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SubjectRe: [PATCH v3 0/2] io_uring: add support for IORING_OP_GETDENTS
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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

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