linux-kernel - 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

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Message-ID: <20100920095239.GE1998@csn.ul.ie>
Date:	Mon, 20 Sep 2010 10:52:39 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: 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

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@....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,

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
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