[dm-devel] [PATCH 4/4] dm: support I/O polling

[JeffleXu]

On 3/3/21 6:09 PM, Mikulas Patocka wrote:
> 
> 
> On Wed, 3 Mar 2021, JeffleXu wrote:
> 
>>
>>
>> On 3/3/21 3:05 AM, Mikulas Patocka wrote:
>>
>>> Support I/O polling if submit_bio_noacct_mq_direct returned non-empty
>>> cookie.
>>>
>>> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
>>>
>>> ---
>>>  drivers/md/dm.c |    5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> Index: linux-2.6/drivers/md/dm.c
>>> ===================================================================
>>> --- linux-2.6.orig/drivers/md/dm.c	2021-03-02 19:26:34.000000000 +0100
>>> +++ linux-2.6/drivers/md/dm.c	2021-03-02 19:26:34.000000000 +0100
>>> @@ -1682,6 +1682,11 @@ static void __split_and_process_bio(stru
>>>  		}
>>>  	}
>>>  
>>> +	if (ci.poll_cookie != BLK_QC_T_NONE) {
>>> +		while (atomic_read(&ci.io->io_count) > 1 &&
>>> +		       blk_poll(ci.poll_queue, ci.poll_cookie, true)) ;
>>> +	}
>>> +
>>>  	/* drop the extra reference count */
>>>  	dec_pending(ci.io, errno_to_blk_status(error));
>>>  }
>>
>> It seems that the general idea of your design is to
>> 1) submit *one* split bio
>> 2) blk_poll(), waiting the previously submitted split bio complets
> 
> No, I submit all the bios and poll for the last one.
> 
>> and then submit next split bio, repeating the above process. I'm afraid
>> the performance may be an issue here, since the batch every time
>> blk_poll() reaps may decrease.
> 
> Could you benchmark it?

I only tested dm-linear.

The configuration (dm table) of dm-linear is:
0 1048576 linear /dev/nvme0n1 0
1048576 1048576 linear /dev/nvme2n1 0
2097152 1048576 linear /dev/nvme5n1 0


fio script used is:
```
$cat fio.conf
[global]
name=iouring-sqpoll-iopoll-1
ioengine=io_uring
iodepth=128
numjobs=1
thread
rw=randread
direct=1
registerfiles=1
hipri=1
runtime=10
time_based
group_reporting
randrepeat=0
filename=/dev/mapper/testdev
bs=4k

[job-1]
cpus_allowed=14
```

IOPS (IRQ mode) | IOPS (iopoll mode (hipri=1))
--------------- | --------------------
           213k |		   19k

At least, it doesn't work well with io_uring interface.


-- 
Thanks,
Jeffle