linux-kernel - Re: regression in page writeback

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: <1253601612.8439.274.camel@twins>
Date:	Tue, 22 Sep 2009 08:40:12 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	linux-kernel@...r.kernel.org, richard@....demon.co.uk,
	jens.axboe@...cle.com, akpm@...ux-foundation.org,
	Chris Mason <chris.mason@...cle.com>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: regression in page writeback

On Tue, 2009-09-22 at 13:49 +0800, Shaohua Li wrote:
> Hi,
> Commit d7831a0bdf06b9f722b947bb0c205ff7d77cebd8 causes disk io regression
> in my test.
> My system has 12 disks, each disk has two partitions. System runs fio sequence
> write on all partitions, each partion has 8 jobs.
> 2.6.31-rc1, fio gives 460m/s disk io
> 2.6.31-rc2, fio gives about 400m/s disk io. Revert the patch, speed back to
> 460m/s
> 
> Under latest git: fio gives 450m/s disk io; If reverting the patch, the speed
> is 484m/s.
> 
> With the patch, fio reports less io merge and more interrupts. My naive
> analysis is the patch makes balance_dirty_pages_ratelimited_nr() limits
> write chunk to 8 pages and then soon go to sleep in balance_dirty_pages(),
> because most time the bdi_nr_reclaimable < bdi_thresh, and so when write
> the pages out, the chunk is 8 pages long instead of 4M long. Without the patch,
> thread can write 8 pages and then move some pages to writeback, and then
> continue doing write. The patch seems to break this.
> 
> Unfortunatelly I can't figure out a fix for this issue, hopefully you have more
> ideas.

This whole writeback business is very fragile, the patch does indeed
cure a few cases and compounds a few other cases, typical trade off.

People are looking at it.

--
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/