Re: [PATCH] btrfs: lower metadata writeback threshold on low dirty threshold

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On Thu 03-05-12 11:43:11, Wu Fengguang wrote:
> This helps write performance when setting the dirty threshold to tiny numbers.
> 
>      3.4.0-rc2         3.4.0-rc2-btrfs4+
>   ------------  ------------------------
>          96.92        -0.4%        96.54  bay/thresh=1000M/btrfs-100dd-1-3.4.0-rc2
>          98.47        +0.0%        98.50  bay/thresh=1000M/btrfs-10dd-1-3.4.0-rc2
>          99.38        -0.3%        99.06  bay/thresh=1000M/btrfs-1dd-1-3.4.0-rc2
>          98.04        -0.0%        98.02  bay/thresh=100M/btrfs-100dd-1-3.4.0-rc2
>          98.68        +0.3%        98.98  bay/thresh=100M/btrfs-10dd-1-3.4.0-rc2
>          99.34        -0.0%        99.31  bay/thresh=100M/btrfs-1dd-1-3.4.0-rc2
>   ==>    88.98        +9.6%        97.53  bay/thresh=10M/btrfs-10dd-1-3.4.0-rc2
>   ==>    86.99       +13.1%        98.39  bay/thresh=10M/btrfs-1dd-1-3.4.0-rc2
>   ==>     2.75     +2442.4%        69.88  bay/thresh=1M/btrfs-10dd-1-3.4.0-rc2
>   ==>     3.31     +2634.1%        90.54  bay/thresh=1M/btrfs-1dd-1-3.4.0-rc2
> 
> Signed-off-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
> ---
>  fs/btrfs/disk-io.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> --- linux-next.orig/fs/btrfs/disk-io.c	2012-05-02 14:04:00.989262395 +0800
> +++ linux-next/fs/btrfs/disk-io.c	2012-05-02 14:04:01.773262414 +0800
> @@ -930,7 +930,8 @@ static int btree_writepages(struct addre
>  
>  		/* this is a bit racy, but that's ok */
>  		num_dirty = root->fs_info->dirty_metadata_bytes;
> -		if (num_dirty < thresh)
> +		if (num_dirty < min(thresh,
> +				    global_dirty_limit << (PAGE_CACHE_SHIFT-2)))
>  			return 0;
>  	}
>  	return btree_write_cache_pages(mapping, wbc);
  Frankly, that whole condition on WB_SYNC_NONE in btree_writepages() looks
like a hack. I think we also had problems with this condition when we tried
to change b_more_io list handling. I found rather terse commit message
explaining the code:
Btrfs: Limit btree writeback to prevent seeks

  Which I kind of understand but is it that bad? Also I think last time we
stumbled over this code we were discussing that these dirty metadata would
be simply hidden from mm which would solve the problem of flusher thread
trying to outsmart the filesystem... But I guess noone had time to
implement this for btrfs.

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