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

[Peter Lieven <pl@kamp.de>]

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.

Peter