Re: [PATCH 8/8] writeback: Do not sleep on the congestion queue if there are no congested BDIs or if significant congestion is not being encountered in the current zone
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2010-09-16 22:28:10
Also in:
linux-fsdevel, lkml
On Wed, 15 Sep 2010 13:27:51 +0100 Mel Gorman [off-list ref] wrote:
If wait_iff_congested() is called with no BDI congested, the function simply calls cond_resched(). In the event there is significant writeback happening in the zone that is being reclaimed, this can be a poor decision as reclaim would succeed once writeback was completed. Without any backoff logic, younger clean pages can be reclaimed resulting in more reclaim overall and poor performance.
This is because cond_resched() is a no-op, and we skip around the under-writeback pages and go off and look further along the LRU for younger clean pages, yes?
This patch tracks how many pages backed by a congested BDI were found during scanning. If all the dirty pages encountered on a list isolated from the LRU belong to a congested BDI, the zone is marked congested until the zone reaches the high watermark.
High watermark, or low watermark? The terms are rather ambiguous so let's avoid them. Maybe "full" watermark and "empty"?
quoted hunk ↗ jump to hunk
...@@ -706,6 +726,7 @@ static unsigned long shrink_page_list(struct list_head *page_list, goto keep; VM_BUG_ON(PageActive(page)); + VM_BUG_ON(page_zone(page) != zone);
?
quoted hunk ↗ jump to hunk
sc->nr_scanned++; ...@@ -903,6 +928,15 @@ keep_lumpy: VM_BUG_ON(PageLRU(page) || PageUnevictable(page)); } + /* + * Tag a zone as congested if all the dirty pages encountered were + * backed by a congested BDI. In this case, reclaimers should just + * back off and wait for congestion to clear because further reclaim + * will encounter the same problem + */ + if (nr_dirty == nr_congested) + zone_set_flag(zone, ZONE_CONGESTED);
The implicit "100%" there is a magic number. hrm. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>