linux-kernel - Re: [RFC 2/3] mm: throttle on IO only when there are too many dirty and writeback pages

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Message-ID: <20151030083805.GE18429@dhcp22.suse.cz>
Date:	Fri, 30 Oct 2015 09:38:05 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/3] mm: throttle on IO only when there are too many dirty
 and writeback pages

On Fri 30-10-15 14:48:40, KAMEZAWA Hiroyuki wrote:
[...]
> > @@ -3191,8 +3191,23 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >   		 */
> >   		if (__zone_watermark_ok(zone, order, min_wmark_pages(zone),
> >   				ac->high_zoneidx, alloc_flags, target)) {
> > -			/* Wait for some write requests to complete then retry */
> > -			wait_iff_congested(zone, BLK_RW_ASYNC, HZ/50);
> > +			unsigned long writeback = zone_page_state(zone, NR_WRITEBACK),
> > +				      dirty = zone_page_state(zone, NR_FILE_DIRTY);
> > +
> > +			if (did_some_progress)
> > +				goto retry;
> > +
> > +			/*
> > +			 * If we didn't make any progress and have a lot of
> > +			 * dirty + writeback pages then we should wait for
> > +			 * an IO to complete to slow down the reclaim and
> > +			 * prevent from pre mature OOM
> > +			 */
> > +			if (2*(writeback + dirty) > reclaimable)
> 
> Doesn't this add unnecessary latency if other zones have enough clean memory ?

We know we haven't made any progress the last reclaim round so any zone
with a clean memory is rather unlikely.

-- 
Michal Hocko
SUSE Labs
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