]   DMA32    0x00001000 -> 0x00100000
> > > > [    0.000000]   Normal   0x00100000 -> 0x00130000
> 
> Ok. A consequence of this is that kswapd balancing a node will still try
> to balance Normal even if DMA32 has enough memory. This could account
> for some of kswapd being mean.

This seemed to be the case.  DMA32 would be freed until watermarks were
met, and then it would fight order-0 allocations in Normal to try to meet
the watermark there, even though DMA32 had more than enough free.

> > > > The fact that so much stuff is evicted just because order-3 hits 0 is
> > > > crazy, especially when larger order pages are still free.  It seems like
> > > > we're trying to keep large orders free here.  Why? 
> 
> Watermarks. The steady stream of order-3 allocations is telling the
> allocator and kswapd that these size pages must be available. It doesn't
> know that slub can happily fall back to smaller pages because that
> information is lost. Even removing __GFP_WAIT won't help because kswapd
> still gets woken up for atomic allocation requests.

Since slub has a fallback to s->min (order 0 in this case), I think it
makes sense to make a GFP_NOBALANCE / GFP_NOKSWAPD, or GFP_NORECLAIM
(allow compaction if compiled, since slub's purpose is to reduce object
container waste).

> > The funny thing here is that slub.c's allocate_slab() calls alloc_pages()
> > with flags | __GFP_NOWARN | __GFP_NORETRY, and intentionally tries a
> > lower order allocation automatically if it fails.  This is why there is
> > no allocation failure warning when this happens.  However, it is too late
> > -- kswapd is woken and it ties to bring order 3 up to the watermark. 
> > If we hacked __alloc_pages_slowpath() to not wake kswapd when
> > __GFP_NOWARN is set, we would never see this problem and the slub
> > optimization might still mostly work. 
> 
> Yes, but we'd see more high-order atomic allocation (e.g. jumbo frames)
> failures as a result so that fix would cause other regressions.

The part of your patch that fixes the SWAP_CLUSTER_MAX also increases
this chance.

> I think there are at least two fixes required here.
> 
> 1. sleeping_prematurely() must be aware that balance_pgdat() has dropped
>    the order.
> 2. kswapd is trying to balance all zones for higher orders even though
>    it doesn't really have to.
> 
> This patch has potential fixes for both of these problems. I have a split-out
> series but I'm posting it as a single patch so see if it allows kswapd to
> go to sleep as expected for you and whether it stops hammering the Normal
> zone unnecessarily. I tested it locally here (albeit with compaction
> enabled) and it did reduce the amount of time kswapd spent awake.

Ok, so this patch has been running in production since Friday, and there
is a definite improvement in our case.  The reading of daemons and core
system stuff back into memory all the time from /dev/sda has stopped,
since kswapd now actually sleeps.  Since this allows caching to work, we
can keep swap enabled.  However, my userspace reimplementation of
zone_watermark_ok(order=3) shows that order-3 is almost never reached
now.  It's not freeing The Right Stuff, or not enough of it before
SWAP_CLUSTER_MAX.

Also, as the days go by, it is still keeping more and more free memory. 
my whitespace-compressed buddyinfo+extras right now shows:

 DMA32  234402    493     2     3    2   2  2 0 0 0 0  235644 249 <= 512
Normal     369      5     3     7    1   0  0 0 0 0 0     463 87 <= 238

So, the order-3 watermarks are still not met, and the free order-0 free
pages seems to keep rising.  Here is the munin memory graph of the week:

	http://0x.ca/sim/ref/2.6.36/memory_mel_patch_4days.png

Things seem to be drifting to kswapd reclaiming order-0 in a way that
doesn't add up to more order-3 blocks available, so fragmentation is
increasing.

You mentioned that there was some mechanism to have it actually target
allocations that could be made into higher order allocations?

