Re: [PATCH] ext4: group cache is added in ext4_mb_discard_preallocations()

["Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>]

On Tue, 30 Mar 2010 20:36:17 +0800, jing zhang <zj.barak@gmail.com> wrote:
> From: Jing Zhang <zj.barak@gmail.com>
> 
> Date: Tue Mar 30 20:35:22     2010
> 
> With the added cache, better group locality may be earned when
> allocating blocks.
> 
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: Andreas Dilger <adilger@sun.com>
> Cc: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
> Cc: "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>
> Signed-off-by: Jing Zhang <zj.barak@gmail.com>
> 
> ---
> 
> --- linux-2.6.32/fs/ext4/mballoc.c	2009-12-03 11:51:22.000000000 +0800
> +++ ext4_mm_leak/mballoc-13.c	2010-03-30 20:28:08.000000000 +0800
> @@ -4183,12 +4183,20 @@ static int ext4_mb_discard_preallocation
>  	ext4_group_t i, ngroups = ext4_get_groups_count(sb);
>  	int ret;
>  	int freed = 0;
> +	static ext4_group_t grp_cache = 0;
> 
>  	trace_ext4_mb_discard_preallocations(sb, needed);
> -	for (i = 0; i < ngroups && needed > 0; i++) {
> -		ret = ext4_mb_discard_group_preallocations(sb, i, needed);
> +	if (needed <= 0)
> +		return freed;
> +	for (i = 0; i < ngroups; i++) {
> +		if (grp_cache >= ngroups)
> +			grp_cache -= ngroups;
> +		ret = ext4_mb_discard_group_preallocations(sb, grp_cache, needed);
>  		freed += ret;
>  		needed -= ret;
> +		if (needed <= 0)
> +			break;
> +		grp_cache++;
>  	}
> 
>  	return freed;

can you explain this further ?

-aneesh