On Thu, Sep 16, 2010 at 03:28:10PM -0700, Andrew Morton wrote: > On Wed, 15 Sep 2010 13:27:51 +0100 > Mel Gorman <mel@xxxxxxxxx> 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, Can be a no-op surely. There is an expectation that it will sometimes schedule. > and we skip around the > under-writeback pages and go off and look further along the LRU for > younger clean pages, yes? > 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? > High watermark. The check is made by kswapd. > The terms are rather ambiguous so let's avoid them. Maybe "full" > watermark and "empty"? > Unfortunately they are ambiguous to me. I know what the high watermark is but not what the full or empty watermarks are. > > > > ... > > > > @@ -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); > > ? > It should not be the case that pages from multiple zones exist on the list passed to shrink_page_list(). Lets say someone broke that assumption in the future, which one should be marked congested? No way to know, so lets catch the bug if the assumptions is ever broken. > > 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. > It is but any other value for that number would be very specific to a workload or a machine. A sysctl would have to be maintained and I couldn't convince myself that anyone could do something sensible with the value. Rather than introducing a new tunable for this, I was toying with the idea over the weekend on tracking the scanned/reclaimed ratio within the scan control - possibly on a per-zone basis but more likely globally. When this ratio drops below a given threshold, start increasing the time it backs off for up to a maximum of HZ/10. There are a lot of details to iron out but it's possibly a better long-term direction than adding a tunable for this implicit magic number because it would be adaptive to what is happening for the current workload. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
