Re: io_uring performance with block sizes > 128k

[Bijan Mottahedeh <bijan.mottahedeh@oracle.com>]

On 3/2/2020 9:01 PM, Jens Axboe wrote:
> On 3/2/20 4:57 PM, Jens Axboe wrote:
>> On 3/2/20 4:55 PM, Bijan Mottahedeh wrote:
>>> I'm seeing a sizeable drop in perf with polled fio tests for block sizes
>>>   > 128k:
>>>
>>> filename=/dev/nvme0n1
>>> rw=randread
>>> direct=1
>>> time_based=1
>>> randrepeat=1
>>> gtod_reduce=1
>>>
>>> fio --readonly --ioengine=io_uring --iodepth 1024 --fixedbufs --hipri
>>> --numjobs=16
>>> fio --readonly --ioengine=pvsync2 --iodepth 1024 --hipri --numjobs=16
>>>
>>>
>>> Compared with the pvsync2 engine, the only major difference I could see
>>> was the dio path, __blkdev_direct_IO() for io_uring vs.
>>> __blkdev_direct_IO_simple() for pvsync2 because of the is_sync_kiocb()
>>> check.
>>>
>>>
>>> static ssize_t
>>> blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>>> {
>>>           ...
>>>           if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
>>>                   return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
>>>
>>>           return __blkdev_direct_IO(iocb, iter, min(nr_pages,
>>> BIO_MAX_PAGES));
>>> }
>>>
>>> Just for an experiment, I hacked io_uring code to force it through the
>>> _simple() path and I get better numbers though the variance is fairly
>>> high, but the drop at bs > 128k seems consistent:
>>>
>>>
>>> # baseline
>>> READ: bw=3167MiB/s (3321MB/s), 186MiB/s-208MiB/s (196MB/s-219MB/s)   #128k
>>> READ: bw=898MiB/s (941MB/s), 51.2MiB/s-66.1MiB/s (53.7MB/s-69.3MB/s) #144k
>>> READ: bw=1576MiB/s (1652MB/s), 81.8MiB/s-109MiB/s (85.8MB/s-114MB/s) #256k
>>>
>>> # hack
>>> READ: bw=2705MiB/s (2836MB/s), 157MiB/s-174MiB/s (165MB/s-183MB/s) #128k
>>> READ: bw=2901MiB/s (3042MB/s), 174MiB/s-194MiB/s (183MB/s-204MB/s) #144k
>>> READ: bw=4194MiB/s (4398MB/s), 252MiB/s-271MiB/s (265MB/s-284MB/s) #256k
>> A quick guess would be that the IO is being split above 128K, and hence
>> the polling only catches one of the parts?
> Can you try and see if this makes a difference?
>
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 571b510ef0e7..cf7599a2c503 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1725,8 +1725,10 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, unsigned int *nr_events,
>   		if (ret < 0)
>   			break;
>   
> +#if 0
>   		if (ret && spin)
>   			spin = false;
> +#endif
>   		ret = 0;
>   	}
>   
>
I didn't see a difference.

If the request is split into two bios, is REQ_F_IOPOLL_COMPLETED set 
only when the 2nd bio completes?

I think you mentioned before that the request is split with 
__blk_queue_split() but I haven't yet been able to see how that happens 
exactly.  I see that the request size nvme_queue_rq() is the same as the 
original (e.g. 256k), is that expected?