linux-kernel - Re: Free memory never fully used, swapping

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Date:	Mon, 29 Nov 2010 18:31:04 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Simon Kirby <sim@...tway.ca>
Cc:	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: Free memory never fully used, swapping

Hi

> On Tue, Nov 23, 2010 at 12:35:31AM -0800, Dave Hansen wrote:
> 
> > I wish.  :)  The best thing to do is to watch stuff like /proc/vmstat
> > along with its friends like /proc/{buddy,meminfo,slabinfo}.  Could you
> > post some samples of those with some indication of where the bad
> > behavior was seen?
> > 
> > I've definitely seen swapping in the face of lots of free memory, but
> > only in cases where I was being a bit unfair about the numbers of
> > hugetlbfs pages I was trying to reserve.
> 
> So, Dave and I spent quite some time today figuring out was going on
> here.  Once load picked up during the day, kswapd actually never slept
> until late in the afternoon.  During the evening now, it's still waking
> up in bursts, and still keeping way too much memory free:
> 
> 	http://0x.ca/sim/ref/2.6.36/memory_tonight.png
> 
> 	(NOTE: we did swapoff -a to keep /dev/sda from overloading)
> 
> We have a much better idea on what is happening here, but more questions.
> 
> 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
> ...
> [    0.000000] On node 0 totalpages: 1047279  
> [    0.000000]   DMA zone: 56 pages used for memmap
> [    0.000000]   DMA zone: 0 pages reserved   
> [    0.000000]   DMA zone: 3943 pages, LIFO batch:0
> [    0.000000]   DMA32 zone: 14280 pages used for memmap
> [    0.000000]   DMA32 zone: 832392 pages, LIFO batch:31
> [    0.000000]   Normal zone: 2688 pages used for memmap
> [    0.000000]   Normal zone: 193920 pages, LIFO batch:31

This machine's zone size are

	DMA32:  3250MB
	NORMAL:  750MB

This inbalance zone size is one of root cause of the strange swapping 
issue. I'm sure we certinally need to fix our VM heuristics. However 
there is no perfect heuristics in the real world and we can't make it. 
Also, I guess a bug reporter need practical workaround.

Then, I wrote following patch.

if you pass a following boot parameter, zone division change to
dma32=1G + normal=3G.

in grub.conf

	 kernel /boot/vmlinuz ro root=foobar .... zone_dma32_size=1G 


I bet this one reduce your head pain a lot. Can you please try this?
Of cource, this is only workaround. not truth fix.


>From 1446c915fd59a5f123c2619d1f1f3b4e1bd0c648 Mon Sep 17 00:00:00 2001
From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Date: Thu, 23 Dec 2010 08:57:27 +0900
Subject: [PATCH] x86: implement zone_dma32_size boot parameter

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
---
 Documentation/kernel-parameters.txt |    5 +++++
 arch/x86/mm/init_64.c               |   17 ++++++++++++++++-
 2 files changed, 21 insertions(+), 1 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index a5966c0..25b4a53 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2686,6 +2686,11 @@ and is between 256 and 4096 characters. It is defined in the file
 			Format:
 			<irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]
 
+	zone_dma32_size=nn[KMG]		[KNL,BOOT,X86-64]
+			forces the dma32 zone to have an exact size of <nn>.
+			This works to reduce dma32 zone (In other word, to
+			increase normal zone) size.
+
 ______________________________________________________________________
 
 TODO:
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 71a5929..12d813d 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -95,6 +95,21 @@ static int __init nonx32_setup(char *str)
 }
 __setup("noexec32=", nonx32_setup);
 
+static unsigned long max_dma32_pfn = MAX_DMA32_PFN;
+static int __init parse_zone_dma32_size(char *arg)
+{
+	unsigned long dma32_pages;
+
+	if (!arg)
+		return -EINVAL;
+
+	dma32_pages = memparse(arg, &arg) >> PAGE_SHIFT;
+	max_dma32_pfn = min(MAX_DMA_PFN + dma32_pages, MAX_DMA32_PFN);
+
+	return 0;
+}
+early_param("zone_dma32_size", parse_zone_dma32_size);
+
 /*
  * When memory was added/removed make sure all the processes MM have
  * suitable PGD entries in the local PGD level page.
@@ -625,7 +640,7 @@ void __init paging_init(void)
 
 	memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
 	max_zone_pfns[ZONE_DMA] = MAX_DMA_PFN;
-	max_zone_pfns[ZONE_DMA32] = MAX_DMA32_PFN;
+	max_zone_pfns[ZONE_DMA32] = max_dma32_pfn;
 	max_zone_pfns[ZONE_NORMAL] = max_pfn;
 
 	sparse_memory_present_with_active_regions(MAX_NUMNODES);
-- 
1.6.5.2




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