linux-ext4 - Re: [RFC] Flex_BG ialloc awareness.

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Date:	Mon, 3 Dec 2007 13:42:47 -0700
From:	Andreas Dilger <adilger@....com>
To:	"Jose R. Santos" <jrs@...ibm.com>
Cc:	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [RFC] Flex_BG ialloc awareness.

On Dec 03, 2007  13:05 -0600, Jose R. Santos wrote:
> @@ -600,6 +600,7 @@ void ext4_free_blocks_sb(handle_t *handle, struct super_block *sb,
>  	ext4_grpblk_t group_freed;
> +	ext4_group_t meta_group;

Please do not call these meta_groups.  This already means something very
specific (i.e. desc_per_block groups) and using it for FLEX_BG is confusing.
One possibly desirable relation is if the FLEX_BG count is some integer or
power-of-two multiple of the metabg count.  That would allow the FLEX_BG
code to share the same in-memory group struct as the mballoc code and save
on some memory overhead.

> +			meta_group = ext4_meta_group(sbi, block_group);
> +			spin_lock(&sbi->s_meta_groups[meta_group].meta_group_lock);
> +			sbi->s_meta_groups[meta_group].free_inodes++;
> +			if (is_directory)
> +				sbi->s_meta_groups[meta_group].num_dirs--;
> +			spin_unlock(&sbi->s_meta_groups[meta_group].meta_group_lock);

This can be as many as hundreds or thousands of spin locks.  Why not use
the same hashed locking code as the group descriptors themselves?

		spin_lock(sb_bgl_lock(sbi, meta_group));
		spin_unlock(sb_bgl_lock(sbi, meta_group));

This scales with the number of CPUs and chance of contention is very low.

> +int find_group_meta(struct super_block *sb, struct inode *parent)
> +{
> +	ext4_group_t parent_mgroup = parent_group / sbi->s_groups_per_meta;

This could use ext4_meta_group(sbi, parent_group)?

> +static inline ext4_group_t ext4_meta_group(struct ext4_sb_info *sbi,
> +					     ext4_group_t block_group)
> +{
> +	return block_group/sbi->s_groups_per_meta;
> +}

It would be preferable to limit s_groups_per_meta to be a power-of-two
so that this can become a shift instead of a divide.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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