Re: Re: [patch v2]raid5: fix directio regression

["Jianpeng Ma" <majianpeng@gmail.com>]

On 2012-08-23 15:55 Shaohua Li <shli@kernel.org> Wrote:
>2012/8/23 Jianpeng Ma <majianpeng@gmail.com>:
>> On 2012-08-23 14:08 Shaohua Li <shli@kernel.org> Wrote:
>>>2012/8/16 Shaohua Li <shli@kernel.org>:
>>>> 2012/8/16 Jianpeng Ma <majianpeng@gmail.com>:
>>>>> On 2012-08-15 09:44 Shaohua Li <shli@kernel.org> Wrote:
>>>>>>On Wed, Aug 15, 2012 at 10:56:10AM +1000, NeilBrown wrote:
>>>>>>> On Tue, 14 Aug 2012 14:33:43 +0800 Shaohua Li <shli@kernel.org> wrote:
>>>>>>>
>>>>>>> > On Thu, Aug 09, 2012 at 01:07:01PM +0800, Shaohua Li wrote:
>>>>>>> > > 2012/8/9 NeilBrown <neilb@suse.de>:
>>>>>>> > > > On Thu, 9 Aug 2012 09:20:05 +0800 "Jianpeng Ma" <majianpeng@gmail.com> wrote:
>>>>>>> > > >
>>>>>>> > > >> On 2012-08-08 20:53 Shaohua Li <shli@kernel.org> Wrote:
>>>>>>> > > >> >2012/8/8 Jianpeng Ma <majianpeng@gmail.com>:
>>>>>>> > > >> >> On 2012-08-08 10:58 Shaohua Li <shli@kernel.org> Wrote:
>>>>>>> > > >> >>>2012/8/7 Jianpeng Ma <majianpeng@gmail.com>:
>>>>>>> > > >> >>>> On 2012-08-07 13:32 Shaohua Li <shli@kernel.org> Wrote:
>>>>>>> > > >> >>>>>2012/8/7 Jianpeng Ma <majianpeng@gmail.com>:
>>>>>>> > > >> >>>>>> On 2012-08-07 11:22 Shaohua Li <shli@kernel.org> Wrote:
>>>>>>> > > >> >>>>>>>My directIO randomwrite 4k workload shows a 10~20% regression caused by commit
>>>>>>> > > >> >>>>>>>895e3c5c58a80bb. directIO usually is random IO and if request size isn't big
>>>>>>> > > >> >>>>>>>(which is the common case), delay handling of the stripe hasn't any advantages.
>>>>>>> > > >> >>>>>>>For big size request, delay can still reduce IO.
>>>>>>> > > >> >>>>>>>
>>>>>>> > > >> >>>>>>>Signed-off-by: Shaohua Li <shli@fusionio.com>
>>>>>>> > > >> >>>> [snip]
>>>>>>> > > >> >>>>>>>--
>>>>>>> > > >> >>>>>> May be used size to judge is not a good method.
>>>>>>> > > >> >>>>>> I firstly sended this patch, only want to control direct-write-block,not for reqular file.
>>>>>>> > > >> >>>>>> Because i think if someone used direct-write-block for raid5,he should know the feature of raid5 and he can control
>>>>>>> > > >> >>>>>> for write to full-write.
>>>>>>> > > >> >>>>>> But at that time, i did know how to differentiate between regular file and block-device.
>>>>>>> > > >> >>>>>> I thik we should do something to do this.
>>>>>>> > > >> >>>>>
>>>>>>> > > >> >>>>>I don't think it's possible user can control his write to be a
>>>>>>> > > >> >>>>>full-write even for
>>>>>>> > > >> >>>>>raw disk IO. Why regular file and block device io matters here?
>>>>>>> > > >> >>>>>
>>>>>>> > > >> >>>>>Thanks,
>>>>>>> > > >> >>>>>Shaohua
>>>>>>> > > >> >>>> Another problem is the size. How to judge the size is large or not?
>>>>>>> > > >> >>>> A syscall write is a dio and a dio may be split more bios.
>>>>>>> > > >> >>>> For my workload, i usualy write chunk-size.
>>>>>>> > > >> >>>> But your patch is judge by bio-size.
>>>>>>> > > >> >>>
>>>>>>> > > >> >>>I'd ignore workload which does sequential directIO, though
>>>>>>> > > >> >>>your workload is, but I bet no real workloads are. So I'd like
>>>>>>> > > >> >> Sorry,my explain maybe not corcrect. I write data once which size is almost chunks-size * devices,in order to full-write
>>>>>>> > > >> >> and as possible as to no pre-read operation.
>>>>>>> > > >> >>>only to consider big size random directio. I agree the size
>>>>>>> > > >> >>>judge is arbitrary. I can optimize it to be only consider stripe
>>>>>>> > > >> >>>which hits two or more disks in one bio, but not sure if it's
>>>>>>> > > >> >>>worthy doing. Not ware big size directio is common, and even
>>>>>>> > > >> >>>is, big size request IOPS is low, a bit delay maybe not a big
>>>>>>> > > >> >>>deal.
>>>>>>> > > >> >> If add a acc_time for 'striep_head' to control?
>>>>>>> > > >> >> When get_active_stripe() is ok, update acc_time.
>>>>>>> > > >> >> For some time, stripe_head did not access and it shold pre-read.
>>>>>>> > > >> >
>>>>>>> > > >> >Do you want to add a timer for each stripe? This is even ugly.
>>>>>>> > > >> >How do you choose the expire time? A time works for harddisk
>>>>>>> > > >> >definitely will not work for a fast SSD.
>>>>>>> > > >> A time is like the size which is arbitrary.
>>>>>>> > > >> How about add a interface in sysfs to control by user?
