linux-ext4 - Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to

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Date:	Tue, 20 Jul 2010 10:13:56 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Andreas Gruenbacher <agruen@...e.de>
CC:	Wang Sheng-Hui <crosslonelyover@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	linux-mm@...ck.org, linux-ext4 <linux-ext4@...r.kernel.org>,
	kernel-janitors <kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan
 > 0

Andreas Gruenbacher wrote:
> On Sunday 18 July 2010 08:36:59 Wang Sheng-Hui wrote:
>> I regenerated the patch. Please check it.
> 
> The logic for calculating how many objects to free is still wrong: 
> mb_cache_shrink_fn returns the number of entries scaled by 
> sysctl_vfs_cache_pressure / 100.  It should also scale nr_to_scan by the 
> inverse of that.  The sysctl_vfs_cache_pressure == 0 case (never scale) may 
> require special attention.

I don't think that's right:

vfs_cache_pressure
------------------

Controls the tendency of the kernel to reclaim the memory which is used for
caching of directory and inode objects.

At the default value of vfs_cache_pressure=100 the kernel will attempt to
reclaim dentries and inodes at a "fair" rate with respect to pagecache and
swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
never reclaim dentries and inodes due to memory pressure and this can easily
lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes.

0 means "never reclaim," it doesn't mean "never scale."

As for nr_to_scan, after the first call, the shrinker has a scaled
version of the total count, so the requested nr_to_scan on the
next call is already scaled based on that.

I think the logic in the mbcache shrinker is fine.

-Eric

> See dcache_shrinker() in fs/dcache.c.

> 
> Thanks,
> Andreas

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