2012/8/16 Jianpeng Ma <majianpeng@xxxxxxxxx>: > On 2012-08-15 09:44 Shaohua Li <shli@xxxxxxxxxx> 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@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? >> > 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. > BTW, why are you and neil not to answer my option which adding REQ_NOIDLE for last bio of one dio? I'm not quite positive to this. each io_submit can submit several requests, and each request has a dio. Setting the flag for the first dio doesn't make sense, for example Thanks, Shaohua -- 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
