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: <20110817134936.GA22484@localhost>
Date:	Wed, 17 Aug 2011 21:49:36 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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

> > > +		if (x_intercept < limit) {
> > > +			x_intercept = limit;	/* auxiliary control line */
> > > +			setpoint += span;
> > > +			pos_ratio >>= 1;
> > > +		}
> >   And here you stretch the control area upto the global dirty limit. I
> > understand you maybe don't want to be really strict and cut control area at
> > bdi_thresh but your choice looks like too benevolent - when you have
> > several active bdi's with different speeds this will effectively erase
> > difference between them, won't it? E.g. with 10 bdi's (x_intercept -
> > bdi_dirty) / (x_intercept - setpoint) is going to be close to 1 unless
> > bdi_dirty really heavily exceeds bdi_thresh.
> 
> Yes the auxiliary control line could be very flat (small slope).
> 
> However it's not normal for the bdi dirty pages to fall into the
> range of auxiliary control line at all. And once it takes effect, 
> the pos_ratio is at most 0.5 (which is the value at the connection
> point with the main bdi control line) which is strong enough to pull
> the dirty pages off the auxiliary bdi control line and into the scope
> of main bdi control line.
> 
> The auxiliary control line is intended for bringing down the bdi_dirty
> of the USB key before 250s (where the "pos bandwidth" line keeps low): 
> 
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/1UKEY+1HDD-3G/ext4-4dd-1M-8p-2945M-20%25-2.6.38-rc5-dt6+-2011-02-22-09-46/balance_dirty_pages-pages.png
> 
> After that the bdi_dirty will fluctuate around bdi_thresh and won't
> grow high and step into the scope of the auxiliary control line.

Note that the main/auxiliary bdi control lines won't take effect at
the same time: the main bdi control lines works around and under the
bdi setpoint, and the auxiliary line takes over in the higher scope up
to @limit.

In the 1UKEY+1HDD test case, the bdi_dirty of the UKEY rushes at the
free run stage when global dirty pages are smaller than (thresh+bg_thresh)/2.

So it will be initially under the control the auxiliary line. Hence the
dirtier task will progress at 1/4 to 1/2 of the UKEY's write bandwidth. 
This will bring down the bdi_dirty reasonably fast while still allowing
the dirtier task to make some progress.

The connection point of the main/auxiliary control lines has pos_ratio=0.5.

After 250 second, the main bdi control line takes over, indicated by
the bdi_dirty fluctuating around bdi setpoint and the position rate
(green line) fluctuating around the base ratelimit(blue line).

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/