The change is virtually a no-op for the majority users that use the default 10/20 background/dirty ratios. For others don't know why they are setting background ratio close enough to dirty ratio. Someone must set background ratio equal to dirty ratio, but no one seems to notice or complain that it's then silently halved under the hood.. Signed-off-by: Wu Fengguang --- mm/page-writeback.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) --- linux-next.orig/mm/page-writeback.c 2010-12-08 22:44:25.000000000 +0800 +++ linux-next/mm/page-writeback.c 2010-12-08 22:44:25.000000000 +0800 @@ -403,8 +403,15 @@ void global_dirty_limits(unsigned long * else background = (dirty_background_ratio * available_memory) / 100; - if (background >= dirty) - background = dirty / 2; + /* + * Ensure at least 1/4 gap between background and dirty thresholds, so + * that when dirty throttling starts at (background + dirty)/2, it's at + * the entrance of bdi soft throttle threshold, so as to avoid being + * hard throttled. + */ + if (background > dirty - dirty * 2 / BDI_SOFT_DIRTY_LIMIT) + background = dirty - dirty * 2 / BDI_SOFT_DIRTY_LIMIT; + tsk = current; if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) { background += background / 4; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/