linux-kernel - Re: dm-crypt parallelization patches

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Date:	Tue, 9 Apr 2013 14:02:01 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Vivek Goyal <vgoyal@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>,
	Milan Broz <gmazyland@...il.com>, dm-devel@...hat.com,
	Andi Kleen <andi@...stfloor.org>, dm-crypt@...ut.de,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Christian Schmidt <schmidt@...add.de>
Subject: Re: dm-crypt parallelization patches

Hey,

On Tue, Apr 09, 2013 at 04:32:28PM -0400, Mikulas Patocka wrote:
> > and we probably need to change that to bio_associate_task().
> 
> Generally, we shouldn't associate bios with "current" task in device 
> mapper targets. For example suppose that we have two stacked dm-crypt 
> targets:

It only follows the first association so it doesn't matter how many
layers it goes through.  That said, yeah, there could be situations
where @task is availalbe but the bio's already in the hand of a
different task.  If that's the case, change it to
associate_task(@task).

> It doesn't work because device mapper on the underlying layers ignores 
> bi_ioc and bi_css.
> 
> If I make device mapper forward bi_ioc and bi_css to outgoing bios, it 
> improves performance (from 2:30 to 1:30), but it is still far from 
> perfect.

For testing, copying bi_ioc and bi_css directly is fine but please add
another interface to copy those for the actual code.  Say,
bio_copy_association(@to_bio, @from_bio) or whatever.

As for the performance loss, I'm somewhat confident in saying the
remaining difference would be from ignoring plugging boundaries.

Thanks.

-- 
tejun
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