Re: ext4: try to improve unwritten extents merges

[Jan Kara <jack@suse.cz>]

Hello!

On Tue 20-11-18 17:01:25, Xiaoguang Wang wrote:
> First sorry to bother you again, recently we meet a
> "dioread_nolock,nodelalloc" slow writeback issue, Liu Bo has sent a patch to
> fix this issue. But here I also wonder whether we can merge unwritten
> extents as far as possible.
> In current codes:
> 
> int
> ext4_can_extents_be_merged(struct inode *inode, struct ext4_extent *ex1,
> 				struct ext4_extent *ex2)
> {
> ...
> 	if (ext4_ext_is_unwritten(ex1) &&
> 	    (ext4_test_inode_state(inode, EXT4_STATE_DIO_UNWRITTEN) ||
> 	     atomic_read(&EXT4_I(inode)->i_unwritten) ||
> 	     (ext1_ee_len + ext2_ee_len > EXT_UNWRITTEN_MAX_LEN)))
> 		return 0;
> ...
> }
> This was added by Darrick in 2014:
> commit a9b8241594adda0a7a4fb3b87bf29d2dff0d997d
> Author: Darrick J. Wong <darrick.wong@oracle.com>
> Date:   Thu Feb 20 21:17:35 2014 -0500
> 
>     ext4: merge uninitialized extents
> 
>     Allow for merging uninitialized extents.
> 
>     Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
>     Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> 
> So long as we have a unwritten extents under io(which also means i_unwritten
> is not zero), then we can not do merge work for unwritten extents, I wonder
> whether this conditon is too strict. Assume that the
> begin of the file is under io, but middle or end of the file could not
> merge unwritten extetns, though they could be.
> 
> I'm not sure whether we could directly remove
> "atomic_read(&EXT4_I(inode)->i_unwritten)",if not, here I make a simple
> patch to respect same semantics. The idea is simple, I use a red-black
> tree to record unwritten extents under io, when trying to merging
> unwritten extents, we search this per-inode tree, it not hit, we can
> merge. I have also run "xfstests quick group test cases", look like that
> it works well. dio maybe also go to this way.

The reason why we don't merge unwritten extents if there is IO to unwritten
extents running is that we split unwritten extents to match exactly the IO
range on submission and then convert it to written extents on IO
completion. So we must avoid merging these split out extents while the IO
is running.

I agree that the condition in ext4_can_extents_be_merged() is rather coarse
so it would be nice to improve it so that unwritten extents on which IO is
not running can be merged. I've also observed that unwritten extents get
fragmented relatively easily under some workloads.

Rather than introducing new RB-tree for this (which costs additional memory
and its maintenance costs also CPU time), I'd use extent status tree to
identify unwritten extent that got split out when preparing the IO (you
should mark such extent in ext4_map_blocks() when EXT4_GET_BLOCKS_IO_SUBMIT
flag is set). Then the flag would get cleared on extent conversion to
written one.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR