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
Date: Tue, 12 Jan 2021 14:06:53 +0000 [thread overview]
Message-ID: <68567cb68fec4d79be79a655301effe1@AcuMS.aculab.com> (raw)
In-Reply-To: <2041983017.5681521.1610459100858.JavaMail.zimbra@sjtu.edu.cn>
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)
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next prev parent reply other threads:[~2021-01-12 14:07 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-07 13:15 [RFC v2] nvfs: a filesystem for persistent memory Mikulas Patocka
2021-01-07 15:11 ` Expense of read_iter Matthew Wilcox
2021-01-07 16:43 ` Mingkai Dong
[not found] ` <2041983017.5681521.1610459100858.JavaMail.zimbra@sjtu.edu.cn>
2021-01-12 14:06 ` David Laight [this message]
2021-01-13 16:44 ` Mikulas Patocka
[not found] ` <1224425872.715547.1610703643424.JavaMail.zimbra@sjtu.edu.cn>
2021-01-20 4:47 ` Dave Chinner
2021-01-20 14:18 ` Jan Kara
2021-01-20 15:12 ` Mikulas Patocka
2021-01-20 15:44 ` David Laight
2021-01-21 15:47 ` Matthew Wilcox
2021-01-21 16:06 ` Mikulas Patocka
2021-01-07 18:59 ` Mikulas Patocka
2021-01-10 6:13 ` Matthew Wilcox
2021-01-10 21:19 ` Mikulas Patocka
2021-01-11 0:18 ` Matthew Wilcox
2021-01-11 21:10 ` Mikulas Patocka
2021-01-11 10:11 ` David Laight
2021-01-10 16:20 ` [RFC v2] nvfs: a filesystem for persistent memory Al Viro
2021-01-10 16:51 ` Al Viro
2021-01-10 21:14 ` Mikulas Patocka
2021-01-10 23:40 ` Al Viro
2021-01-11 11:41 ` Mikulas Patocka
2021-01-11 10:29 ` David Laight
2021-01-11 11:44 ` Mikulas Patocka
2021-01-11 11:57 ` David Laight
2021-01-11 14:43 ` Al Viro
2021-01-11 14:54 ` David Laight
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