RE: Expense of read_iter

[David Laight <David.Laight@ACULAB.COM>]

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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