Re: [PATCH v4] io_uring: reduce latency by reissueing the operation

[Pavel Begunkov]

On 6/22/21 1:17 PM, Olivier Langlois wrote:
> It is quite frequent that when an operation fails and returns EAGAIN,
> the data becomes available between that failure and the call to
> vfs_poll() done by io_arm_poll_handler().
> 
> Detecting the situation and reissuing the operation is much faster
> than going ahead and push the operation to the io-wq.
> 
> Performance improvement testing has been performed with:
> Single thread, 1 TCP connection receiving a 5 Mbps stream, no sqpoll.
> 
> 4 measurements have been taken:
> 1. The time it takes to process a read request when data is already available
> 2. The time it takes to process by calling twice io_issue_sqe() after vfs_poll() indicated that data was available
> 3. The time it takes to execute io_queue_async_work()
> 4. The time it takes to complete a read request asynchronously
> 
> 2.25% of all the read operations did use the new path.
> 
> ready data (baseline)
> avg	3657.94182918628
> min	580
> max	20098
> stddev	1213.15975908162
> 
> reissue	completion
> average	7882.67567567568
> min	2316
> max	28811
> stddev	1982.79172973284
> 
> insert io-wq time
> average	8983.82276995305
> min	3324
> max	87816
> stddev	2551.60056552038
> 
> async time completion
> average	24670.4758861127
> min	10758
> max	102612
> stddev	3483.92416873804
> 
> Conclusion:
> On average reissuing the sqe with the patch code is 1.1uSec faster and
> in the worse case scenario 59uSec faster than placing the request on
> io-wq
> 
> On average completion time by reissuing the sqe with the patch code is
> 16.79uSec faster and in the worse case scenario 73.8uSec faster than
> async completion.
> 
> Signed-off-by: Olivier Langlois <olivier@xxxxxxxxxxxxxx>
> ---
>  fs/io_uring.c | 31 ++++++++++++++++++++++---------
>  1 file changed, 22 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index fc8637f591a6..5efa67c2f974 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c

[...]

>  static bool __io_poll_remove_one(struct io_kiocb *req,
> @@ -6437,6 +6445,7 @@ static void __io_queue_sqe(struct io_kiocb *req)
>  	struct io_kiocb *linked_timeout = io_prep_linked_timeout(req);
>  	int ret;
>  
> +issue_sqe:
>  	ret = io_issue_sqe(req, IO_URING_F_NONBLOCK|IO_URING_F_COMPLETE_DEFER);
>  
>  	/*
> @@ -6456,12 +6465,16 @@ static void __io_queue_sqe(struct io_kiocb *req)
>  			io_put_req(req);
>  		}
>  	} else if (ret == -EAGAIN && !(req->flags & REQ_F_NOWAIT)) {
> -		if (!io_arm_poll_handler(req)) {
> +		switch (io_arm_poll_handler(req)) {
> +		case IO_APOLL_READY:
> +			goto issue_sqe;
> +		case IO_APOLL_ABORTED:
>  			/*
>  			 * Queued up for async execution, worker will release
>  			 * submit reference when the iocb is actually submitted.
>  			 */
>  			io_queue_async_work(req);
> +			break;

Hmm, why there is a new break here? It will miscount @linked_timeout
if you do that. Every io_prep_linked_timeout() should be matched with
io_queue_linked_timeout().


>  		}
>  	} else {
>  		io_req_complete_failed(req, ret);
> 

-- 
Pavel Begunkov