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Date:   Tue, 12 Jan 2021 14:06:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Zhongwei Cai' <>,
        Mingkai Dong <>,
        Matthew Wilcox <>,
        "" <>
CC:     Mikulas Patocka <>,
        Andrew Morton <>,
        Jan Kara <>,
        Steven Whitehouse <>,
        Eric Sandeen <>,
        Dave Chinner <>,
        Theodore Ts'o <>,
        Wang Jianchao <>,
        "Tadakamadla, Rajesh" <>,
        "" <>,
        "" <>,
        "" <>
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.


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