Re: [PATCH] writeback: write_cache_pages doesn't terminate at nr_to_write <= 0

Wu Fengguang
2010-08-23T08:27:28Z

On Mon, Aug 23, 2010 at 02:43:40PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> I noticed XFS writeback in 2.6.36-rc1 was much slower than it should have
> been. Enabling writeback tracing showed:
>
> flush-253:16-8516 [007] 1342952.351608: wbc_writepage: bdi 253:16: towrt=1024 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
> flush-253:16-8516 [007] 1342952.351654: wbc_writepage: bdi 253:16: towrt=1023 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
> flush-253:16-8516 [000] 1342952.369520: wbc_writepage: bdi 253:16: towrt=0 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
> flush-253:16-8516 [000] 1342952.369542: wbc_writepage: bdi 253:16: towrt=-1 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
> flush-253:16-8516 [000] 1342952.369549: wbc_writepage: bdi 253:16: towrt=-2 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
>
> Writeback is not terminating in background writeback if ->writepage is
> returning with wbc->nr_to_write == 0, resulting in sub-optimal single page
> writeback on XFS.

The new code looks more simple, and more robust in case ->writepage
modifies wbc->nr_to_write (as in xfs_vm_writepage).

Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>

Thanks,
Fengguang

> Fix the write_cache_pages loop to terminate correctly when this situation
> occurs and so prevent this sub-optimal background writeback pattern. This
> improves sustained sequential buffered write performance from around
> 250MB/s to 750MB/s for a 100GB file on an XFS filesystem on my 8p test VM.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> mm/page-writeback.c | 26 ++++++++++----------------
> 1 files changed, 10 insertions(+), 16 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 7262aac..f069782 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -984,22 +984,16 @@ continue_unlock:
> }
> }
>
> - if (wbc->nr_to_write > 0) {
> - if (--wbc->nr_to_write == 0 &&
> - wbc->sync_mode == WB_SYNC_NONE) {
> - /*
> - * We stop writing back only if we are
> - * not doing integrity sync. In case of
> - * integrity sync we have to keep going
> - * because someone may be concurrently
> - * dirtying pages, and we might have
> - * synced a lot of newly appeared dirty
> - * pages, but have not synced all of the
> - * old dirty pages.
> - */
> - done = 1;
> - break;
> - }
> + /*
> + * We stop writing back only if we are not doing
> + * integrity sync. In case of integrity sync we have to
> + * keep going until we have written all the pages
> + * we tagged for writeback prior to entering this loop.
> + */
> + if (--wbc->nr_to_write <= 0 &&
> + wbc->sync_mode == WB_SYNC_NONE) {
> + done = 1;
> + break;
> }
> }
> pagevec_release(&pvec);
> --
> 1.7.1
>
> --
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