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Subject: Re: [PATCH 4/5] Use add_page_to_lru_list() helper function
From: Peter Zijlstra <peterz@infradead.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       Andrew Morton <akpm@linux-foundation.org>
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Date: Mon, 20 Jul 2009 09:28:31 +0200
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On Mon, 2009-07-20 at 14:37 +0900, KOSAKI Motohiro wrote:
> > > @@ -1241,7 +1241,6 @@ static void move_active_pages_to_lru(str
> > >  			spin_lock_irq(&zone->lru_lock);
> > >  		}
> > >  	}
> > > -	__mod_zone_page_state(zone, NR_LRU_BASE + lru, pgmoved);
> > >  	if (!is_active_lru(lru))
> > >  		__count_vm_events(PGDEACTIVATE, pgmoved);
> > >  }
> > 
> > This is a net loss, you introduce pgmoved calls to __inc_zone_state,
> > instead of the one __mod_zone_page_state() call.
> 
> max pgmoved is 32. 32 times __inc_zone_state() make 0 or 1 time
> atomic operation (not much than two).
> I don't think it reduce performance.

its not just atomics, count calls and branches too. It simply adds a ton
of code for no particular reason.