Re: [PATCH] ext4/jbd2: drop jbd2_transaction_committed()

[Jan Kara]

On Mon 13-05-24 15:21:19, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 
> jbd2_transaction_committed() is used to check whether a transaction with
> the given tid has already committed, it hold j_state_lock in read mode
> and check the tid of current running transaction and committing
> transaction, but holding the j_state_lock is expensive.
> 
> We have already stored the sequence number of the most recently
> committed transaction in journal t->j_commit_sequence, we could do this
> check by comparing it with the given tid instead. If the given tid isn't
> smaller than j_commit_sequence, we can ensure that the given transaction
> has been committed. That way we could drop the expensive lock and
> achieve about 10% ~ 20% performance gains in concurrent DIOs on may
> virtual machine with 100G ramdisk.
> 
> fio -filename=/mnt/foo -direct=1 -iodepth=10 -rw=$rw -ioengine=libaio \
>     -bs=4k -size=10G -numjobs=10 -runtime=60 -overwrite=1 -name=test \
>     -group_reporting
> 
> Before:
>   overwrite       IOPS=88.2k, BW=344MiB/s
>   read            IOPS=95.7k, BW=374MiB/s
>   rand overwrite  IOPS=98.7k, BW=386MiB/s
>   randread        IOPS=102k, BW=397MiB/s
> 
> After:
>   verwrite:       IOPS=105k, BW=410MiB/s
>   read:           IOPS=112k, BW=436MiB/s
>   rand overwrite: IOPS=104k, BW=404MiB/s
>   randread:       IOPS=111k, BW=432MiB/s
> 
> CC: Dave Chinner <david@xxxxxxxxxxxxx>
> Suggested-by: Dave Chinner <david@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/linux-ext4/493ab4c5-505c-a351-eefa-7d2677cdf800@xxxxxxxxxxxxxxx/T/#m6a14df5d085527a188c5a151191e87a3252dc4e2
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>

I agree this is workable solution and the performance benefits are nice. But
I have some comments regarding the implementation:

> @@ -3199,8 +3199,8 @@ static bool ext4_inode_datasync_dirty(struct inode *inode)
>  	journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
>  
>  	if (journal) {
> -		if (jbd2_transaction_committed(journal,
> -			EXT4_I(inode)->i_datasync_tid))
> +		if (tid_geq(journal->j_commit_sequence,
> +			    EXT4_I(inode)->i_datasync_tid))

Please leave the helper jbd2_transaction_committed(), just make the
implementation more efficient. Also accessing j_commit_sequence without any
lock is theoretically problematic wrt compiler optimization. You should have
READ_ONCE() there and the places modifying j_commit_sequence need to use
WRITE_ONCE().

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR