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Date:   Wed, 9 Dec 2020 14:39:35 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     brookxu <brookxu.cn@...il.com>
Cc:     adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org
Subject: Re: [PATCH RESEND 4/8] ext4: add the gdt block of meta_bg to
 system_zone

On Wed, Dec 09, 2020 at 07:48:09PM +0800, brookxu wrote:
> 
> Maybe I missed something. If i% meta_bg_size is used instead, if
> flex_size <64, then we will miss some flex_bg. There seems to be
> a contradiction here. In the scenario where only flex_bg is
> enabled, it may not be appropriate to use meta_bg_size. In the
> scenario where only meta_bg is enabled, it may not be appropriate
> to use flex_size.
> 
> As you said before, it maybe better to remove
> 
> 	if ((i <5) || ((i% flex_size) == 0))
> 
> and do it for all groups.

I don't think the original (i % flex_size) made any sense in the first
place.

What flex_bg does is that it collects the allocation bitmaps and inode
tables for each block group and locates them within the first block
group in a flex_bg.  It doesn't have anything to do with whether or
not a particular block group has a backup copy of the superblock and
block group descriptor table --- in non-meta_bg file systems and the
meta_bg file systems where the block group is less than
s_first_meta_bg * EXT4_DESC_PER_BLOCK(sb).  And the condition in
question is only about whether or not to add the backup superblock and
backup block group descriptors.  So checking for i % flex_size made no
sense, and I'm not sure that check was there in the first place.

> In this way weh won't miss some flex_bg, meta_bg, and sparse_bg.
> I tested it on an 80T disk and found that the performance loss
> was small:
> 
>  unpatched kernel:
>  ext4_setup_system_zone() takes 524ms, 
> 
>  patched kernel:
>  ext4_setup_system_zone() takes 552ms, 

I don't really care that much about the time it takes to execute
ext4_setup_system_zone().

The really interesting question is how large is the rb_tree
constructed by that function, and what is the percentage increase of
time that the ext4_inode_block_valid() function takes.  (e.g., how
much additional memory is the system_blks tree taking, and how deep is
that tree, since ext4_inode_block_valid() gets called every time we
allocate or free a block, and every time we need to validate an extent
tree node.

Cheers,

						- Ted

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