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 | Fri, 23 Sep 2022 19:32:46 +0800 |
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Hi, Jan!
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. > I see it now, thanks a lot for your expiations, that really helps a lot.
I misunderstand about the how the bfq works. I'll remove this patch in next version.
Thanks, Kuai
> Honza >
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