Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   David Laight <David.Laight@ACULAB.COM>
To:     'Zhongwei Cai' <sunrise_l@sjtu.edu.cn>,
        Mingkai Dong <mingkaidong@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>
CC:     Mikulas Patocka <mpatocka@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Eric Sandeen <esandeen@redhat.com>,
        Dave Chinner <dchinner@redhat.com>,
        Theodore Ts'o <tytso@mit.edu>,
        Wang Jianchao <jianchao.wan9@gmail.com>,
        "Tadakamadla, Rajesh" <rajesh.tadakamadla@hpe.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>
Subject: RE: Expense of read_iter
Thread-Topic: Expense of read_iter
Thread-Index: Adbo7C52XXt2HSHdSaqZSwQZFrXi9Q==
Date:   Tue, 12 Jan 2021 14:06:53 +0000
Message-ID: <68567cb68fec4d79be79a655301effe1@AcuMS.aculab.com>
References: <alpine.LRH.2.02.2101061245100.30542@file01.intranet.prod.int.rdu2.redhat.com>
 <20210107151125.GB5270@casper.infradead.org>
 <17045315-CC1F-4165-B8E3-BA55DD16D46B@gmail.com>
 <2041983017.5681521.1610459100858.JavaMail.zimbra@sjtu.edu.cn>
In-Reply-To: <2041983017.5681521.1610459100858.JavaMail.zimbra@sjtu.edu.cn>
Accept-Language: en-GB, en-US
MIME-Version: 1.0
Content-Language: en-US
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Precedence: bulk

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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)