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)
