linux-ext4 - Re: [PATCH] ext4: speed up group trim with the right free block count.

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Date:	Fri, 21 Jan 2011 11:49:21 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Tao Ma <tm@....ma>
cc:	linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] ext4: speed up group trim with the right free block
 count.

On Fri, 21 Jan 2011, Tao Ma wrote:

> From: Tao Ma <boyu.mt@...bao.com>
> 
> When we trim some free blocks in a group of ext4, we should
> calculate the free blocks properly and check whether there are
> enough freed blocks left for us to trim. Current solution will
> only calculate free spaces if they are large for a trim which
> isn't appropriate.
> 
> Let us see a small example:
> a group has 1.5M free which are 300k, 300k, 300k, 300k, 300k.
> And minblocks is 1M. With current solution, we have to iterate
> the whole group since these 300k will never be subtracted from
> 1.5M. But actually we should exit after we find the first 2
> free spaces since the left 3 chunks only sum up to 900K if we
> subtract the first 600K although they can't be trimed.
> 
> Cc: "Theodore Ts'o" <tytso@....edu>
> Signed-off-by: Tao Ma <boyu.mt@...bao.com>
> ---
>  fs/ext4/mballoc.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 851f49b..970e471 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4737,7 +4737,7 @@ ext4_grpblk_t ext4_trim_all_free(struct super_block *sb, struct ext4_buddy *e4b,
>  		ext4_grpblk_t start, ext4_grpblk_t max, ext4_grpblk_t minblocks)
>  {
>  	void *bitmap;
> -	ext4_grpblk_t next, count = 0;
> +	ext4_grpblk_t next, count = 0, free_count = 0;
>  	ext4_group_t group;
>  	int ret = 0;
>  
> @@ -4762,6 +4762,7 @@ ext4_grpblk_t ext4_trim_all_free(struct super_block *sb, struct ext4_buddy *e4b,
>  				break;
>  			count += next - start;
>  		}
> +		free_count += next - start;
>  		start = next + 1;
>  
>  		if (fatal_signal_pending(current)) {
> @@ -4775,7 +4776,7 @@ ext4_grpblk_t ext4_trim_all_free(struct super_block *sb, struct ext4_buddy *e4b,
>  			ext4_lock_group(sb, group);
>  		}
>  
> -		if ((e4b->bd_info->bb_free - count) < minblocks)
> +		if ((e4b->bd_info->bb_free - free_count) < minblocks)
>  			break;
>  	}
>  	ext4_unlock_group(sb, group);
> 

Looks good, thanks.

-Lukas
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