Re: Very low write throughput on file opened with O_SYNC/O_DSYNC

[Dmitry Shulyak <yashulyak@gmail.com>]

it worked, but there are some issues.
with o_dsync and even moderate submission rate threads are stuck in
some cpu task (99.9% cpu consumption), and make very slow progress.
have you expected it? it must be something specific to uring, i can't
reproduce this condition by writing from 2048 threads.




On Mon, 17 Aug 2020 at 19:17, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 8/17/20 8:49 AM, Dmitry Shulyak wrote:
> > With 48 threads i am getting 200 mb/s, about the same with 48 separate
> > uring instances.
> > With single uring instance (or with shared pool) - 60 mb/s.
> > fs - ext4, device - ssd.
>
> You could try something like this kernel addition:
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 4b102d9ad846..8909a1d37801 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1152,7 +1152,7 @@ static void io_prep_async_work(struct io_kiocb *req)
>         io_req_init_async(req);
>
>         if (req->flags & REQ_F_ISREG) {
> -               if (def->hash_reg_file)
> +               if (def->hash_reg_file && !(req->flags & REQ_F_FORCE_ASYNC))
>                         io_wq_hash_work(&req->work, file_inode(req->file));
>         } else {
>                 if (def->unbound_nonreg_file)
>
> and then set IOSQE_IO_ASYNC on your writes. That'll parallelize them in
> terms of execution.
>
> --
> Jens Axboe
>