linux-ext4 - find_group

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Date:	Tue, 23 Dec 2008 01:21:07 +0530
From:	"Rohit Sharma" <imreckless@...il.com>
To:	Kernelnewbies <kernelnewbies@...linux.org>,
	ext4 <linux-ext4@...r.kernel.org>,
	maecompprojects@...glegroups.com,
	technical_mae_alumini@...glegroups.com
Subject: find_group_orlov()

This is a code snippet from find_group_orlov() in ialloc.c of ext2 fs

I am not able to figure out the percpu counters, why it is used.


if (ndirs == 0)
		ndirs = 1;	/* percpu_counters are approximate... */

	blocks_per_dir = (le32_to_cpu(es->s_blocks_count)-free_blocks) / ndirs;

	max_dirs = ndirs / ngroups + inodes_per_group / 16;
               // Why 16??
	min_inodes = avefreei - inodes_per_group / 4;
                   // Why 4 ??
	min_blocks = avefreeb - EXT2_BLOCKS_PER_GROUP(sb) / 4;
    // Why 4 ??

	max_debt = EXT2_BLOCKS_PER_GROUP(sb) / max(blocks_per_dir, BLOCK_COST);
	if (max_debt * INODE_COST > inodes_per_group)
		max_debt = inodes_per_group / INODE_COST;
	if (max_debt > 255)
		max_debt = 255;
	if (max_debt == 0)
		max_debt = 1;

What is the use of debts in allocation. ??

Is it done to make efficient use of free disk space ??
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