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