linux-kernel - Re: [PATCH 04/10] block: initial patch for on-stack per-task plugging

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Message-ID: <20110310194625.GI29464@redhat.com>
Date:	Thu, 10 Mar 2011 14:46:25 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	hch@...radead.org, Mike Snitzer <snitzer@...hat.com>
Subject: Re: [PATCH 04/10] block: initial patch for on-stack per-task plugging

On Thu, Mar 10, 2011 at 08:32:05PM +0100, Jens Axboe wrote:
> On 2011-03-10 17:54, Vivek Goyal wrote:
> > On Sat, Jan 22, 2011 at 01:17:23AM +0000, Jens Axboe wrote:
> > 
> > [..]
> >> -/*
> >> - * Only disabling plugging for non-rotational devices if it does tagging
> >> - * as well, otherwise we do need the proper merging
> >> - */
> >> -static inline bool queue_should_plug(struct request_queue *q)
> >> -{
> >> -	return !(blk_queue_nonrot(q) && blk_queue_tagged(q));
> >> -}
> >> -
> > 
> > Jens,
> > 
> > While discussing stack plug with Mike Snitzer, this occurred to us that in
> > new code we seem to be plugging even if underlying device is SSD with NCQ.
> > Should we continue to maintain the old behavior of not plugging for NCQ SSD?
> 
> The main reason plugging was turned off for SSD's previously was because
> it ended up hammering on the queue lock a lot. So it was turned off to
> speed them up.
> 
> The new plugging scheme is faster than hitting the queue directly, so
> now it would be a good idea to do the plugging in fact. Plus even for
> high performance SSD's, things like merging are still a good idea.
> 
> So yes, it's on now and on purpose.

Ok. Thanks for the explanation. That helps.

Vivek
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