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

[Dmitry Shulyak <yashulyak@gmail.com>]

Forgot to mention, that this is on 5.7.12, with writev/read during operations.

On Mon, 17 Aug 2020 at 14:46, Dmitry Shulyak <yashulyak@gmail.com> wrote:
>
> Hi everyone,
>
> I noticed in iotop that all writes are executed by the same thread
> (io_wqe_worker-0). This is a significant problem if I am using files
> with mentioned flags. Not the case with reads, requests are
> multiplexed over many threads (note the different name
> io_wqe_worker-1). The problem is not specific to O_SYNC, in the
> general case I can get higher throughput with thread pool and regular
> system calls, but specifically with O_SYNC the throughput is the same
> as if I were using a single thread for writing.
>
> The setup is always the same, ring per thread with shared workers pool
> (IORING_SETUP_ATTACH_WQ), and high submission rate. Also, it is
> possible to get around this performance issue by using separate worker
> pools, but then I have to load balance workload between many rings for
> perf gains.
>
> I thought that it may have something to do with the IOSQE_ASYNC flag,
> but setting it had no effect.
>
> Is it expected behavior? Are there any other solutions, except
> creating many rings with isolated worker pools?