ext2/3/4 performance issue

From: Bernhard Kraft
Date: Sun May 10 2015 - 04:28:45 EST

Hello folks,

I work on implementing the ext2 filesystem for a PIC microcontroller and while reading the sources of it in the linux kernel I stumbled upon the following performance issue. I don't know if it is really important but I tought I will ask:

In super.c in function "ext2_statfs" there is a for loop [1] which iterates over all block groups to determine the overhead each block group has. For this it determines if the block group has a superblock (by calling "ext2_bg_has_super") and how many blocks are occupied by the group descriptor blocks (by calling "ext2_bg_num_gdb").

Now it is the case, that "ext2_bg_num_gdb" itself calls "ext2_bg_has_super" in balloc.c [2]. See ext2_bg_num_gdb at the very bottom of balloc.c

There will only be a group descriptor if there is a superblock. The overhead generated by calling "ext2_bg_has_super" twice is not quite minimal. At least if the sparse superblock feature is used as it involves checking if the block number equals any power of 3, 5 or 7.

So as every block which has a superblock must also have a group descriptor it would be fine to replace the for loop mentioned in [1] by a for loop like:

------------ snip -------------------
for (i = 0; i < sbi->s_groups_count; i++)
if (ext2_bg_has_super(sb, i))
overhead += 1 + sbi->s_gdb_count;
------------ snip -------------------

The call to "ext_bg_num_gb" is avoided an by this the redundant call to "ext2_bg_has_super". As the function isn't used anywhere else it could get removed at all.

I don't know if "ext2_statfs" is called often enough that such an optimization would make sense. And I am also not involved in kernel development so I am not quite into kernel coding guidelines.

The same issue is also valid for ext3 and ext4.

[1] https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/fs/ext2/super.c#n1395

[2] https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/fs/ext2/balloc.c#n1532

Wer nicht gelegentlich auch einmal kausalwidrige Dinge zu denken vermag,
wird seine Wissenschaft nie um eine neue Idee bereichern kÃnnen.
Max Planck (1858-1947)
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/