Messages in this thread | | | Date | Tue, 27 Sep 2011 08:07:29 +0200 | Subject | Re: [patch]cfq-iosched: delete deep seeky queue idle logic | From | Corrado Zoccolo <> |
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On Mon, Sep 26, 2011 at 2:55 AM, Shaohua Li <shaohua.li@intel.com> wrote: > On Fri, 2011-09-23 at 13:50 +0800, Corrado Zoccolo wrote: >> Il giorno 21/set/2011 13:16, "Shaohua Li" <shaohua.li@intel.com> ha >> scritto: >> > >> > On Sat, 2011-09-17 at 03:25 +0800, Corrado Zoccolo wrote: >> > > On Fri, Sep 16, 2011 at 8:40 AM, Shaohua Li <shaohua.li@intel.com> >> wrote: >> > > > On Fri, 2011-09-16 at 14:04 +0800, Corrado Zoccolo wrote: >> > > >> On Fri, Sep 16, 2011 at 5:09 AM, Shaohua Li >> <shaohua.li@intel.com> wrote: >> > > >> > Recently Maxim and I discussed why his aiostress workload >> performs poorly. If >> > > >> > you didn't follow the discussion, here are the issues we >> found: >> > > >> > 1. cfq seeky dection isn't good. Assume a task accesses >> sector A, B, C, D, A+1, >> > > >> > B+1, C+1, D+1, A+2...Accessing A, B, C, D is random. cfq will >> detect the queue >> > > >> > as seeky, but since when accessing A+1, A+1 is already in >> disk cache, this >> > > >> > should be detected as sequential really. Not sure if any real >> workload has such >> > > >> > access patern, and seems not easy to have a clean fix too. >> Any idea for this? >> > > >> >> > > >> Not all disks will cache 4 independent streams, we can't make >> that >> > > >> assumption in cfq. >> > > > sure thing. we can't make such assumption. I'm thinking if we >> should >> > > > move the seeky detection in request finish. If time between two >> requests >> > > > finish is short, we thought the queue is sequential. This will >> make the >> > > > detection adaptive. but seems time measurement isn't easy. >> > > > >> > > >> The current behaviour of assuming it as seeky should work well >> enough, >> > > >> in fact it will be put in the seeky tree, and it can enjoy the >> seeky >> > > >> tree quantum of time. If the second round takes a short time, >> it will >> > > >> be able to schedule a third round again after the idle time. >> > > >> If there are other seeky processes competing for the tree, the >> cache >> > > >> can be cleared by the time it gets back to your 4 streams >> process, so >> > > >> it will behave exactly as a seeky process from cfq point of >> view. >> > > >> If the various accesses were submitted in parallel, the deep >> seeky >> > > >> queue logic should kick in and make sure the process gets a >> sequential >> > > >> quantum, rather than sharing it with other seeky processes, so >> > > >> depending on your disk, it could perform better. >> > > > yes, the idle logic makes it ok, but sounds like "make things >> wrong >> > > > first (in seeky detection) and then fix it later (the idle >> logic)". >> > > > >> > > >> > 2. deep seeky queue idle. This makes raid performs poorly. I >> would think we >> > > >> > revert the logic. Deep queue is more popular with high end >> hardware. In such >> > > >> > hardware, we'd better not do idle. >> > > >> > Note, currently we set a queue's slice after the first >> request is finished. >> > > >> > This means the drive already idles a little time. If the >> queue is truely deep, >> > > >> > new requests should already come in, so idle isn't required. >> > > > What did you think about this? Assume seeky request takes long >> time, so >> > > > the queue is already idling for a little time. >> > > I don't think I understand. If cfq doesn't idle, it will dispatch >> an >> > > other request from the same or an other queue (if present) >> > > immediately, until all possible in-flight requests are sent. Now, >> you >> > > depend on NCQ for the order requests are handled, so you cannot >> > > guarantee fairness any more. >> > > >> > > > >> > > >> > Looks Vivek used to post a patch to rever it, but it gets >> ignored. >> > > >> > >> http://us.generation-nt.com/patch-cfq-iosched-revert-logic-deep-queues-help-198339681.html >> > > >> I get a 404 here. I think you are seeing only one half of the >> medal. >> > > >> That logic is there mainly to ensure fairness between deep >> seeky >> > > >> processes and normal seeky processes that want low latency. >> > > > didn't understand it. The logic doesn't protect non-deep >> process. how >> > > > could it make the normal seeky process have low latency? or did >> you have >> > > > a test case for this, so I can analyze? >> > > > I tried a workload with one task drives depth 4 and one task >> drives >> > > > depth 16. Appears the behavior isn't changed w/wo the logic. >> > sorry for the delay. >> > >> > > Try a workload with one shallow seeky queue and one deep (16) one, >> on >> > > a single spindle NCQ disk. >> > > I think the behaviour when I submitted my patch was that both were >> > > getting 100ms slice (if this is not happening, probably some >> > > subsequent patch broke it). >> > > If you remove idling, they will get disk time roughly in >> proportion >> > > 16:1, i.e. pretty unfair. >> > I thought you are talking about a workload with one thread depth 4, >> and >> > the other thread depth 16. I did some tests here. In an old kernel, >> > without the deep seeky idle logic, the threads have disk time in >> > proportion 1:5. With it, they get almost equal disk time. SO this >> > reaches your goal. In a latest kernel, w/wo the logic, there is no >> big >> > difference (the 16 depth thread get about 5x more disk time). With >> the >> > logic, the depth 4 thread gets equal disk time in first several >> slices. >> > But after an idle expiration(mostly because current block plug hold >> > requests in task list and didn't add them to elevator), the queue >> never >> > gets detected as deep, because the queue dispatch request one by >> one. So >> > the logic is already broken for some time (maybe since block plug is >> > added). >> Could be that dispatching requests one by one is harming the >> performance, then? > Not really. Say 4 requests are running, the task dispatches a request > after one previous request is completed. requests are dispatching one by > one but there are still 4 requests running at any time. Checking the > in_flight requests are more precise for the deep detection. > What happens if there are 4 tasks, all that could dispatch 4 requests in parallel? Will we reach and sustain 16 in flight requests, or it will bounce around 4 in flight? I think here we could get a big difference. Probably it is better to move the deep queue detection logic in the per-task queue? Then cfq will decide if it should dispatch few requests from every task (shallow case) or all requests from a single task (deep), and then idle.
Thanks Corrado
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