Re: 32GB SSD on USB1.1 P3/700 == ___HELL___ (2.6.34-rc3)

From: Wu Fengguang <fengguang.wu@intel.com>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andreas Mohr <andi@lisas.de>, Jens Axboe <axboe@kernel.dk>,
	Minchan Kim <minchan.kim@gmail.com>,
	Linux Memory Management List <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Rik van Riel <riel@redhat.com>
Subject: Re: 32GB SSD on USB1.1 P3/700 == ___HELL___ (2.6.34-rc3)
Date: Thu, 15 Apr 2010 12:19:31 +0800	[thread overview]
Message-ID: <20100415041931.GA14215@localhost> (raw)
In-Reply-To: <20100415122928.D168.A69D9226@jp.fujitsu.com>

On Thu, Apr 15, 2010 at 11:31:52AM +0800, KOSAKI Motohiro wrote:
> > > Many applications (this one and below) are stuck in
> > > wait_on_page_writeback(). I guess this is why "heavy write to
> > > irrelevant partition stalls the whole system".  They are stuck on page
> > > allocation. Your 512MB system memory is a bit tight, so reclaim
> > > pressure is a bit high, which triggers the wait-on-writeback logic.
> > 
> > I wonder if this hacking patch may help.
> > 
> > When creating 300MB dirty file with dd, it is creating continuous
> > region of hard-to-reclaim pages in the LRU list. priority can easily
> > go low when irrelevant applications' direct reclaim run into these
> > regions..
> 
> Sorry I'm confused not. can you please tell us more detail explanation?
> Why did lumpy reclaim cause OOM? lumpy reclaim might cause
> direct reclaim slow down. but IIUC it's not cause OOM because OOM is
> only occur when priority-0 reclaim failure.

No I'm not talking OOM. Nor lumpy reclaim.

I mean the direct reclaim can get stuck for long time, when we do
wait_on_page_writeback() on lumpy_reclaim=1.

> IO get stcking also prevent priority reach to 0.

Sure. But we can wait for IO a bit later -- after scanning 1/64 LRU
(the below patch) instead of the current 1/1024.

In Andreas' case, 512MB/1024 = 512KB, this is way too low comparing to
the 22MB writeback pages. There can easily be a continuous range of
512KB dirty/writeback pages in the LRU, which will trigger the wait
logic.

Thanks,
Fengguang

> 
> 
> > 
> > Thanks,
> > Fengguang
> > ---
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index e0e5f15..f7179cf 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1149,7 +1149,7 @@ static unsigned long shrink_inactive_list(unsigned long max_scan,
> >  	 */
> >  	if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> >  		lumpy_reclaim = 1;
> > -	else if (sc->order && priority < DEF_PRIORITY - 2)
> > +	else if (sc->order && priority < DEF_PRIORITY / 2)
> >  		lumpy_reclaim = 1;
> >  
> >  	pagevec_init(&pvec, 1);
> > --
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> 