Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933276Ab1DMWEt (ORCPT ); Wed, 13 Apr 2011 18:04:49 -0400 Received: from cantor2.suse.de ([195.135.220.15]:45908 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933257Ab1DMWEq (ORCPT ); Wed, 13 Apr 2011 18:04:46 -0400 Date: Thu, 14 Apr 2011 00:04:44 +0200 From: Jan Kara To: Wu Fengguang Cc: Andrew Morton , Peter Zijlstra , Richard Kennedy , Jan Kara , Hugh Dickins , Rik van Riel , Dave Chinner , LKML , Linux Memory Management List , linux-fsdevel@vger.kernel.org Subject: Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up time Message-ID: <20110413220444.GF4648@quack.suse.cz> References: <20110413085937.981293444@intel.com> <20110413090415.763161169@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110413090415.763161169@intel.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1695 Lines: 42 On Wed 13-04-11 16:59:41, Wu Fengguang wrote: > Reduce the dampening for the control system, yielding faster > convergence. The change is a bit conservative, as smaller values may > lead to noticeable bdi threshold fluctuates in low memory JBOD setup. > > CC: Peter Zijlstra > CC: Richard Kennedy > Signed-off-by: Wu Fengguang Well, I have nothing against this change as such but what I don't like is that it just changes magical +2 for similarly magical +0. It's clear that this will lead to more rapid updates of proportions of bdi's share of writeback and thread's share of dirtying but why +0? Why not +1 or -1? So I'd prefer to get some understanding of why do we need to update the proportion period and why 4-times faster is just the right amount of faster :) If I remember right you had some numbers for this, didn't you? Honza > --- > mm/page-writeback.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > --- linux-next.orig/mm/page-writeback.c 2011-03-02 14:52:19.000000000 +0800 > +++ linux-next/mm/page-writeback.c 2011-03-02 15:00:17.000000000 +0800 > @@ -145,7 +145,7 @@ static int calc_period_shift(void) > else > dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) / > 100; > - return 2 + ilog2(dirty_total - 1); > + return ilog2(dirty_total - 1); > } > > /* > > -- Jan Kara SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/