> Il giorno 27 set 2022, alle ore 03:02, Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> 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. Thanks, Paolo > I'll try to have a test the combinations. > > Thanks, > Kuai >> Honza
