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 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.

Stefan