linux-kernel - RE: Expense of read

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Date:   Tue, 12 Jan 2021 14:06:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Zhongwei Cai' <sunrise_l@...u.edu.cn>,
        Mingkai Dong <mingkaidong@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>
CC:     Mikulas Patocka <mpatocka@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Eric Sandeen <esandeen@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        Theodore Ts'o <tytso@....edu>,
        Wang Jianchao <jianchao.wan9@...il.com>,
        "Tadakamadla, Rajesh" <rajesh.tadakamadla@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: RE: Expense of read_iter

From: Zhongwei Cai
> Sent: 12 January 2021 13:45
..
> The overhead mainly consists of two parts. The first is constructing
> struct iov_iter and iterating it (i.e., new_sync, _copy_mc_to_iter and
> iov_iter_init). The second is the dax io mechanism provided by VFS (i.e.,
> dax_iomap_rw, iomap_apply and ext4_iomap_begin).

Setting up an iov_iter with a single buffer ought to be relatively
cheap - compared to a file system read.

The iteration should be over the total length
calling copy_from/to_iter() for 'chunks' that don't
depend on the size of the iov[] fragments.

So copy_to/from_iter() should directly replace the copy_to/from_user()
calls in the 'read' method.
For a single buffer this really ought to be noise as well.

Clearly is the iov[] has a lot of short fragments the copy
will be more expensive.

Access to /dev/null and /dev/zero are much more likely to show
the additional costs of the iov_iter code than fs code.

	David

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