Re: [PATCH 5/5] xfs: reverse search directory freespace indexes

[Christoph Hellwig <hch@infradead.org>]

On Thu, Aug 29, 2019 at 08:47:10PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> When a directory is growing rapidly, new blocks tend to get added at
> the end of the directory. These end up at the end of the freespace
> index, and when the directory gets large finding these new
> freespaces gets expensive. The code does a linear search across the
> frespace index from the first block in the directory to the last,
> hence meaning the newly added space is the last index searched.
> 
> Instead, do a reverse order index search, starting from the last
> block and index in the freespace index. This makes most lookups for
> free space on rapidly growing directories O(1) instead of O(N), but
> should not have any impact on random insert workloads because the
> average search length is the same regardless of which end of the
> array we start at.
> 
> The result is a major improvement in large directory grow rates:
> 
> 		create time(sec) / rate (files/s)
>  File count     vanilla             Prev commit		Patched
>   10k	      0.41 / 24.3k	   0.42 / 23.8k       0.41 / 24.3k
>   20k	      0.74 / 27.0k	   0.76 / 26.3k       0.75 / 26.7k
>  100k	      3.81 / 26.4k	   3.47 / 28.8k       3.27 / 30.6k
>  200k	      8.58 / 23.3k	   7.19 / 27.8k       6.71 / 29.8k
>    1M	     85.69 / 11.7k	  48.53 / 20.6k      37.67 / 26.5k
>    2M	    280.31 /  7.1k	 130.14 / 15.3k      79.55 / 25.2k
>   10M	   3913.26 /  2.5k                          552.89 / 18.1k
> 
> Signed-Off-By: Dave Chinner <dchinner@redhat.com>

Same here.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>