lkml.org 
[lkml]   [2010]   [Sep]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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
On Wed, 15 Sep 2010 13:27:51 +0100
Mel Gorman <mel@csn.ul.ie> 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"?

>
> ...
>
> @@ -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);

?

> 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.




\
 
 \ /
  Last update: 2010-09-17 00:31    [W:0.194 / U:0.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site