linux-kernel - Re: [PATCH 03/10] writeback: Do not congestion sleep if there are no congested BDIs or significant writeback

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Message-ID: <AANLkTikz0uv_7tDYQ--WAT5g0SHaMhUeSmo0U7WkHonb@mail.gmail.com>
Date:	Mon, 13 Sep 2010 19:20:37 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 03/10] writeback: Do not congestion sleep if there are no
 congested BDIs or significant writeback

On Mon, Sep 13, 2010 at 7:07 PM, Mel Gorman <mel@....ul.ie> wrote:
> On Mon, Sep 13, 2010 at 06:48:10PM +0900, Minchan Kim wrote:
>> >> > > > <SNIP>
>> >> > > > I'm not saying it is. The objective is to identify a situation where
>> >> > > > sleeping until the next write or congestion clears is pointless. We have
>> >> > > > already identified that we are not congested so the question is "are we
>> >> > > > writing a lot at the moment?". The assumption is that if there is a lot
>> >> > > > of writing going on, we might as well sleep until one completes rather
>> >> > > > than reclaiming more.
>> >> > > >
>> >> > > > This is the first effort at identifying pointless sleeps. Better ones
>> >> > > > might be identified in the future but that shouldn't stop us making a
>> >> > > > semi-sensible decision now.
>> >> > >
>> >> > > nr_bdi_congested is no problem since we have used it for a long time.
>> >> > > But you added new rule about writeback.
>> >> > >
>> >> >
>> >> > Yes, I'm trying to add a new rule about throttling in the page allocator
>> >> > and from vmscan. As you can see from the results in the leader, we are
>> >> > currently sleeping more than we need to.
>> >>
>> >> I can see the about avoiding congestion_wait but can't find about
>> >> (writeback < incative / 2) hueristic result.
>> >>
>> >
>> > See the leader and each of the report sections entitled
>> > "FTrace Reclaim Statistics: congestion_wait". It provides a measure of
>> > how sleep times are affected.
>> >
>> > "congest waited" are waits due to calling congestion_wait. "conditional waited"
>> > are those related to wait_iff_congested(). As you will see from the reports,
>> > sleep times are reduced overall while callers of wait_iff_congested() still
>> > go to sleep. The reports entitled "FTrace Reclaim Statistics: vmscan" show
>> > how reclaim is behaving and indicators so far are that reclaim is not hurt
>> > by introducing wait_iff_congested().
>>
>> I saw  the result.
>> It was a result about effectiveness _both_ nr_bdi_congested and
>> (writeback < inactive/2).
>> What I mean is just effectiveness (writeback < inactive/2) _alone_.
>
> I didn't measured it because such a change means that wait_iff_congested()
> ignored BDI congestion. If we were reclaiming on a NUMA machine for example,
> it could mean that a BDI gets flooded with requests if we only checked the
> ratios of one zone if little writeback was happening in that zone at the
> time. It did not seem like a good idea to ignore congestion.

You seem to misunderstand my word.
Sorry for not clear sentence.

I don't mean ignore congestion.
First of all, we should consider congestion of bdi.
My meant is whether we need adding up (nr_writeback < nr_inacive /2)
heuristic plus congestion bdi.
It wasn't previous version in your patch but it showed up in this version.
So I thought apparently you have any evidence why we should add such heuristic.

>
>> If we remove (writeback < inactive / 2) check and unconditionally
>> return, how does the behavior changed?
>>
>
> Based on just the workload Johannes sent, scanning and completion times both
> increased without any improvement in the scanning/reclaim ratio (a bad result)
> hence why this logic was introduced to back off where there is some
> writeback taking place even if the BDI is not congested.

Yes. That's what I want. At least, comment of function should have it
to understand the logic.  In addition, It would be better to add the
number to show how it back off well.


>
> --
> Mel Gorman
> Part-time Phd Student                          Linux Technology Center
> University of Limerick                         IBM Dublin Software Lab
>



-- 
Kind regards,
Minchan Kim
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