Re: [PATCH] block_write_full_page: switch synchronous writes to use WRITE_SYNC_PLUG

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Apr 07 2009, Theodore Tso wrote:
> Now that we have a distinction between WRITE_SYNC and WRITE_SYNC_PLUG,
> use WRITE_SYNC_PLUG in __block_write_full_page() to avoid unplugging
> the block device I/O queue between each page that gets flushed out.
> 
> The upstream callers of block_write_full_page() which wait for the
> writes to finish call wait_on_buffer(), wait_on_writeback_range()
> (which ultimately calls sync_page(), which calls
> blk_run_backing_dev(), which will unplug the device queue), and so on.
> 
> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> ---
> 
> We should get this applied to avoid any performance regressions
> resulting from commit a64c8610.
> 
>  fs/buffer.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 977e12a..95b5390 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1646,7 +1646,8 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
>  	struct buffer_head *bh, *head;
>  	const unsigned blocksize = 1 << inode->i_blkbits;
>  	int nr_underway = 0;
> -	int write_op = (wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE);
> +	int write_op = (wbc->sync_mode == WB_SYNC_ALL ?
> +			WRITE_SYNC_PLUG : WRITE);
>  
>  	BUG_ON(!PageLocked(page));

I think you should comment on why we don't need to do the actual unplug.
See what I added in fs/jbd/commit.c:journal_commit_transaction():

        /*
         * Use plugged writes here, since we want to submit several
         * before we unplug the device. We don't do explicit
         * unplugging in here, instead we rely on sync_buffer() doing
         * the unplug for us.
         */

-- 
Jens Axboe