On 4/28/21 7:32 AM, Hao Xu wrote:
> sqes are submitted by sqthread when it is leveraged, which means there
> is IO latency when waking up sqthread. To wipe it out, submit limited
> number of sqes in the original task context.
> Tests result below:
>
> 99th latency:
> iops\idle 10us 60us 110us 160us 210us 260us 310us 360us 410us 460us 510us
> with this patch:
> 2k 13 13 12 13 13 12 12 11 11 10.304 11.84
> without this patch:
> 2k 15 14 15 15 15 14 15 14 14 13 11.84
>
> fio config:
> ./run_fio.sh
> fio \
> --ioengine=io_uring --sqthread_poll=1 --hipri=1 --thread=1 --bs=4k \
> --direct=1 --rw=randread --time_based=1 --runtime=300 \
> --group_reporting=1 --filename=/dev/nvme1n1 --sqthread_poll_cpu=30 \
> --randrepeat=0 --cpus_allowed=35 --iodepth=128 --rate_iops=${1} \
> --io_sq_thread_idle=${2}
Interesting concept! One question:
> @@ -9304,8 +9311,18 @@ static int io_get_ext_arg(unsigned flags, const void __user *argp, size_t *argsz
> if (unlikely(ctx->sq_data->thread == NULL)) {
> goto out;
> }
> - if (flags & IORING_ENTER_SQ_WAKEUP)
> + if (flags & IORING_ENTER_SQ_WAKEUP) {
> wake_up(&ctx->sq_data->wait);
> + if ((flags & IORING_ENTER_SQ_DEPUTY) &&
> + !(ctx->flags & IORING_SETUP_IOPOLL)) {
> + ret = io_uring_add_task_file(ctx);
> + if (unlikely(ret))
> + goto out;
> + mutex_lock(&ctx->uring_lock);
> + io_submit_sqes(ctx, min(to_submit, 8U));
> + mutex_unlock(&ctx->uring_lock);
> + }
> + }
Do we want to wake the sqpoll thread _post_ submitting these ios? The
idea being that if we're submitting now after a while (since the thread
is sleeping), then we're most likely going to be submitting more than
just this single batch. And the wakeup would do the same if done after
the submit, it'd just not interfere with this submit. You could imagine
a scenario where we do the wake and the sqpoll thread beats us to the
submit, and now we're just stuck waiting for the uring_lock and end up
doing nothing.
Maybe you guys already tested this? Also curious if you did, what kind
of requests are being submitted? That can have quite a bit of effect on
how quickly the submit is done.
--
Jens Axboe
