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

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Date:	Tue, 26 May 2015 16:34:14 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Ming Lin <mlin@...nel.org>
Cc:	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>, Neil Brown <neilb@...e.de>,
	Alasdair Kergon <agk@...hat.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, 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?

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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