[6/6] e2fsck: optimize pass 5 for CPU utilization

Add a fast past optimization in e2fsck's pass 5 for the common case
where the block bitmap is correct.  The optimization works by
extracting each block group's block allocation bitmap into a memory
buffer, and comparing it with the expected allocation bitmap using
memcmp().  If it matches, then we can just update the free block
counts and be on our way, and skip checking each bit individually.

Addresses-Google-Bug: #7534813

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
 e2fsck/pass5.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 53 insertions(+), 2 deletions(-)

On Sat, 24 Nov 2012, Theodore Ts'o wrote:

> Date: Sat, 24 Nov 2012 19:36:34 -0500
> From: Theodore Ts'o <tytso@mit.edu>
> To: Ext4 Developers List <linux-ext4@vger.kernel.org>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Subject: [PATCH 6/6] e2fsck: optimize pass 5 for CPU utilization
> 
> Add a fast past optimization in e2fsck's pass 5 for the common case
> where the block bitmap is correct.  The optimization works by
> extracting each block group's block allocation bitmap into a memory
> buffer, and comparing it with the expected allocation bitmap using
> memcmp().  If it matches, then we can just update the free block
> counts and be on our way, and skip checking each bit individually.
> 
> Addresses-Google-Bug: #7534813

Looks good, we can always move the discard operation at the end of
the function before we free the bitmap, but that's further
optimization.

Reviewed-by: Lukas Czerner <lczerner@redhat.com>

> 
> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> ---
>  e2fsck/pass5.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 53 insertions(+), 2 deletions(-)
> 
> diff --git a/e2fsck/pass5.c b/e2fsck/pass5.c
> index 8312fe0..5aff55c 100644
> --- a/e2fsck/pass5.c
> +++ b/e2fsck/pass5.c
> @@ -212,6 +212,12 @@ static void check_block_bitmaps(e2fsck_t ctx)
>  	int	cmp_block = 0;
>  	int	redo_flag = 0;
>  	blk64_t	super_blk, old_desc_blk, new_desc_blk;
> +	char *actual_buf, *bitmap_buf;
> +
> +	actual_buf = (char *) e2fsck_allocate_memory(ctx, fs->blocksize,
> +						     "actual bitmap buffer");
> +	bitmap_buf = (char *) e2fsck_allocate_memory(ctx, fs->blocksize,
> +						     "bitmap block buffer");
>  
>  	clear_problem_context(&pctx);
>  	free_array = (unsigned int *) e2fsck_allocate_memory(ctx,
> @@ -259,11 +265,53 @@ redo_counts:
>  	for (i = B2C(fs->super->s_first_data_block);
>  	     i < ext2fs_blocks_count(fs->super);
>  	     i += EXT2FS_CLUSTER_RATIO(fs)) {
> +		int first_block_in_bg = (B2C(i) -
> +					 B2C(fs->super->s_first_data_block)) %
> +			fs->super->s_clusters_per_group == 0;
> +		int n, nbytes = fs->super->s_clusters_per_group / 8;
> +
>  		actual = ext2fs_fast_test_block_bitmap2(ctx->block_found_map, i);
>  
> +		/*
> +		 * Try to optimize pass5 by extracting a bitmap block
> +		 * as expected from what we have on disk, and then
> +		 * comparing the two.  If they are identical, then
> +		 * update the free block counts and go on to the next
> +		 * block group.  This is much faster than doing the
> +		 * individual bit-by-bit comparison.  The one downside
> +		 * is that this doesn't work if we are asking e2fsck
> +		 * to do a discard operation.
> +		 */
> +		if (!first_block_in_bg ||
> +		    (group == (int)fs->group_desc_count - 1) ||
> +		    (ctx->options & E2F_OPT_DISCARD))
> +			goto no_optimize;
> +
> +		retval = ext2fs_get_block_bitmap_range2(ctx->block_found_map,
> +				B2C(i), fs->super->s_clusters_per_group,
> +				actual_buf);
> +		if (retval)
> +			goto no_optimize;
> +		if (ext2fs_bg_flags_test(fs, group, EXT2_BG_BLOCK_UNINIT))
> +			memset(bitmap_buf, 0, nbytes);
> +		else {
> +			retval = ext2fs_get_block_bitmap_range2(fs->block_map,
> +					B2C(i), fs->super->s_clusters_per_group,
> +					bitmap_buf);
> +			if (retval)
> +				goto no_optimize;
> +		}
> +		if (memcmp(actual_buf, bitmap_buf, nbytes) != 0)
> +			goto no_optimize;
> +		n = ext2fs_bitcount(actual_buf, nbytes);
> +		group_free = fs->super->s_clusters_per_group - n;
> +		free_blocks += group_free;
> +		i += fs->super->s_clusters_per_group - 1;
> +		goto next_group;
> +	no_optimize:
> +
>  		if (skip_group) {
> -			if ((B2C(i) - B2C(fs->super->s_first_data_block)) %
> -			    fs->super->s_clusters_per_group == 0) {
> +			if (first_block_in_bg) {
>  				super_blk = 0;
>  				old_desc_blk = 0;
>  				new_desc_blk = 0;
> @@ -401,6 +449,7 @@ redo_counts:
>  			if (!bitmap && i >= first_free)
>  				e2fsck_discard_blocks(ctx, first_free,
>  						      (i - first_free) + 1);
> +		next_group:
>  			first_free = ext2fs_blocks_count(fs->super);
>  
>  			free_array[group] = group_free;
> @@ -475,6 +524,8 @@ redo_counts:
>  	}
>  errout:
>  	ext2fs_free_mem(&free_array);
> +	ext2fs_free_mem(&actual_buf);
> +	ext2fs_free_mem(&bitmap_buf);
>  }
>  
>  static void check_inode_bitmaps(e2fsck_t ctx)
> 
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