Re: [PATCH 18/35] writeback: start background writeback earlier

Wu Fengguang
2010-12-16T05:38:06Z

On Mon, Dec 13, 2010 at 10:47:04PM +0800, Wu, Fengguang wrote:
> It's possible for some one to suddenly eat lots of memory,
> leading to sudden drop of global dirty limit. So a dirtier
> task may get hard throttled immediately without some previous
> balance_dirty_pages() call to invoke background writeback.
>
> In this case we need to check for background writeback earlier in the
> loop to avoid stucking the application for very long time. This was not
> a problem before the IO-less balance_dirty_pages() because it will try
> to write something and then break out of the loop regardless of the
> global limit.
>
> Another scheme this check will help is, the dirty limit is too close to
> the background threshold, so that someone manages to jump directly into
> the pause threshold (background+dirty)/2.
>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> mm/page-writeback.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> --- linux-next.orig/mm/page-writeback.c 2010-12-13 21:46:16.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2010-12-13 21:46:17.000000000 +0800
> @@ -748,6 +748,9 @@ static void balance_dirty_pages(struct a
> bdi_stat(bdi, BDI_WRITEBACK);
> }
>
> + if (unlikely(!writeback_in_progress(bdi)))
> + bdi_start_background_writeback(bdi);
> +
> bdi_update_bandwidth(bdi, start_time, bdi_dirty, bdi_thresh);
>
> /*
>

The above patch allows this simplification.
---
Subject: writeback: start background writeback earlier - handle laptop mode
Date: Wed Dec 15 20:15:54 CST 2010

The laptop mode handling can be simplified since we've kick background
writeback inside the balance_dirty_pages() loop on dirty_exceeded.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/page-writeback.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- linux-next.orig/mm/page-writeback.c 2010-12-15 20:14:33.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-12-15 20:15:39.000000000 +0800
@@ -891,8 +891,10 @@ pause:
* In normal mode, we start background writeout at the lower
* background_thresh, to keep the amount of dirty memory low.
*/
- if ((laptop_mode && dirty_exceeded) ||
- (!laptop_mode && (nr_reclaimable > background_thresh)))
+ if (laptop_mode)
+ return;
+
+ if (nr_reclaimable > background_thresh)
bdi_start_background_writeback(bdi);
}