linux-kernel - Re: [PATCH] cfq-iosched: non-rot devices do not need queue merging

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: <20091230211138.GN4489@kernel.dk>
Date:	Wed, 30 Dec 2009 22:11:38 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH] cfq-iosched: non-rot devices do not need queue merging

On Wed, Dec 30 2009, Corrado Zoccolo wrote:
> On Wed, Dec 30, 2009 at 7:45 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
> > On Wed, Dec 30 2009, Corrado Zoccolo wrote:
> >> Non rotational devices' performances are not affected by
> >> distance of requests, so there is no point in having overhead
> >> to merge queues of nearby requests.
> >
> > If the distance is zero, it may still make a big difference (at least
> > for writes). This check would be better as "ncq and doesn't suck", ala
> >
> >        blk_queue_nonrot(q) && tagged
> >
> > like we do elsewhere.
> 
> For reads, though, even flash cards and netbook ssds are completely
> unaffected. I have done few experiments on my available disks:
> * http://dl.dropbox.com/u/3525644/service_time.png (I used the
> program: http://dl.dropbox.com/u/3525644/stride.c to get the graphs).

Completely agree, it's writes that matter (as mentioned).

> For distance 0, I think request merging will be more effective than
> queue merging, moreover I think the multi-thread trick to have large

Definitely true, but we don't allow cross cfqq merges to begin with.

> I/O depth is used for reads, not writes (where simply issuing buffered
> writes already achieves a similar effect), so I think it is safe to
> disable it for all non-rotational devices.

That still leaves direct writes. Granted it's a problem with a huge
scope, but still.

-- 
Jens Axboe

--
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/