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

["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?
Ok.
>