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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 12:06:10 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 30.05.2016 um 11:53 hat Peter Lieven geschrieben:
> 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.

The test case already uses blkdebug to enforce a specific align value
(which is 4096 in this test case, not 512):

    echo "open -o driver=$IMGFMT,file.align=4k blkdebug::$TEST_IMG"


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