Re: [Qemu-devel] [PATCH V2] block/io: optimize bdrv_co_pwritev for small requests

[Peter Lieven <pl@kamp.de>]

Am 30.05.2016 um 11:47 schrieb Kevin Wolf:
> Am 30.05.2016 um 11:30 hat Peter Lieven geschrieben:
>> Am 30.05.2016 um 10:24 schrieb Kevin Wolf:
>>> Am 30.05.2016 um 08:25 hat Peter Lieven geschrieben:
>>>> Am 27.05.2016 um 10:55 schrieb Kevin Wolf:
>>>>> Am 27.05.2016 um 02:36 hat Fam Zheng geschrieben:
>>>>>> On Thu, 05/26 11:20, Paolo Bonzini wrote:
>>>>>>> On 26/05/2016 10:30, Fam Zheng wrote:
>>>>>>>>>> This doesn't look too wrong...  Should the right sequence of events be
>>>>>>>>>> head/after_head or head/after_tail?  It's probably simplest to just emit
>>>>>>>>>> all four events.
>>>>>>>> I've no idea. (That's why I leaned towards fixing the test case).
>>>>>>> Well, fixing the testcase means knowing what events should be emitted.
>>>>>>>
>>>>>>> QEMU with Peter's patch emits head/after_head.  If the right one is
>>>>>>> head/after_tail, _both QEMU and the testcase_ need to be adjusted.  Your
>>>>>>> patch keeps the backwards-compatible route.
>>>>>> Yes, I mean I was not very convinced in tweaking the events at all: each pair
>>>>>> of them has been emitted around bdrv_aligned_preadv(), and the new branch
>>>>>> doesn't do it anymore. So I don't see a reason to add events here.
>>>>> Yes, if you can assume that anyone who uses the debug events know
>>>>> exactly what the code looks like, adding the events here is pointless
>>>>> because TAIL, AFTER_TAIL and for the greatest part also AFTER_HEAD are
>>>>> essentially the same then.
>>>>>
>>>>> Having TAIL before the qiov change and AFTER_TAIL afterwards doesn't
>>>>> make any difference, they could (and should) be called immediately one
>>>>> after another if we wanted to keep the behaviour.
>>>>>
>>>>> I would agree that we should take a look at the test case and what it
>>>>> actually wants to achieve before we can decide whether AFTER_HEAD and
>>>>> TAIL/AFTER_TAIL would be the same (the former could trigger earlier if
>>>>> there are two requests and only one is unaligned at the tail). Maybe we
>>>>> even need to extend the test case now so that both paths (explicit read
>>>>> of the tail and the shortcut) are covered.
>>>> The part that actually blocks in 077 is
>>>>
>>>> # Sequential RMW requests on the same physical sector
>>>>
>>>> its expecting all 4 events around the RMW cycle.
>>>>
>>>> However, it seems that also other parts of 077 would need an adjustment
>>>> and the output might differ depending on the alignment. So I guess we
>>>> have to emit the events if we don't want to recode the whole 077 and make
>>>> it aware of the alignment.
>>> Yes, but my point is that we may need to rework 077 anyway if we don't
>>> only want to make it pass again, but to cover all relevant paths, too.
>>> We got a new code path and it's unlikely that the existing tests covered
>>> both the old code path and the new one.
>> So you would postpone this patch until 077 is reworked?
>> I found this one a nice improvement and 077 might take some time.
> The problem with "we'll rework the tests later" is always that it
> doesn't happen if the patches for the functional parts and a workaround
> for the test case are merged.
>
> I don't think that making 077 cover both cases should be hard or take
> much time, it just needs to be done. If all the time for writing emails
> in this thread had been used to work on the test case, it would already
> be done.

Understood. If you can give a hint how to get the value of the align
parameter into the test script I can try. Otherwise the test will fail
also if any block driver has an align value that is not equal to 512.

Peter