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

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH V2] block/io: optimize bdrv_co_pwritev for small requests
Date: Mon, 30 May 2016 11:47:19 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

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.


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