From: Matthew Wilcox <willy@infradead.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: 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-fsdevel@vger.kernel.org,
linux-nvdimm@lists.01.org
Subject: Expense of read_iter
Date: Thu, 7 Jan 2021 15:11:25 +0000 [thread overview]
Message-ID: <20210107151125.GB5270@casper.infradead.org> (raw)
In-Reply-To: <alpine.LRH.2.02.2101061245100.30542@file01.intranet.prod.int.rdu2.redhat.com>
On Thu, Jan 07, 2021 at 08:15:41AM -0500, Mikulas Patocka wrote:
> I'd like to ask about this piece of code in __kernel_read:
> if (unlikely(!file->f_op->read_iter || file->f_op->read))
> return warn_unsupported...
> and __kernel_write:
> if (unlikely(!file->f_op->write_iter || file->f_op->write))
> return warn_unsupported...
>
> - It exits with an error if both read_iter and read or write_iter and
> write are present.
>
> I found out that on NVFS, reading a file with the read method has 10%
> better performance than the read_iter method. The benchmark just reads the
> same 4k page over and over again - and the cost of creating and parsing
> the kiocb and iov_iter structures is just that high.
Which part of it is so expensive? Is it worth, eg adding an iov_iter
type that points to a single buffer instead of a single-member iov?
+++ b/include/linux/uio.h
@@ -19,6 +19,7 @@ struct kvec {
enum iter_type {
/* iter types */
+ ITER_UBUF = 2,
ITER_IOVEC = 4,
ITER_KVEC = 8,
ITER_BVEC = 16,
@@ -36,6 +36,7 @@ struct iov_iter {
size_t iov_offset;
size_t count;
union {
+ void __user *buf;
const struct iovec *iov;
const struct kvec *kvec;
const struct bio_vec *bvec;
and then doing all the appropriate changes to make that work.
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next prev parent reply other threads:[~2021-01-07 15:14 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 ` Matthew Wilcox [this message]
2021-01-07 16:43 ` Expense of read_iter Mingkai Dong
[not found] ` <2041983017.5681521.1610459100858.JavaMail.zimbra@sjtu.edu.cn>
2021-01-12 14:06 ` David Laight
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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