>>>>>>> > > >> Only user can judge the workload, which sequatial write or random write.
>>>>>>> > > >
>>>>>>> > > > This is getting worse by the minute.  A sysfs interface for this is
>>>>>>> > > > definitely not a good idea.
>>>>>>> > > >
>>>>>>> > > > The REQ_NOIDLE flag is a pretty clear statement that no more requests that
>>>>>>> > > > merge with this one are expected.  If some use cases sends random requests,
>>>>>>> > > > maybe it should be setting REQ_NOIDLE.
>>>>>>> > > >
>>>>>>> > > > Maybe someone should do some research and find out why WRITE_ODIRECT doesn't
>>>>>>> > > > include REQ_NOIDLE.  Understanding that would help understand the current
>>>>>>> > > > problem.
>>>>>>> > >
>>>>>>> > > A quick search shows only cfq-iosched uses REQ_NOIDLE. In
>>>>>>> > > cfq, a queue is idled to avoid losing its share. REQ_NOIDLE
>>>>>>> > > tells cfq to avoid idle, since the task will not dispatch further
>>>>>>> > > requests any more. Note this isn't no merge.
>>>>>>> >
>>>>>>> > Since REQ_NOIDLE has no relationship with request merge, we'd better remove it.
>>>>>>> > I came out a new patch, which doesn't depend on request size any more. With
>>>>>>> > this patch, sequential directio will still introduce unnecessary raid5 preread
>>>>>>> > (especially for small size IO), but I bet no app does sequential small size
>>>>>>> > directIO.
>>>>>>> >
>>>>>>> > Thanks,
>>>>>>> > Shaohua
>>>>>>> >
>>>>>>> > Subject: raid5: fix directio regression
>>>>>>> >
>>>>>>> > My directIO randomwrite 4k workload shows a 10~20% regression caused by commit
>>>>>>> > 895e3c5c58a80bb. This commit isn't friendly for small size random IO, because
>>>>>>> > delaying such request hasn't any advantages.
>>>>>>> >
>>>>>>> > DirectIO usually is random IO. I thought we can ignore request merge between
>>>>>>> > bios from different io_submit. So we only consider one bio which can drive
>>>>>>> > unnecessary preread in raid5, which is large request. If a bio is large enough
>>>>>>> > and some of its stripes will access two or more disks, such stripes should be
>>>>>>> > delayed to avoid unnecessary preread till bio for the last disk of the strips
>>>>>>> > is added.
>>>>>>> >
>>>>>>> > REQ_NOIDLE doesn't mean about request merge, I deleted it.
>>>>>>>
>>>>>>> Hi,
>>>>>>>  Have you tested what effect this has on large sequential direct writes?
>>>>>>>  Because it don't make sense to me and I would be surprised if it improves
>>>>>>>  things.
>>>>>>>
>>>>>>>  You are delaying setting the STRIPE_PREREAD_ACTIVE bit until you think you
>>>>>>>  have submitted all the writes from this bio that apply to the give stripe.
>>>>>>>  That does make some sense, however it doesn't seem to deal with the
>>>>>>>  possibility that the one bio covers parts of two different stripes.  In that
>>>>>>>  case the first stripe never gets STRIPE_PREREAD_ACTIVE set, so it is delayed
>>>>>>>  despite having 'REQ_SYNC' set.
>>>>>>
>>>>>>I didn't get your point. Isn't last_sector - logical_sector < chunk_sectors true
>>>>>>in the case?
>>>>>>
>>>>>>>  Also, and more significantly, plugging should mean that the various
>>>>>>>  stripe_heads are not even looked at until all of the original bio is
>>>>>>>  processed, so while STRIPE_PREREAD_ACTIVE might get set early, it should not
>>>>>>>  get processed until the whole bio is processed and the queue is unplugged.
>>>>>>>
>>>>>>>  So I don't think this patch should make a difference on large direct writes,
>>>>>>>  and if it does then something strange is going on that I'd like to
>>>>>>>  understand first.
>>>>>>
>>>>>>Aha, ok, this makes sense. recent delayed stripe release should make the
>>>>>>problem go away. So Jianpeng, can you try your workload with the commit
>>>>>>reverted with a recent kernel please?
>>>>>>
>>>>> I tested used your patch in my workload.
>>>>> Like the neil said, the performance does not regress.
>>>>> But if the code is :
>>>>>>                       if (test_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
>>>>>>                               release_stripe(sh);
>>>>>>                       else
>>>>>>                               release_stripe_plug(mddev, sh);
>>>>> The speed is about 76MB/s.With those code the speed is 200MB/s.
>>>>
>>>> Hmm, what I want to test is upstream kernel with commit 895e3c5c58a80bb
>>>> reverted. don't apply my patch. We want to just revert the commit.
>>>
>>>Did you have data for your original workload with 895e3c5c58a80bb
>>>reverted now?
>> our raid5 which had 14 SATA HDDs.
>>
>> with  895e3c5c58a80bb reverted:
>> using dd to test 55MB/s
>> using our-fs 200-250Mb/s
>>
>> with  895e3c5c58a80bb:
>> using dd to test 275MB/s
>> using our-fs 500-550Mb/s
>
>what's block size of dd in this test? In your original test, your
>BS covers chunk_sector*data_disks. In that case,
>895e3c5c58a80bb is likely not required.
Maybe it's my fault. Kerel version is 3.4.4 which did not contain the patch 8811b5968f6216e97c.
So i will retest it.
>
>I guess you are using a smaller bs this time, so we need
>merge different bios to a stripe overwrite?
At present, in our situation,we control our-fs to do it.
If md can support this function,like io-schedule. it's better for all who can't contorl use-date to cover
chunk_sector*data_disks.