Re: [PATCH] fs: buffer_head, remove kmem_cache constructor to reduce memory usage under slub

[Nick Piggin <npiggin@xxxxxxx>]

On Tue, Feb 09, 2010 at 01:29:51PM +0000, Richard Kennedy wrote:
> fs: Remove the buffer_head kmem_cache constructor to reduce memory usage
> under slub.
> 
> When using slub, having a kmem_cache constructor forces slub to add a
> free pointer to the size of the cached object, which can have a
> significant impact to the number of small objects that can fit into a
> slab. 
> 
> As buffer_head is relatively small and we can have large numbers of
> them, removing the constructor is a definite win.
> 
> On x86_64 removing the constructor gives me 39 objects/slab, 3 more than
> without the patch. And on x86_32 73 objects/slab, which is 9 more.
> 
> As alloc_buffer_head() already initializes each new object there is very
> little difference in actual code run.  
> 
> Signed-off-by: Richard Kennedy <richard@xxxxxxxxxxxxxxx>

Looks fine to me. Seems like it should reduce temporal cache footprint
too by just touching the objects as they are used.

Acked-by: Nick Piggin <npiggin@xxxxxxx>

> ----
> This patch against 2.6.33-rc7
> 
> I've been running this patch for over a week on both a x86_64 desktop & 
> a x86_32 laptop with no problems, only having fewer pages in the
> buffer_head cache :)
> 
> regards
> Richard
> 
> 
> 
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 6fa5302..bc3212e 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -3265,7 +3265,7 @@ static void recalc_bh_state(void)
>  	
>  struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
>  {
> -	struct buffer_head *ret = kmem_cache_alloc(bh_cachep, gfp_flags);
> +	struct buffer_head *ret = kmem_cache_zalloc(bh_cachep, gfp_flags);
>  	if (ret) {
>  		INIT_LIST_HEAD(&ret->b_assoc_buffers);
>  		get_cpu_var(bh_accounting).nr++;
> @@ -3352,15 +3352,6 @@ int bh_submit_read(struct buffer_head *bh)
>  }
>  EXPORT_SYMBOL(bh_submit_read);
>  
> -static void
> -init_buffer_head(void *data)
> -{
> -	struct buffer_head *bh = data;
> -
> -	memset(bh, 0, sizeof(*bh));
> -	INIT_LIST_HEAD(&bh->b_assoc_buffers);
> -}
> -
>  void __init buffer_init(void)
>  {
>  	int nrpages;
> @@ -3369,7 +3360,7 @@ void __init buffer_init(void)
>  			sizeof(struct buffer_head), 0,
>  				(SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|
>  				SLAB_MEM_SPREAD),
> -				init_buffer_head);
> +				NULL);
>  
>  	/*
>  	 * Limit the bh occupancy to 10% of ZONE_NORMAL
> 
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