linux-kernel - Re: [PATCH 2/5] writeback: dirty position control

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110829133729.GA27871@localhost>
Date:	Mon, 29 Aug 2011 21:37:29 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] writeback: dirty position control

On Mon, Aug 29, 2011 at 09:12:07PM +0800, Peter Zijlstra wrote:
> On Wed, 2011-08-24 at 14:00 -0400, Vivek Goyal wrote:
> > 
> > Ok, I think I am beginning to see your point. Let me just elaborate on
> > the example you gave.
> > 
> > Assume a system is completely balanced and a task is writing at 100MB/s
> > rate.
> > 
> > write_bw = dirty_rate = 100MB/s, pos_ratio = 1; N=1
> > 
> > bdi->dirty_ratelimit = 100MB/s
> > 
> > Now another tasks starts dirtying the page cache on same bdi. Number of 
> > dirty pages should go up pretty fast and likely position ratio feedback
> > will kick in to reduce the dirtying rate. (rate based feedback does not
> > kick in till next 200ms) and pos_ratio feedback seems to be instantaneous.
> > Assume new pos_ratio is .5
> > 
> > So new throttle rate for both the tasks is 50MB/s.
> > 
> > bdi->dirty_ratelimit = 100MB/s (a feedback has not kicked in yet)
> > task_ratelimit = bdi->dirty_ratelimit * pos_ratio = 100 *.5 = 50MB/s
> > 
> > Now lets say 200ms have passed and rate base feedback is reevaluated.
> > 
> >                                                       write_bw  
> > bdi->dirty_ratelimit_(i+1) = bdi->dirty_ratelimit_i * ---------
> >                                                       dirty_bw
> > 
> > bdi->dirty_ratelimit_(i+1) = 100 * 100/100 = 100MB/s
> > 
> > Ideally bdi->dirty_ratelimit should have now become 50MB/s as N=2 but 
> > that did not happen. And reason being that there are two feedback control
> > loops and pos_ratio loops reacts to imbalances much more quickly. Because
> > previous loop has already reacted to the imbalance and reduced the
> > dirtying rate of task, rate based loop does not try to adjust anything
> > and thinks everything is just fine.
> > 
> > Things are fine in the sense that still dirty_rate == write_bw but
> > system is not balanced in terms of number of dirty pages and pos_ratio=.5
> > 
> > So you are trying to make one feedback loop aware of second loop so that
> > if second loop is unbalanced, first loop reacts to that as well and not
> > just look at dirty_rate and write_bw. So refining new balanced rate by
> > pos_ratio helps.
> >                                                       write_bw  
> > bdi->dirty_ratelimit_(i+1) = bdi->dirty_ratelimit_i * --------- * pos_ratio
> >                                                       dirty_bw
> > 
> > Now if global dirty pages are imbalanced, balanced rate will still go
> > down despite the fact that dirty_bw == write_bw. This will lead to
> > further reduction in task dirty rate. Which in turn will lead to reduced
> > number of dirty rate and should eventually lead to pos_ratio=1.
> 
> 
> Ok so this argument makes sense, is there some formalism to describe
> such systems where such things are more evident?

I find the most easy and clean way to describe it is,

(1) the below formula
                                                          write_bw  
    bdi->dirty_ratelimit_(i+1) = bdi->dirty_ratelimit_i * --------- * pos_ratio
                                                          dirty_bw
is able to yield

    dirty_ratelimit_(i) ~= (write_bw / N)

as long as

- write_bw, dirty_bw and pos_ratio are not changing rapidly
- dirty pages are not around @freerun or @limit

Otherwise there will be larger estimation errors.

(2) based on (1), we get

    task_ratelimit ~= (write_bw / N) * pos_ratio

So the pos_ratio feedback is able to drive dirty count to the
setpoint, where pos_ratio = 1.

That interpretation based on _real values_ can neatly decouple the two
feedback loops :) It makes full utilization of the fact "the
dirty_ratelimit _value_ is independent on pos_ratio except for
possible impacts on estimation errors".

Thanks,
Fengguang
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/