Simon-

> ==== CUT HERE ====
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 39c24eb..25fe08d 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -645,6 +645,7 @@ typedef struct pglist_data {
>  	wait_queue_head_t kswapd_wait;
>  	struct task_struct *kswapd;
>  	int kswapd_max_order;
> +	enum zone_type high_zoneidx;
>  } pg_data_t;
>  
>  #define node_present_pages(nid)	(NODE_DATA(nid)->node_present_pages)
> @@ -660,7 +661,7 @@ typedef struct pglist_data {
>  
>  extern struct mutex zonelists_mutex;
>  void build_all_zonelists(void *data);
> -void wakeup_kswapd(struct zone *zone, int order);
> +void wakeup_kswapd(struct zone *zone, int order, enum zone_type high_zoneidx);
>  int zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>  		int classzone_idx, int alloc_flags);
>  enum memmap_context {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 07a6544..344b597 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1921,7 +1921,7 @@ void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
>  	struct zone *zone;
>  
>  	for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
> -		wakeup_kswapd(zone, order);
> +		wakeup_kswapd(zone, order, high_zoneidx);
>  }
>  
>  static inline int
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d31d7ce..00529a0 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2118,15 +2118,17 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
>  #endif
>  
>  /* is kswapd sleeping prematurely? */
> -static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
> +static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
>  {
>  	int i;
> +	bool all_zones_ok = true;
> +	bool any_zone_ok = false;
>  
>  	/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
>  	if (remaining)
>  		return 1;
>  
> -	/* If after HZ/10, a zone is below the high mark, it's premature */
> +	/* Check the watermark levels */
>  	for (i = 0; i < pgdat->nr_zones; i++) {
>  		struct zone *zone = pgdat->node_zones + i;
>  
> @@ -2138,10 +2140,20 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
>  
>  		if (!zone_watermark_ok(zone, order, high_wmark_pages(zone),
>  								0, 0))
> -			return 1;
> +			all_zones_ok = false;
> +		else
> +			any_zone_ok = true;
>  	}
>  
> -	return 0;
> +	/*
> +	 * For high-order requests, any zone meeting the watermark is enough
> +	 *   to allow kswapd go back to sleep
> +	 * For order-0, all zones must be balanced
> +	 */
> +	if (order)
> +		return !any_zone_ok;
> +	else
> +		return !all_zones_ok;
>  }
>  
>  /*
> @@ -2168,6 +2180,7 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
>  static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
>  {
>  	int all_zones_ok;
> +	int any_zone_ok;
>  	int priority;
>  	int i;
>  	unsigned long total_scanned;
> @@ -2201,6 +2214,7 @@ loop_again:
>  			disable_swap_token();
>  
>  		all_zones_ok = 1;
> +		any_zone_ok = 0;
>  
>  		/*
>  		 * Scan in the highmem->dma direction for the highest
> @@ -2310,10 +2324,12 @@ loop_again:
>  				 * spectulatively avoid congestion waits
>  				 */
>  				zone_clear_flag(zone, ZONE_CONGESTED);
> +				if (i <= pgdat->high_zoneidx)
> +					any_zone_ok = 1;
>  			}
>  
>  		}
> -		if (all_zones_ok)
> +		if (all_zones_ok || (order && any_zone_ok))
>  			break;		/* kswapd: all done */
>  		/*
>  		 * OK, kswapd is getting into trouble.  Take a nap, then take
> @@ -2336,7 +2352,7 @@ loop_again:
>  			break;
>  	}
>  out:
> -	if (!all_zones_ok) {
> +	if (!(all_zones_ok || (order && any_zone_ok))) {
>  		cond_resched();
>  
>  		try_to_freeze();
> @@ -2361,7 +2377,13 @@ out:
>  		goto loop_again;
>  	}
>  
> -	return sc.nr_reclaimed;
> +	/*
> +	 * Return the order we were reclaiming at so sleeping_prematurely()
> +	 * makes a decision on the order we were last reclaiming at. However,
> +	 * if another caller entered the allocator slow path while kswapd
> +	 * was awake, order will remain at the higher level
> +	 */
> +	return order;
>  }
>  
>  /*
> @@ -2417,6 +2439,7 @@ static int kswapd(void *p)
>  		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
>  		new_order = pgdat->kswapd_max_order;
>  		pgdat->kswapd_max_order = 0;
> +		pgdat->high_zoneidx = MAX_ORDER;
>  		if (order < new_order) {
>  			/*
>  			 * Don't sleep if someone wants a larger 'order'
> @@ -2464,7 +2487,7 @@ static int kswapd(void *p)
>  		 */
>  		if (!ret) {
>  			trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
> -			balance_pgdat(pgdat, order);
> +			order = balance_pgdat(pgdat, order);
>  		}
>  	}
>  	return 0;
> @@ -2473,7 +2496,7 @@ static int kswapd(void *p)
>  /*
>   * A zone is low on free memory, so wake its kswapd task to service it.
>   */
> -void wakeup_kswapd(struct zone *zone, int order)
> +void wakeup_kswapd(struct zone *zone, int order, enum zone_type high_zoneidx)
>  {
>  	pg_data_t *pgdat;
>  
> @@ -2483,8 +2506,10 @@ void wakeup_kswapd(struct zone *zone, int order)
>  	pgdat = zone->zone_pgdat;
>  	if (zone_watermark_ok(zone, order, low_wmark_pages(zone), 0, 0))
>  		return;
> -	if (pgdat->kswapd_max_order < order)
> +	if (pgdat->kswapd_max_order < order) {
>  		pgdat->kswapd_max_order = order;
> +		pgdat->high_zoneidx = min(pgdat->high_zoneidx, high_zoneidx);
> +	}
>  	trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
>  	if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
>  		return;
--
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