lkml.org 
[lkml]   [2021]   [Feb]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH v3 0/2] io_uring: add support for IORING_OP_GETDENTS
Date
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

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

\
 
 \ /
  Last update: 2021-02-20 18:47    [W:0.121 / U:5.176 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site