Re: [PATCH] btrfs: add block plug for .writepages

[Chris Mason <chris.mason@oracle.com>]

On Tue, Nov 01, 2011 at 12:45:07AM +0900, Namjae Jeon wrote:
> Add block plug for btrfs .writepages. Block plug is helpful to reduce block lock contention.
> 
> Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
> ---
>  fs/btrfs/extent_io.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index d418164..8f2d6bd 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -2388,6 +2388,7 @@ static int extent_write_cache_pages(struct extent_io_tree *tree,
>  	pgoff_t end;		/* Inclusive */
>  	int scanned = 0;
>  	int tag;
> +	struct blk_plug plug;
>  
>  	pagevec_init(&pvec, 0);
>  	if (wbc->range_cyclic) {
> @@ -2405,6 +2406,8 @@ static int extent_write_cache_pages(struct extent_io_tree *tree,
>  retry:
>  	if (wbc->sync_mode == WB_SYNC_ALL)
>  		tag_pages_for_writeback(mapping, index, end);
> +
> +	blk_start_plug(&plug);
>  	while (!done && !nr_to_write_done && (index <= end) &&
>  	       (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
>  			min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
> @@ -2468,6 +2471,7 @@ retry:
>  		pagevec_release(&pvec);
>  		cond_resched();
>  	}
> +	blk_finish_plug(&plug);
>  	if (!scanned && !done) {
>  		/*
>  		 * We hit the last page and there is more work to be done: wrap

Thanks for the patch, but we actually do this at a lower level in btrfs.
The actual IO can be done to multiple block devices, so we try to plug
for each device individually.

-chris