linux-kernel - Re: [PATCH v4 01/11] block: make generic_make

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Date:	Wed, 27 May 2015 09:06:40 +1000
From:	NeilBrown <neilb@...e.de>
To:	Alasdair G Kergon <agk@...hat.com>
Cc:	Ming Lin <mlin@...nel.org>, Mike Snitzer <snitzer@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Jens Axboe <axboe@...nel.dk>, Dongsu Park <dpark@...teo.net>,
	Christoph Hellwig <hch@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Ming Lei <ming.lei@...onical.com>, dm-devel@...hat.com,
	Lars Ellenberg <drbd-dev@...ts.linbit.com>,
	drbd-user@...ts.linbit.com, Jiri Kosina <jkosina@...e.cz>,
	Geoff Levand <geoff@...radead.org>, Jim Paris <jim@...n.com>,
	Joshua Morris <josh.h.morris@...ibm.com>,
	Philip Kelleher <pjk1939@...ux.vnet.ibm.com>,
	Minchan Kim <minchan@...nel.org>,
	Nitin Gupta <ngupta@...are.org>,
	Oleg Drokin <oleg.drokin@...el.com>,
	Andreas Dilger <andreas.dilger@...el.com>,
	Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH v4 01/11] block: make generic_make_request handle
 arbitrarily sized bios

On Tue, 26 May 2015 16:34:14 +0100 Alasdair G Kergon <agk@...hat.com> wrote:

> On Tue, May 26, 2015 at 08:02:08AM -0700, Ming Lin wrote:
> > Now bio_add_page() can build lager bios.
> > And blk_queue_split() can split the bios in ->make_request() if needed.
> 
> But why not try to make the bio the right size in the first place so you
> don't have to incur the performance impact of splitting?

Because we don't know what the "right" size is.  And the "right" size can
change when array reconfiguration happens.

Splitting has to happen somewhere, if only in bio_addpage where it decides to
create a new bio rather than add another page to the current one.  So moving
the split to a different level of the stack shouldn't necessarily change the
performance profile.

Obviously testing is important to confirm that.

NeilBrown

> 
> What performance testing have you yet done to demonstrate the *actual* impact
> of this patchset in situations where merge_bvec_fn is currently a net benefit?
> 
> Alasdair
> 
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