Re: [PATCH] ext2 statfs improvement for block and inode free count

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 13 Jul 2007 18:36:54 -0700 Badari Pulavarty <pbadari@us.ibm.com> wrote:

> More statfs() improvements for ext2. ext2 already maintains
> percpu counters for free blocks and inodes. Derive free
> block count and inode count by summing up percpu counters,
> instead of counting up all the groups in the filesystem
> each time.
> 

hm, another speedup patch with no measurements which demonstrate its
benefit.

> 
> Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
> Acked-by: Andreas Dilger <adilger@clusterfs.com>
> 
>  fs/ext2/super.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6.22/fs/ext2/super.c
> ===================================================================
> --- linux-2.6.22.orig/fs/ext2/super.c	2007-07-13 20:06:38.000000000 -0700
> +++ linux-2.6.22/fs/ext2/super.c	2007-07-13 20:06:51.000000000 -0700
> @@ -1136,12 +1136,12 @@ static int ext2_statfs (struct dentry * 
>  	buf->f_type = EXT2_SUPER_MAGIC;
>  	buf->f_bsize = sb->s_blocksize;
>  	buf->f_blocks = le32_to_cpu(es->s_blocks_count) - overhead;
> -	buf->f_bfree = ext2_count_free_blocks(sb);
> + 	buf->f_bfree = percpu_counter_sum(&sbi->s_freeblocks_counter);
>  	buf->f_bavail = buf->f_bfree - le32_to_cpu(es->s_r_blocks_count);
>  	if (buf->f_bfree < le32_to_cpu(es->s_r_blocks_count))
>  		buf->f_bavail = 0;
>  	buf->f_files = le32_to_cpu(es->s_inodes_count);
> -	buf->f_ffree = ext2_count_free_inodes(sb);
> + 	buf->f_ffree = percpu_counter_sum(&sbi->s_freeinodes_counter);
>  	buf->f_namelen = EXT2_NAME_LEN;
>  	fsid = le64_to_cpup((void *)es->s_uuid) ^
>  	       le64_to_cpup((void *)es->s_uuid + sizeof(u64));
> 

Well there's a tradeoff here.  At large CPU counts, percpu_counter_sum()
becomes quite expensive - it takes a global lock and then goes off fishing
in every CPU's percpu_alloced memory.

So there is some value of (num_online_cpus / sb->s_groups_count) at which
this change becomes a loss.  Where does that value lie?

Bear in mind that the global lock in percpu_counter_sum() will tilt the
scales quite a bit.

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