Messages in this thread |  | | Date | Thu, 14 Apr 2011 00:04:44 +0200 | From | Jan Kara <> | Subject | Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up time |
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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 <a.p.zijlstra@chello.nl> > CC: Richard Kennedy <richard@rsk.demon.co.uk> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> 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 <jack@suse.cz> SUSE Labs, CR
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