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From: Jens Axboe <[email protected]>
To: David Laight <[email protected]>,
	'Lennert Buytenhek' <[email protected]>,
	Al Viro <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Subject: Re: [PATCH v3 0/2] io_uring: add support for IORING_OP_GETDENTS
Date: Sat, 20 Feb 2021 11:29:22 -0700	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 2/20/21 10:44 AM, David Laight wrote:
> 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.

What 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.

While that may be true (find doing stat as well), the runtime is
clearly dominated by IO. Adding a stat on top would be an extra
copy, but no extra IO.

> 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.

But this is what take the most offense to - it's _trivial_ to
write that program with io_uring, especially compared to managing
threads. Threads are certainly a more known paradigm at this point,
but an io_uring submit + reap loop is definitely not "much of the
complexity". If you're referring to the kernel change itself, that's
trivial, as the diffstat shows.

-- 
Jens Axboe


  reply	other threads:[~2021-02-20 18:30 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-18 12:26 [PATCH v3 0/2] io_uring: add support for IORING_OP_GETDENTS Lennert Buytenhek
2021-02-18 12:27 ` [PATCH v3 1/2] readdir: split the core of getdents64(2) out into vfs_getdents() Lennert Buytenhek
2021-02-18 12:27 ` [PATCH v3 2/2] io_uring: add support for IORING_OP_GETDENTS Lennert Buytenhek
2021-02-19 12:05   ` Pavel Begunkov
2021-02-19 12:10     ` Pavel Begunkov
2021-02-19 18:06     ` Lennert Buytenhek
2021-02-19 12:34   ` Matthew Wilcox
2021-02-19 18:07     ` Lennert Buytenhek
2021-02-19 18:59       ` Lennert Buytenhek
2021-02-20 17:44 ` [PATCH v3 0/2] " David Laight
2021-02-20 18:29   ` Jens Axboe [this message]
2021-02-21 19:38     ` David Laight
2021-02-21 21:12       ` Jens Axboe

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