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 18:23:03 +0800 |
| |
Hi, Jan
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.
Thanks, Kuai
> So I'm kind of wondering why you'd like to use blk-wbt and BFQ together... > > Honza >
| |