linux-kernel - Re: [PATCH 2/2] block: loop: avoiding too many pending per work I/O

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]

Date:	Tue, 5 May 2015 12:55:41 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Justin M. Forbes" <jforbes@...oraproject.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	"v4.0" <stable@...r.kernel.org>
Subject: Re: [PATCH 2/2] block: loop: avoiding too many pending per work I/O

Hello, Ming.

On Tue, May 05, 2015 at 10:46:10PM +0800, Ming Lei wrote:
> On Tue, May 5, 2015 at 9:59 PM, Tejun Heo <tj@...nel.org> wrote:
> > It's a bit weird to hard code this to 16 as this effectively becomes a
> > hidden bottleneck for concurrency.  For cases where 16 isn't a good
> > value, hunting down what's going on can be painful as it's not visible
> > anywhere.  I still think the right knob to control concurrency is
> > nr_requests for the loop device.  You said that for linear IOs, it's
> > better to have higher nr_requests than concurrency but can you
> > elaborate why?
> 
> I mean, in case of sequential IO, the IO may hit page cache a bit easier,
> so handling the IO may be quite quick, then it is often more efficient to
> handle them in one same context(such as, handle one by one from IO
> queue) than from different contexts(scheduled from different worker
> threads). And that can be made by setting a bigger nr_requests(queue_depth).

Ah, so, it's about the queueing latency.  Blocking the issuer from
get_request side for the same level of concurrency would incur a lot
longer latency before the next IO can be dispatched.  The arbitrary 16
is still bothering but for now it's fine I guess, but we need to
revisit the whole thing including WQ_HIGHPRI thing.  Maybe it made
sense when we had only one thread servicing all IOs but w/ high
concurrency I don't think it's a good idea.

Please feel free to add

 Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun
--
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/