linux-kernel - Re: 32GB SSD on USB1.1 P3/700 == ___HELL__

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Date:	Thu, 15 Apr 2010 12:41:11 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andreas Mohr <andi@...as.de>, Jens Axboe <axboe@...nel.dk>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: 32GB SSD on USB1.1 P3/700 == ___HELL___ (2.6.34-rc3)

On Thu, Apr 15, 2010 at 12:32:50PM +0800, KOSAKI Motohiro wrote:
> > 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.
> 
> In my feeling from your explanation, we need auto adjustment mechanism
> instead change default value for special machine. no?

You mean the dumb DEF_PRIORITY/2 may be too large for a 1TB memory box?

However for such boxes, whether it be DEF_PRIORITY-2 or DEF_PRIORITY/2
shall be irrelevant: it's trivial anyway to reclaim an order-1 or
order-2 page. In other word, lumpy_reclaim will hardly go 1.  Do you
think so?

Thanks,
Fengguang
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