linux-kernel - Re: [RFC][PATCH] mm: cut down __GFP

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Date:	Mon, 2 May 2011 21:49:54 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@...ux.vnet.ibm.com>,
	Dave Young <hidave.darkstar@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	Dave Chinner <david@...morbit.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC][PATCH] mm: cut down __GFP_NORETRY page allocation
 failures

On Mon, May 02, 2011 at 09:29:58PM +0800, Wu Fengguang wrote:
> > > +                     if (preferred_zone &&
> > > +                         zone_watermark_ok_safe(preferred_zone, sc->order,
> > > +                                     high_wmark_pages(preferred_zone),
> > > +                                     zone_idx(preferred_zone), 0))
> > > +                             goto out;
> > > +             }
> > 
> > As I said, I think direct reclaim path sould be fast if possbile and
> > it should not a function of min_free_kbytes.
> 
> It can be made not a function of min_free_kbytes by simply changing
> high_wmark_pages() to low_wmark_pages() in the above chunk, since
> direct reclaim is triggered when ALLOC_WMARK_LOW cannot be satisfied,
> ie. it just dropped below low_wmark_pages().
> 
> But still, it costs 62ms reclaim latency (base kernel is 29ms).

I got new findings: the CPU schedule delays are much larger than
reclaim delays. It does make the "direct reclaim until low watermark
OK" latency less a problem :)

1000 dd test case:
                RECLAIM delay   CPU delay       nr_alloc_fail   CAL (last CPU)
base kernel     29ms            244ms           14586           218440
patched         62ms            215ms           5004            325

Thanks,
Fengguang
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