Re: [Qemu-devel] [PATCH RFC v2 0/4] block/qapi: refactor and optimize the qmp_query_blockstats()

[Stefan Hajnoczi]

On Tue, Dec 20, 2016 at 10:26 AM, Dou Liyang <address@hidden> wrote:
> At 12/20/2016 06:01 PM, Stefan Hajnoczi wrote:
>>
>> On Tue, Dec 20, 2016 at 9:54 AM, Dou Liyang <address@hidden>
>> wrote:
>>>
>>> At 12/20/2016 05:39 PM, Stefan Hajnoczi wrote:
>>>>
>>>>
>>>> On Tue, Dec 20, 2016 at 12:32:40AM +0800, Fam Zheng wrote:
>>>>>
>>>>>
>>>>> On Mon, 12/19 15:02, Stefan Hajnoczi wrote:
>>>>>>
>>>>>>
>>>>>> On Mon, Dec 19, 2016 at 04:51:22PM +0800, Dou Liyang wrote:
>>>>>>>
>>>>>>>
>>>>>>> These patches aim to refactor the qmp_query_blockstats() and
>>>>>>> improve the performance by reducing the running time of it.
>>>>>>>
>>>>>>> qmp_query_blockstats() is used to monitor the blockstats, it
>>>>>>> querys all the graph_bdrv_states or monitor_block_backends.
>>>>>>>
>>>>>>> There are the two jobs:
>>>>>>>
>>>>>>> 1 For the performance:
>>>>>>>
>>>>>>> 1.1 the time it takes(ns) in each time:
>>>>>>> the disk numbers     | 10    | 500
>>>>>>> -------------------------------------
>>>>>>> before these patches | 19429 | 667722
>>>>>>> after these patches  | 17516 | 557044
>>>>>>>
>>>>>>> 1.2 the I/O performance is degraded(%) during the monitor:
>>>>>>>
>>>>>>> the disk numbers     | 10    | 500
>>>>>>> -------------------------------------
>>>>>>> before these patches | 1.3   | 14.2
>>>>>>> after these patches  | 0.8   | 9.1
>>>>>>
>>>>>>
>>>>>>
>>>>>> Do you know what is consuming the remaining 9.1%?
>>>>>>
>>>>>> I'm surprised to see such a high performance impact caused by a QMP
>>>>>> command.
>>>>>
>>>>>
>>>>>
>>>>> If it's "performance is 9.1% worse only during the 557044 ns when the
>>>>> QMP
>>>>> command is being processed", it's probably becaues the main loop is
>>>>> stalled a
>>>>> bit, and it's not a big problem. I'd be very surprised if the
>>>>> degradation
>>>>> is
>>>>> more longer than that.
>>>>
>>>>
>>>>
>>>> It would be interesting to compare against virtio-blk dataplane.  That
>>>> way the QMP command can execute without interfering with disk I/O
>>>> activity.
>>>>
>>>
>>> Yes, I will try to do it for you.
>>>
>>> I have compared against the disk of IDE/SATA, I think the I/O requests
>>> is handled by the vcpu threads. it isn't be disturbed like virtio-blk
>>> dataplane. but its performance also degraded.
>>
>>
>> IDE/SATA does I/O request *submission* in the vcpu thread.  Completion
>> is processed by the main loop.  Therefore it is also affected by slow
>> monitor commands.
>>
>
> Can you tell me one of the completion method in Qemu? I want to learn
> more.

How completion works depends on the -drive aio= option.

With aio=threads you need to look at thread-pool.c and block/raw-posix.c.

With aio=native you need to look at block/linux-aio.c

Since disk I/O is asynchronous there will be a completion event in the
AioContext to check the result of a completed I/O request and let the
device model (e.g. virtio-blk) process it.

Stefan