[PATCH 1/1] shrink dentry cache before inode cache

[PATCH 1/1] shrink dentry cache before inode cache

[Cliff Wickman]

The shrink_slab() function must often be called twice to get significant
slab cache reduction.

shrink_slab() walks the shrinker_list to call functions that can
release kernel slab memory.

The shrinker_list is walked head to tail and, as it is now, comes across the
inode cache shrinker first.  This releases inodes found on the inode_unused 
list.  Afterwards the dentry cache shrinker moves many freeable inodes to 
the list.  But those inodes are not freed until a second invocation of 
shrink_slab().

The dentry cache shrinker (shrink_dcache_memory()) should run before 
the inode cache shrinker (shrink_icache_memory()).

This can be accomplished by queuing the dentry cache shrinker earlier -
simply calling inode_init() before dcache_init().

Diffed against 2.6.15-rc5

Signed-off-by: Cliff Wickman <cpw@sgi.com>
---
 fs/dcache.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletion(-)

Index: linux-2.6.16-rc5/fs/dcache.c
===================================================================
--- linux-2.6.16-rc5.orig/fs/dcache.c
+++ linux-2.6.16-rc5/fs/dcache.c
@@ -1738,8 +1738,11 @@ void __init vfs_caches_init(unsigned lon
 	filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC, filp_ctor, filp_dtor);
 
-	dcache_init(mempages);
 	inode_init(mempages);
+	dcache_init(mempages); /* place after inode_init so that the dentry
+				  cache shrink goes onto the shrinker list
+				  before the inode cache shrink;
+				  freeing dentry's does iput's of inodes */
 	files_init(mempages);
 	mnt_init(mempages);
 	bdev_cache_init();
-- 
Cliff Wickman
Silicon Graphics, Inc.
cpw@sgi.com
(651) 683-3824

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Re: [PATCH 1/1] shrink dentry cache before inode cache

[Andrew Morton]

Cliff Wickman <cpw@sgi.com> wrote:
>
> The shrink_slab() function must often be called twice to get significant
> slab cache reduction.
> 
> shrink_slab() walks the shrinker_list to call functions that can
> release kernel slab memory.
> 
> The shrinker_list is walked head to tail and, as it is now, comes across the
> inode cache shrinker first.  This releases inodes found on the inode_unused 
> list.  Afterwards the dentry cache shrinker moves many freeable inodes to 
> the list.  But those inodes are not freed until a second invocation of 
> shrink_slab().
> 
> The dentry cache shrinker (shrink_dcache_memory()) should run before 
> the inode cache shrinker (shrink_icache_memory()).
> 
> This can be accomplished by queuing the dentry cache shrinker earlier -
> simply calling inode_init() before dcache_init().

Is logical, although the implementation is fragile.  This came up a year or
so ago and we decided it wouldn't make any difference - iirc because slab
shrinking tends to occur in little bites across the whole list.

Do you have any measurements or instrumentation which show improvement?

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