Messages in this thread | | | Subject | Re: [PATCH v3 3/5] block, bfq: don't disable wbt if CONFIG_BFQ_GROUP_IOSCHED is disabled | From | Yu Kuai <> | Date | Wed, 28 Sep 2022 11:30:05 +0800 |
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Hi,
在 2022/09/28 0:14, Paolo Valente 写道: > > >> Il giorno 27 set 2022, alle ore 03:02, Yu Kuai <yukuai1@huaweicloud.com> ha scritto: >> >> Hi, Jan >> >> 在 2022/09/26 22:22, Jan Kara 写道: >>> Hi Kuai! >>> On Mon 26-09-22 21:00:48, Yu Kuai wrote: >>>> 在 2022/09/23 19:03, Jan Kara 写道: >>>>> Hi Kuai! >>>>> >>>>> On Fri 23-09-22 18:23:03, Yu Kuai wrote: >>>>>> 在 2022/09/23 18:06, Jan Kara 写道: >>>>>>> On Fri 23-09-22 17:50:49, Yu Kuai wrote: >>>>>>>> Hi, Christoph >>>>>>>> >>>>>>>> 在 2022/09/23 16:56, Christoph Hellwig 写道: >>>>>>>>> On Thu, Sep 22, 2022 at 07:35:56PM +0800, Yu Kuai wrote: >>>>>>>>>> wbt and bfq should work just fine if CONFIG_BFQ_GROUP_IOSCHED is disabled. >>>>>>>>> >>>>>>>>> Umm, wouldn't this be something decided at runtime, that is not >>>>>>>>> if CONFIG_BFQ_GROUP_IOSCHED is enable/disable in the kernel build >>>>>>>>> if the hierarchical cgroup based scheduling is actually used for a >>>>>>>>> given device? >>>>>>>>> . >>>>>>>>> >>>>>>>> >>>>>>>> That's a good point, >>>>>>>> >>>>>>>> Before this patch wbt is simply disabled if elevator is bfq. >>>>>>>> >>>>>>>> With this patch, if elevator is bfq while bfq doesn't throttle >>>>>>>> any IO yet, wbt still is disabled unnecessarily. >>>>>>> >>>>>>> It is not really disabled unnecessarily. Have you actually tested the >>>>>>> performance of the combination? I did once and the results were just >>>>>>> horrible (which is I made BFQ just disable wbt by default). The problem is >>>>>>> that blk-wbt assumes certain model of underlying storage stack and hardware >>>>>>> behavior and BFQ just does not fit in that model. For example BFQ wants to >>>>>>> see as many requests as possible so that it can heavily reorder them, >>>>>>> estimate think times of applications, etc. On the other hand blk-wbt >>>>>>> assumes that if request latency gets higher, it means there is too much IO >>>>>>> going on and we need to allow less of "lower priority" IO types to be >>>>>>> submitted. These two go directly against one another and I was easily >>>>>>> observing blk-wbt spiraling down to allowing only very small number of >>>>>>> requests submitted while BFQ was idling waiting for more IO from the >>>>>>> process that was currently scheduled. >>>>>>> >>>>>> >>>>>> Thanks for your explanation, I understand that bfq and wbt should not >>>>>> work together. >>>>>> >>>>>> However, I wonder if CONFIG_BFQ_GROUP_IOSCHED is disabled, or service >>>>>> guarantee is not needed, does the above phenomenon still exist? I find >>>>>> it hard to understand... Perhaps I need to do some test. >>>>> >>>>> Well, BFQ implements for example idling on sync IO queues which is one of >>>>> the features that upsets blk-wbt. That does not depend on >>>>> CONFIG_BFQ_GROUP_IOSCHED in any way. Also generally the idea that BFQ >>>>> assigns storage *time slots* to different processes and IO from other >>>>> processes is just queued at those times increases IO completion >>>>> latency (for IOs of processes that are not currently scheduled) and this >>>>> tends to confuse blk-wbt. >>>>> >>>> Hi, Jan >>>> >>>> Just to be curious, have you ever think about or tested wbt with >>>> io-cost? And even more, how bfq work with io-cost? >>>> >>>> I haven't tested yet, but it seems to me some of them can work well >>>> together. >>> No, I didn't test these combinations. I actually expect there would be >>> troubles in both cases under high IO load but you can try :) >> >> Just realize I made a clerical error, I actually want to saied that >> *can't* work well together. >> > > You are right, they can't work together, conceptually. Their logics would simply keep conflicting, and none of the two would make ti to control IO as desired.
Yes, I just run some simple tests, test result is very bad...
Perhaps we can do something like bfq does to disable wbt.
Thanks, Kuai > > Thanks, > Paolo > >> I'll try to have a test the combinations. >> >> Thanks, >> Kuai >>> Honza > > . >
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