On Wed, Aug 15, 2012 at 10:56:10AM +1000, NeilBrown wrote: > On Tue, 14 Aug 2012 14:33:43 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote: > > > On Thu, Aug 09, 2012 at 01:07:01PM +0800, Shaohua Li wrote: > > > 2012/8/9 NeilBrown <neilb@xxxxxxx>: > > > > On Thu, 9 Aug 2012 09:20:05 +0800 "Jianpeng Ma" <majianpeng@xxxxxxxxx> wrote: > > > > > > > >> On 2012-08-08 20:53 Shaohua Li <shli@xxxxxxxxxx> Wrote: > > > >> >2012/8/8 Jianpeng Ma <majianpeng@xxxxxxxxx>: > > > >> >> On 2012-08-08 10:58 Shaohua Li <shli@xxxxxxxxxx> Wrote: > > > >> >>>2012/8/7 Jianpeng Ma <majianpeng@xxxxxxxxx>: > > > >> >>>> On 2012-08-07 13:32 Shaohua Li <shli@xxxxxxxxxx> Wrote: > > > >> >>>>>2012/8/7 Jianpeng Ma <majianpeng@xxxxxxxxx>: > > > >> >>>>>> On 2012-08-07 11:22 Shaohua Li <shli@xxxxxxxxxx> 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@xxxxxxxxxxxx> > > > >> >>>> [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? -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
