Re: [RFC PATCH 1/1] ext4: Try to better reuse recently freed space

[Jan Kara <jack@suse.cz>]

On Thu 04-07-13 11:11:54, Lukas Czerner wrote:
> Currently if the block allocator can not find the goal to allocate we
> would use global goal for stream allocation. However the global goal
> (s_mb_last_group and s_mb_last_start) will move further every time such
> allocation appears and never move backwards.
> 
> This causes several problems in certain scenarios:
> 
> - the goal will move further and further preventing us from reusing
>   space which might have been freed since then. This is ok from the file
>   system point of view because we will reuse that space eventually,
>   however we're allocating block from slower parts of the spinning disk
>   even though it might not be necessary.
> - The above also causes more serious problem for example for thinly
>   provisioned storage (sparse images backed storage as well), because
>   instead of reusing blocks which are already provisioned we would try
>   to use new blocks. This would unnecessarily drain storage free blocks
>   pool.
> - This will also cause blocks to be allocated further from the given
>   goal than it's necessary. Consider for example truncating, or removing
>   and rewriting the file in the loop. This workload will never reuse
>   freed blocks until we continually claim and free all the block in the
>   file system.
> 
> Note that file systems like xfs, ext3, or btrfs does not have this
> problem. This is simply caused by the notion of global pool.
> 
> Fix this by changing the global goal to be goal per inode. This will
> allow us to invalidate the goal every time the inode has been truncated,
> or newly created, so in those cases we would try to use the proper more
> specific goal which is based on inode position.
  In principle the patch looks fine to me. Just minor style nit below:

> 
> Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> ---
>  fs/ext4/ext4.h    |  7 ++++---
>  fs/ext4/inode.c   |  8 ++++++++
>  fs/ext4/mballoc.c | 20 ++++++++------------
>  3 files changed, 20 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 6ed348d..4dffa92 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -917,6 +917,10 @@ struct ext4_inode_info {
>  
>  	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
>  	__u32 i_csum_seed;
> +
> +	/* where last allocation was done - for stream allocation */
> +	unsigned long i_last_group;
> +	unsigned long i_last_start;
  Can we use proper types here please? I think they should be
ext4_grpblk_t and ext4_group_t. And maybe we should define something like
EXT4_INVAL_GRPNO and use that instead of UINT_MAX.

								Honza
>  };
>  
>  /*
> @@ -1242,9 +1246,6 @@ struct ext4_sb_info {
>  	unsigned int s_mb_order2_reqs;
>  	unsigned int s_mb_group_prealloc;
>  	unsigned int s_max_dir_size_kb;
> -	/* where last allocation was done - for stream allocation */
> -	unsigned long s_mb_last_group;
> -	unsigned long s_mb_last_start;
>  
>  	/* stats for buddy allocator */
>  	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 0188e65..07d0434 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
>  	else
>  		ext4_ind_truncate(handle, inode);
>  
> +	/* Invalidate last allocation counters */
> +	ei->i_last_group = UINT_MAX;
> +	ei->i_last_start = UINT_MAX;
> +
  And 
>  	up_write(&ei->i_data_sem);
>  
>  	if (IS_SYNC(inode))
> @@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
>  	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
>  	ei->i_block_group = iloc.block_group;
>  	ei->i_last_alloc_group = ~0;
> +
> +	/* Invalidate last allocation counters */
> +	ei->i_last_group = UINT_MAX;
> +	ei->i_last_start = UINT_MAX;
>  	/*
>  	 * NOTE! The in-memory inode i_data array is in little-endian order
>  	 * even on big-endian machines: we do NOT byteswap the block numbers!
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a9ff5e5..6c23666 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
>  static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
>  					struct ext4_buddy *e4b)
>  {
> -	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
>  	int ret;
>  
>  	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
> @@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
>  	get_page(ac->ac_buddy_page);
>  	/* store last allocated for subsequent stream allocation */
>  	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> -		spin_lock(&sbi->s_md_lock);
> -		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
> -		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
> -		spin_unlock(&sbi->s_md_lock);
> +		EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
> +		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
>  	}
>  }
>  
> @@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>  			ac->ac_2order = i - 1;
>  	}
>  
> -	/* if stream allocation is enabled, use global goal */
> -	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> -		/* TBD: may be hot point */
> -		spin_lock(&sbi->s_md_lock);
> -		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> -		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> -		spin_unlock(&sbi->s_md_lock);
> +	/* if stream allocation is enabled and per inode goal is
> +	 * set, use it */
> +	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
> +	   (EXT4_I(ac->ac_inode)->i_last_start != UINT_MAX)) {
> +		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
> +		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
>  	}
>  
>  	/* Let's just scan groups to find more-less suitable blocks */
> -- 
> 1.8.3.1
> 
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-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR