linux-kernel - RE: [PATCH v3 0/2] io_uring: add support for IORING_OP

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Message-ID: <247d154f2ba549b88a77daf29ec1791f@AcuMS.aculab.com>
Date:   Sat, 20 Feb 2021 17:44:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Lennert Buytenhek' <buytenh@...tstofly.org>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>
CC:     Matthew Wilcox <willy@...radead.org>
Subject: RE: [PATCH v3 0/2] io_uring: add support for IORING_OP_GETDENTS

From: Lennert Buytenhek
> Sent: 18 February 2021 12:27
> 
> These patches add support for IORING_OP_GETDENTS, which is a new io_uring
> opcode that more or less does an lseek(sqe->fd, sqe->off, SEEK_SET)
> followed by a getdents64(sqe->fd, (void *)sqe->addr, sqe->len).
> 
> A dumb test program for IORING_OP_GETDENTS is available here:
> 
> 	https://krautbox.wantstofly.org/~buytenh/uringfind-v2.c
> 
> This test program does something along the lines of what find(1) does:
> it scans recursively through a directory tree and prints the names of
> all directories and files it encounters along the way -- but then using
> io_uring.  (The io_uring version prints the names of encountered files and
> directories in an order that's determined by SQE completion order, which
> is somewhat nondeterministic and likely to differ between runs.)
> 
> On a directory tree with 14-odd million files in it that's on a
> six-drive (spinning disk) btrfs raid, find(1) takes:
> 
> 	# echo 3 > /proc/sys/vm/drop_caches
> 	# time find /mnt/repo > /dev/null
> 
> 	real    24m7.815s
> 	user    0m15.015s
> 	sys     0m48.340s
> 	#
> 
> And the io_uring version takes:
> 
> 	# echo 3 > /proc/sys/vm/drop_caches
> 	# time ./uringfind /mnt/repo > /dev/null
> 
> 	real    10m29.064s
> 	user    0m4.347s
> 	sys     0m1.677s
> 	#

While there may be uses for IORING_OP_GETDENTS are you sure your
test is comparing like with like?
The underlying work has to be done in either case, so you are
swapping system calls for code complexity.
I suspect that find is actually doing a stat() call on every
directory entry and that your io_uring example is just believing
the 'directory' flag returned in the directory entry for most
modern filesystems.

If you write a program that does openat(), readdir(), close()
for each directory and with a long enough buffer (mostly) do
one readdir() per directory you'll get a much better comparison.

You could even write a program with 2 threads, one does all the
open/readdir/close system calls and the other does the printing
and generating the list of directories to process.
That should get the equivalent overlapping that io_uring gives
without much of the complexity.

	David

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