linux-kernel - Re: [RFC v3 00/13] vfs: hot data tracking

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Date:	Tue, 16 Oct 2012 07:42:17 +1100
From:	Dave Chinner <david@...morbit.com>
To:	zwu.kernel@...il.com
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxram@...ux.vnet.ibm.com, viro@...iv.linux.org.uk,
	dave@...os.cz, tytso@....edu, cmm@...ibm.com,
	Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
Subject: Re: [RFC v3 00/13] vfs: hot data tracking

On Wed, Oct 10, 2012 at 06:07:22PM +0800, zwu.kernel@...il.com wrote:
> From: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
> 
> NOTE:
> 
>   The patchset is currently post out mainly to make sure
> it is going in the correct direction and hope to get some
> helpful comments from other guys.
>   For more infomation, please check hot_tracking.txt in Documentation
> 
> TODO List:

1) Fix OOM issues - the hot inode tracking caches grow very large
and don't get trimmed under memory pressure. From slabtop, after
creating roughly 24 million single byte files(*) on a machine with
8GB RAM:

  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME                   
23859510 23859476  99%    0.12K 795317       30   3181268K hot_range_item
23859441 23859439  99%    0.16K 1037367       23   4149468K hot_inode_item
572530 572530 100%    0.55K  81790        7    327160K radix_tree_node
241706 241406  99%    0.22K  14218       17     56872K xfs_ili
241206 241204  99%    1.06K  80402        3    321608K xfs_inode

The inode tracking is trying to track all 24 million inodes even
though they have been written only once, and there are only 240,000
inodes in the cache at this point in time. That was the last update
that slabtop got, so it is indicative of the impending OOM situation
that occurred.

> Changelog from v2:
>  1.) Converted to Radix trees, not RB-tree [Zhiyong, Dave Chinner]
>  2.) Added memory shrinker [Dave Chinner]

I haven't looked at the shrinker, but clearly it is not working,
otherwise the above OOM situation would not be occurring.

Cheers,

Dave.

(*) Tested on an empty 17TB XFS filesystem with:

$ sudo mkfs.xfs -f -l size=131072b,sunit=8 /dev/vdc
meta-data=/dev/vdc               isize=256    agcount=17, agsize=268435455 blks
         =                       sectsz=512   attr=2, projid32bit=0
data     =                       bsize=4096   blocks=4563402735, imaxpct=5
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=131072, version=2
         =                       sectsz=512   sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
$ sudo mount -o logbsize=256k /dev/vdc /mnt/scratch
$ sudo chmod 777 /mnt/scratch
$ fs_mark  -D  10000  -S0  -n  100000  -s  1  -L  63  -d \
/mnt/scratch/0  -d  /mnt/scratch/1  -d  /mnt/scratch/2  -d \
/mnt/scratch/3  -d  /mnt/scratch/4  -d  /mnt/scratch/5  -d \
/mnt/scratch/6  -d  /mnt/scratch/7
.....
     0     21600000            1      16679.3         12552262
     0     22400000            1      15412.4         12588587
     0     23200000            1      16367.6         14199322
     0     24000000            1      15680.4         15741205
<hangs here w/ OOM>

-- 
Dave Chinner
david@...morbit.com
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