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: <1312910427.1083.68.camel@twins>
Date:	Tue, 09 Aug 2011 19:20:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	"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>,
	Vivek Goyal <vgoyal@...hat.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 Tue, 2011-08-09 at 12:32 +0200, Peter Zijlstra wrote:
> >                     origin - dirty
> >         pos_ratio = --------------
> >                     origin - goal 
> 
> > which comes from the below [*] control line, so that when (dirty == goal),
> > pos_ratio == 1.0:
> 
> OK, so basically you want a linear function for which:
> 
> f(goal) = 1 and has a root somewhere > goal.
> 
> (that one line is much more informative than all your graphs put
> together, one can start from there and derive your function)
> 
> That does indeed get you the above function, now what does it mean? 

So going by:

                                         write_bw
  ref_bw = dirty_ratelimit * pos_ratio * --------
                                         dirty_bw

pos_ratio seems to be the feedback on the deviation of the dirty pages
around its setpoint. So we adjust the reference bw (or rather ratelimit)
to take account of the shift in output vs input capacity as well as the
shift in dirty pages around its setpoint.

>From that we derive the condition that: 

  pos_ratio(setpoint) := 1

Now in order to create a linear function we need one more condition. We
get one from the fact that once we hit the limit we should hard throttle
our writers. We get that by setting the ratelimit to 0, because, after
all, pause = nr_dirtied / ratelimit would yield inf. in that case. Thus:

  pos_ratio(limit) := 0

Using these two conditions we can solve the equations and get your:

                        limit - dirty
  pos_ratio(dirty) =  ----------------
                      limit - setpoint

Now, for some reason you chose not to use limit, but something like
min(limit, 4*thresh) something to do with the slope affecting the rate
of adjustment. This wants a comment someplace.

Now all of the above would seem to suggest:

  dirty_ratelimit := ref_bw

However for that you use:

  if (pos_bw < dirty_ratelimit && ref_bw < dirty_ratelimit)
	dirty_ratelimit = max(ref_bw, pos_bw);

  if (pos_bw > dirty_ratelimit && ref_bw > dirty_ratelimit)
	dirty_ratelimit = min(ref_bw, pos_bw);

You have:

  pos_bw = dirty_ratelimit * pos_ratio

Which is ref_bw without the write_bw/dirty_bw factor, this confuses me..
why are you ignoring the shift in output vs input rate there?

--
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/